Yellow fever vaccine (17D) — Vaccine Data Navigator

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Yellow fever Yellow fever virus (orthoflavivirus; 17D vaccine strain derived from the Asibi strain)

Disease risks

Acute risks of the infection itself: complications, hospitalisation, ICU, death.

Outcome (O)	Population (P)	Estimate	Design (S)	Source
Death among severe yellow fever cases severity: death case definition: Severe cases defined by fever and either jaundice or haemorrhaging	not reported in the pooled estimate vaccination status: not reported country: Africa and South America setting: surveillance data and outbreak investigations period: literature to March 2021 n: 32 proportions from 14 articles, 26 pooled, plus four Brazilian studies All ages	39 (31–47) CFR (%) per severe case	meta-analysis	Servadio et al. 2021 BMC Infect Dis quote RESULTS: The estimated CFR was 39 % (95 % CI: 31 %, 47 %). abstract, RESULTS The first systematic review to estimate yellow fever CFR. High heterogeneity; South America showed a higher CFR than Africa, on fewer studies.
Death during yellow fever outbreaks, range across African countries severity: death case definition: Cases reported in the included African outbreak and surveillance studies	not reported vaccination status: mixed country: African countries (Ghana to Nigeria) setting: outbreaks period: 1975-2020 n: 12 studies All ages	10 CFR (%), lower bound of the country range per reported case	systematic review	Nwaiwu et al. 2021 BMC Infect Dis quote Case fatality rate associated with yellow fever outbreaks ranged from 10% in Ghana to 86% in Nigeria. abstract, RESULTS The source reports a range from 10% in Ghana to 86% in Nigeria; this record holds the low end and its companion the high end. Case ascertainment differs enormously between these surveillance systems, which is most of why the range is so wide.
Death during yellow fever outbreaks, upper end of the African country range severity: death case definition: Cases reported in the included African outbreak and surveillance studies	not reported vaccination status: mixed country: Nigeria setting: outbreaks period: 1975-2020 n: 12 studies All ages	86 CFR (%), upper bound of the country range per reported case	systematic review	Nwaiwu et al. 2021 BMC Infect Dis quote Case fatality rate associated with yellow fever outbreaks ranged from 10% in Ghana to 86% in Nigeria. abstract, RESULTS Almost certainly reflects detection of only the most severe cases rather than a true 86% risk of death per infection. Do not quote without the caveat.
Reported yellow fever incidence across African countries severity: any reported case case definition: Reported cases per 100,000 population in the included studies	not reported vaccination status: mixed country: Nigeria, Uganda, DRC, Kenya, Ethiopia, Gambia, Senegal, Ghana period: 1975-2020 n: 12 studies All ages	1 incidence per 100,000 population, lowest country figure per 100,000 population	systematic review	Nwaiwu et al. 2021 BMC Infect Dis quote The incidence of yellow fever per 100,000 population ranged from < 1 case in Nigeria, < 3 cases in Uganda, 13 cases in Democratic Republic of the Congo, 27 cases in Kenya, 40 cases in Ethiopia, 46 cases in Gambia, 1267 cases in Senegal, and 10,350 cases in Ghana. abstract, RESULTS The sentence gives a range from under 1 case per 100,000 in Nigeria to 10,350 in Ghana (an epidemic-period figure). The spread is driven by outbreak years and surveillance quality, not by stable differences in risk.
Progression from infection to severe (toxic) yellow fever severity: severe/toxic phase case definition: Severe infection with recurrence of high fever, jaundice, bleeding, organ failure and shock	not reported (all ages) vaccination status: not reported country: global All ages	15 proportion of infections progressing to severe disease (%) per infection	expert body	who.int quote About 15% of people infected with yellow fever will have severe infection. Fact sheet, Symptoms WHO fact sheet figure. CDC gives a lower proportion in the companion record; the two expert-body figures should be read together, and neither carries an interval.
Death among patients in the toxic phase of yellow fever severity: death case definition: Patients with severe infection (high fever, jaundice, bleeding, organ failure, shock)	not reported (all ages) vaccination status: not reported country: global All ages	50 CFR of the toxic phase (%) per patient in the toxic phase	expert body	who.int quote About 50% of patients in this phase die within 7–10 days. Fact sheet, Symptoms The denominator is patients who reach the toxic phase, not all infections. Do not multiply this by the progression proportion without saying so explicitly.
Progression to the serious (haemorrhagic, jaundiced) form of yellow fever severity: severe case definition: Infected patients progressing after a brief remission to haemorrhagic symptoms, jaundice, shock and multisystem organ failure	not reported (all ages) vaccination status: not reported country: endemic Africa and tropical South America All ages	12 proportion of infected patients progressing to severe disease (%) per infected patient	expert body	cdc.gov quote After a brief remission of ≤48 hours, approximately 12% of infected patients progress to a more serious form of the disease, characterized by hemorrhagic symptoms, jaundice, and eventually shock and multisystem organ failure. Yellow Book chapter, Clinical presentation Lower than the WHO fact sheet proportion. Both are expert-body estimates without intervals.
Death among severe yellow fever cases severity: death case definition: Severe cases with haemorrhage, jaundice, shock and multisystem organ failure	not reported (all ages) vaccination status: not reported country: endemic Africa and tropical South America All ages	30 CFR of severe cases (%), range as reported per severe case	expert body	cdc.gov quote The case-fatality rate for severe cases is 30–60%. Yellow Book chapter, Clinical presentation The source gives a range of 30-60%, not a point estimate; only the lower bound is filed as the value because the compiler takes one number. The meta-analysed CFR record is the better single figure.
Annual global yellow fever cases severity: clinical disease case definition: Clinical yellow fever disease	not reported (all ages) vaccination status: mixed country: worldwide All ages	200,000 annual cases (count) per year worldwide	expert body	cdc.gov quote Worldwide, yellow fever virus causes an estimated 200,000 cases of clinical disease and 30,000 deaths annually ( 4 ). MMWR 2015, Yellow Fever Epidemiology and Risk for Disease in Travelers The older WHO-derived figure ACIP quoted, used here because it is still the number most often repeated. The same sentence also gives an annual death count; the modelled estimates are the ones to prefer.
Death among severe yellow fever cases severity: death case definition: Severe yellow fever with jaundice and haemorrhage	not reported (all ages) vaccination status: not reported country: worldwide All ages	20 CFR of severe cases (%), lower bound of the reported range per severe case	expert body	cdc.gov quote The case-fatality ratio for severe yellow fever is 20%–50% ( 5 ). MMWR 2015, Yellow Fever Epidemiology and Risk for Disease in Travelers The source gives 20%-50%. Only the lower bound is filed as the value; the meta-analysed CFR record is the better single estimate.
Yellow fever illness in an unvaccinated traveller severity: illness case definition: Illness attributed to yellow fever during a two-week stay	not reported (adult travellers) risk group: unvaccinated international travellers vaccination status: unvaccinated country: West Africa setting: travel to an endemic area All ages	50 risk per 100,000 travellers per 2-week stay per 100,000 unvaccinated travellers, 2-week stay	expert body	cdc.gov quote For a 2-week stay, the estimated risks for illness and for death due to YF for an unvaccinated traveler visiting an endemic area are as follows: for West Africa, risk for illness is 50 per 100,000 and risk for death is 10 per 100,000; for South America, risk for illness is 5 per 100,000 and risk for death is 1 per 100,000. Yellow Book chapter, Risk for travelers Derived from risk to resident populations, often in peak season; CDC says it might not reflect traveller risk, whose immunity, exposure and season differ. The same sentence gives risk of death as 10 per 100,000.
Yellow fever illness in an unvaccinated traveller severity: illness case definition: Illness attributed to yellow fever during a two-week stay	not reported (adult travellers) risk group: unvaccinated international travellers vaccination status: unvaccinated country: South America setting: travel to an endemic area All ages	5 risk per 100,000 travellers per 2-week stay per 100,000 unvaccinated travellers, 2-week stay	expert body	cdc.gov quote For a 2-week stay, the estimated risks for illness and for death due to YF for an unvaccinated traveler visiting an endemic area are as follows: for West Africa, risk for illness is 50 per 100,000 and risk for death is 10 per 100,000; for South America, risk for illness is 5 per 100,000 and risk for death is 1 per 100,000. Yellow Book chapter, Risk for travelers A tenth of the West Africa estimate. Same caveat: extrapolated from resident-population risk. The same sentence gives risk of death as 1 per 100,000.
Death within 60 days of hospital admission for confirmed yellow fever severity: death case definition: Yellow fever confirmed by real-time PCR in blood at admission or at autopsy	adults older than 18 years vaccination status: not reported country: Brazil setting: two hospitals in Sao Paulo, sylvatic outbreak period: 11 January to 10 May 2018 n: 76 patients with confirmed yellow fever 18-49y50-64y65+	36 CFR among hospitalised confirmed cases (%) per hospitalised confirmed case follow-up: 60 days after admission	cohort	Kallas et al. 2019 Lancet Infect Dis quote 27 (36%) of 76 patients died during the 60 day period after hospital admission. abstract, FINDINGS Hospitalised patients only, so this is the fatality of severe disease that reached care, not of infection. Older age, male sex, high neutrophil count, raised AST and high viral load were associated with death.
Death among travel-associated yellow fever cases severity: death case definition: Yellow fever in residents of the United States and Europe who travelled to West Africa or South America	not reported risk group: international travellers, almost all unvaccinated vaccination status: unvaccinated country: United States and Europe (travellers to West Africa and South America) period: 1970-2015 n: 11 cases All ages	73 proportion of travel-associated cases who died (%) per travel-associated case	surveillance	cdc.gov quote During 1970–2015, 11 cases of YF were reported in people from the United States and Europe who traveled to West Africa (6 cases) or South America (5 cases); 8 (73%) died. Yellow Book chapter, Risk for travelers Eight of eleven died. The only vaccinated traveller in the series survived. Tiny numbers: this is a case count, not a rate, and severe cases are the ones that get reported.
Death among travel-associated yellow fever cases acquired in Brazil severity: death case definition: Travel-related yellow fever in international travellers returning from Brazil	not reported risk group: international travellers to Brazil during the 2016-18 outbreak vaccination status: unvaccinated country: Brazil (acquisition); various (reporting) setting: outbreak period: January 2018 onward n: 10 travel-related cases All ages	10 travel-associated cases reported (count) per outbreak period, no traveller denominator available	surveillance	Hamer et al. 2018 MMWR Morb Mortal Wkly Rep quote Since January 2018, 10 travel-related cases of yellow fever, including four deaths, have been reported in international travelers returning from Brazil. abstract Four of these ten travellers died, and MMWR states explicitly that none of the 10 had received yellow fever vaccination. The fatality count is written out in words in the source, so only the case count can be filed as a number. A count, not a rate: no denominator of travellers at risk is available.
Death among confirmed yellow fever cases in Minas Gerais severity: death case definition: Confirmed yellow fever among reported cases in residents of Minas Gerais state	57% of reported cases were men aged 30 to 59 years vaccination status: partially vaccinated country: Brazil (Minas Gerais) setting: sylvatic outbreak in a partially vaccinated state period: 2016-2018 n: 3,304 reported cases, 905 confirmed 18-49y50-64y	202 deaths among confirmed cases (count) per confirmed case	surveillance	Ferreira et al. 2022 PLoS Negl Trop Dis quote Approximately 27% of cases (905) were confirmed, and 22% (202) of these individuals died. abstract, METHODOLOGY AND PRINCIPAL FINDINGS 202 deaths among 905 confirmed cases. The state had had yellow fever vaccine in its routine schedule for decades, and heterogeneous coverage is the explanation the authors give for the outbreaks.
Laboratory-confirmed yellow fever among suspected cases, Angola outbreak severity: confirmed infection case definition: Suspected yellow fever samples positive by ELISA and confirmed by RT-PCR	highest positivity in the 20-29 and 10-19 year age groups sex: incidence about three times higher in males than females aged 20-29 vaccination status: mixed country: Angola setting: national referral laboratory during the 2015-16 urban outbreak period: 5 December 2015 to 23 December 2016 n: 4,618 samples from suspected cases 5-11y12-17y18-49y50-64y	19.1 proportion of suspected cases laboratory-confirmed (%) per suspected case tested	surveillance	Manuel et al. 2024 Health Sci Rep quote RESULTS: We detected 884 (19.1%) cases that were positive for ELISA, which were confirmed by RT-PCR assay. abstract, RESULTS A laboratory-confirmation proportion, not an attack rate. Deaths in this outbreak concentrated in young adults.
Suspected yellow fever cases and deaths, Kongo Central Province, DRC severity: death case definition: Suspected yellow fever cases reported to surveillance; 37 of 393 specimens laboratory-confirmed	not reported vaccination status: partially vaccinated country: Democratic Republic of the Congo (Kongo Central Province) setting: outbreak linked to importation from Angola period: as of 18 August 2016 n: 410 suspected cases All ages	42 deaths among suspected cases (count) per suspected case	surveillance	Otshudiema et al. 2017 MMWR Morb Mortal Wkly Rep quote As of August 18, 2016, a total of 410 suspected yellow fever cases and 42 deaths were reported in Kongo Central Province. abstract Suspected cases, most never laboratory-confirmed: malaria, viral hepatitis and typhoid are common differentials in this setting, so the denominator is not a set of yellow fever infections. Nearly all confirmed cases in this province were imported from Angola.
Suspected cases and deaths in the 2015-16 Angola and DRC epidemic severity: death case definition: Suspected yellow fever cases reported during the epidemic	not reported vaccination status: mixed country: Angola and Democratic Republic of the Congo setting: urban epidemic period: late 2015 to 2016 n: more than 7,334 suspected cases All ages	393 deaths among suspected cases (count) per suspected case	surveillance	Kraemer et al. 2017 Lancet Infect Dis quote BACKGROUND: Since late 2015, an epidemic of yellow fever has caused more than 7334 suspected cases in Angola and the Democratic Republic of the Congo, including 393 deaths. abstract, BACKGROUND Suspected, not confirmed, cases. The epidemic grew with a doubling time of under a week and reached dozens of districts within three months.
Annual yellow fever cases with fever and jaundice or haemorrhage severity: severe case definition: Cases with fever and jaundice or haemorrhage, modelled from outbreak reports and serological surveys	all ages in the modelled endemic population vaccination status: mixed country: Africa, endemic zone period: 2013 All ages	130,000 (51,000–380,000) annual cases (count) per year in the African endemic zone	modelling	Garske et al. 2014 PLoS Med quote The yellow fever burden in Africa was estimated for the year 2013 as 130,000 (95% CI 51,000-380,000) cases with fever and jaundice or haemorrhage including 78,000 (95% CI 19,000-180,000) deaths, taking into account the current level of vaccination coverage. abstract, METHODS AND FINDINGS Modelled, not counted; the authors name sparse surveillance and serosurvey data as the main limitation. Deaths from the same model are a separate record.
Annual yellow fever deaths severity: death case definition: Deaths among modelled cases with fever and jaundice or haemorrhage	all ages in the modelled endemic population vaccination status: mixed country: Africa, endemic zone period: 2013 All ages	78,000 (19,000–180,000) annual deaths (count) per year in the African endemic zone	modelling	Garske et al. 2014 PLoS Med quote The yellow fever burden in Africa was estimated for the year 2013 as 130,000 (95% CI 51,000-380,000) cases with fever and jaundice or haemorrhage including 78,000 (95% CI 19,000-180,000) deaths, taking into account the current level of vaccination coverage. abstract, METHODS AND FINDINGS Same model as the companion case-count record. The interval spans an order of magnitude.
Annual severe yellow fever infections severity: severe case definition: Severe infections estimated from occurrence and serology data	all ages in the modelled endemic population vaccination status: mixed country: Africa and South America period: 2018 All ages	109,000 (67,000–173,000) annual severe infections (count) per year in the endemic populations of Africa and South America	modelling	Gaythorpe et al. 2021 Elife quote We estimate that there were 109,000 (95% credible interval [CrI] [67,000-173,000]) severe infections and 51,000 (95% CrI [31,000-82,000]) deaths due to YF in Africa and South America in 2018. abstract A credible interval, not a confidence interval. Wider geography than the earlier Africa-only model, so the two counts are not directly comparable.
Annual yellow fever deaths severity: death case definition: Deaths estimated from occurrence and serology data	all ages in the modelled endemic population vaccination status: mixed country: Africa and South America period: 2018 All ages	51,000 (31,000–82,000) annual deaths (count) per year in the endemic populations of Africa and South America	modelling	Gaythorpe et al. 2021 Elife quote We estimate that there were 109,000 (95% credible interval [CrI] [67,000-173,000]) severe infections and 51,000 (95% CrI [31,000-82,000]) deaths due to YF in Africa and South America in 2018. abstract Credible interval. Lower than the earlier Africa-only estimate for a single year; method differences as well as vaccination separate the two.
In-hospital death among ICU-admitted yellow fever patients severity: death case definition: Laboratory-confirmed yellow fever admitted to intensive care because of rapid clinical deterioration	not reported (adults) vaccination status: not reported country: Brazil setting: ICU of a tertiary hospital in Sao Paulo, outbreak period: 10 January to 11 March 2018 n: 79 patients 18-49y50-64y65+	67 in-hospital CFR (%) per ICU-admitted confirmed case	case series	Ho et al. 2019 J Travel Med quote The in-hospital fatality rate was 67%. abstract, RESULTS The most severely ill end of the spectrum: nearly three-quarters needed renal replacement therapy. Not comparable to a population CFR.
In-hospital death among ICU yellow fever patients with diabetes severity: death case definition: Laboratory-confirmed yellow fever admitted to intensive care	not reported (adults) risk group: diabetes mellitus vaccination status: not reported country: Brazil setting: ICU, outbreak period: 2018 n: 79 ICU patients 18-49y50-64y65+	80 CFR among ICU patients with diabetes (%) per ICU-admitted confirmed case with diabetes	case series	Ho et al. 2019 J Travel Med quote Patients with diabetes mellitus had a higher case fatality rate (CFR) of 80%, while patients without diabetes had a CFR of 65%. abstract, RESULTS Compared with 65% in patients without diabetes in the same series. No adjustment and very small subgroups, so treat as descriptive.
Confirmed sylvatic yellow fever cases and deaths in Sao Paulo state severity: confirmed case and death case definition: Autochthonous confirmed human sylvatic yellow fever cases	not reported vaccination status: mixed country: Brazil (Sao Paulo state) setting: sylvatic outbreak period: 2016-2019 n: 648 confirmed human cases All ages	230 deaths among confirmed cases (count) per confirmed human case	ecological	Lacerda et al. 2021 Sci Rep quote We confirmed 648 human cases of SYF in SP, with 230 deaths and 843 NHP epizootics. abstract The authors note there were not enough vaccine doses for the population at risk during this outbreak. Confirmed cases only; milder infections are missing from the denominator.

Vaccine efficacy and effectiveness

Every estimate compiled, annotated by population (P), intervention (I), comparator (C), outcome (O) and study design (S).

Outcome (O)	Population (P)	Intervention / comparator	Estimate	Design (S)	Source
Seroprotection 10 or more years after a single dose severity: serological surrogate case definition: Neutralising-antibody seroprotection by each study's own criterion	healthy adults risk group: mostly travellers country: non-endemic settings period: vaccinated 10-60 years earlier n: 2,895 individuals in 39 studies, 20 pooled 18-49y50-64y65+	17D yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 10 or more years vs none (single-arm pooled proportion)	94 (86–99) seroprotection rate (%)	meta-analysis	Schnyder et al. 2024 Lancet Glob Health quote Pooled seroprotection rates were 94% (95% CI 86-99) among healthy adults in a non-endemic setting (mostly travellers) and 76% (65-85) in an endemic setting (all Brazilian studies). abstract, FINDINGS A serological surrogate, not protection against disease, and the criteria for seroprotection were highly heterogeneous. The endemic-setting figure from the same sentence is a separate record.
Seroprotection 10 or more years after a single dose, endemic setting severity: serological surrogate case definition: Neutralising-antibody seroprotection by each study's own criterion	healthy adults country: endemic setting, all Brazilian studies period: vaccinated 10 or more years earlier 18-49y50-64y65+	17DD yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 10 or more years vs none (single-arm pooled proportion)	76 (65–85) seroprotection rate (%)	meta-analysis	Schnyder et al. 2024 Lancet Glob Health quote Pooled seroprotection rates were 94% (95% CI 86-99) among healthy adults in a non-endemic setting (mostly travellers) and 76% (65-85) in an endemic setting (all Brazilian studies). abstract, FINDINGS Lower than the 94% in non-endemic travellers. The review attributes part of the difference to a higher seroprotection cutoff used in Brazil, which is a methodological explanation rather than a biological one.
Seroprotection 10 or more years after a single dose given in infancy severity: serological surrogate case definition: Neutralising-antibody seroprotection in those vaccinated at 9-23 months of age	aged 9-23 months at the time of vaccination period: vaccinated 10 or more years earlier Infant <1y1-4y	17D yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 10 or more years vs none (single-arm pooled proportion)	47 (35–60) seroprotection rate (%)	meta-analysis	Schnyder et al. 2024 Lancet Glob Health quote The pooled seroprotection rate was 47% (35-60) in children (aged 9-23 months at time of vaccination) and 61% (38-82) in people living with HIV. abstract, FINDINGS Under half still seroprotected. This is the evidence the review uses to say booster doses might still be needed for children vaccinated under 2, against the single-dose-for-life recommendation.
Seroprotection 10 or more years after a single dose in people living with HIV severity: serological surrogate case definition: Neutralising-antibody seroprotection by each study's own criterion	not separated by age immune status: people living with HIV period: vaccinated 10 or more years earlier All ages	17D yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 10 or more years vs none (single-arm pooled proportion)	61 (38–82) seroprotection rate (%)	meta-analysis	Schnyder et al. 2024 Lancet Glob Health quote The pooled seroprotection rate was 47% (35-60) in children (aged 9-23 months at time of vaccination) and 61% (38-82) in people living with HIV. abstract, FINDINGS Wide interval on few studies. The review concludes booster doses might still be required in this group.
Seroprotection within 5 years of primary vaccination in early childhood severity: serological surrogate case definition: Pooled seroprotection rate in children vaccinated before age 2	vaccinated before age 2 years n: more than 17,000 participants aged 6 months to 85 years across 36 studies Infant <1y1-4y	17D yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: within 5 years after primary vaccination vs none (single-arm pooled proportion)	52 seroprotection rate (%)	meta-analysis	Kling et al. 2022 Clin Infect Dis quote In children vaccinated before age 2 years, the seroprotection rate was 52% within 5 years after primary vaccination. abstract No interval reported for this stratum. The same review found rates close to universal by three months in healthy adults and children, so this is waning in the youngest, not primary failure alone.
Seropositivity 10 or more years after primary vaccination severity: serological surrogate case definition: Yellow fever virus-specific antibodies detected in a blood sample	not reported n: 1,137 persons in 13 observational studies All ages	yellow fever vaccine platform: live attenuated doses: 1 (primary) time since vaccination: 10 or more years vs none (single-arm pooled proportion)	92 (85–96) seropositivity rate (%)	meta-analysis	cdc.gov quote Using a random effects model, the estimated seropositivity rate for persons vaccinated ≥10 years previously was 92% (95% confidence interval [CI] = 85%–96%). MMWR 2015, Seropositivity The GRADE figure behind the ACIP decision to drop the 10-year booster. Observational studies pooled with a random-effects model.
Seropositivity 20 or more years after primary vaccination severity: serological surrogate case definition: Yellow fever virus-specific antibodies detected in a blood sample	not reported n: 164 persons All ages	yellow fever vaccine platform: live attenuated doses: 1 (primary) time since vaccination: 20 or more years vs none (single-arm pooled proportion)	80 (74–86) seropositivity rate (%)	meta-analysis	cdc.gov quote Of the 164 persons vaccinated ≥20 years previously, the estimated seropositivity rate was 80% (CI = 74%–86%). MMWR 2015, Seropositivity Lower than the pooled rate at a decade or more, on a much smaller sample. ACIP nonetheless judged a booster unnecessary for most travellers.
Seroconversion after primary vaccination in children severity: serological surrogate case definition: Seroconversion in children resident in endemic areas, most co-vaccinated with other antigens	4 months to 10 years country: endemic areas n: 4,675 children Infant <1y1-4y5-11y	yellow fever vaccine platform: live attenuated doses: 1 vs none (single-arm pooled proportion)	93 (88–96) seroconversion rate (%)	meta-analysis	cdc.gov quote Based on a random effects model, the estimated seroconversion rate in 4,675 children was 93% (CI = 88%‒96%). MMWR 2015, Other relevant evidence ACIP reports no difference between children vaccinated under 9 months and those vaccinated at 9 months or older. Short-term seroconversion only; the durability records tell a different story for this age group.
Seroconversion after a one-fifth fractional dose versus the standard dose severity: serological surrogate case definition: Seroconversion at four to five weeks after vaccination	trial populations, not separated here n: 155 participants All ages	WHO-prequalified yellow fever vaccines, one fifth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 4-5 weeks vs standard dose	1.00 (0.98–1.03) RR, fractional versus standard dose	meta-analysis	Nnaji et al. 2020 Vaccine quote Seroconversion rates at four to five weeks following vaccination were similar between participants who received standard doses and participants who received fractional doses containing one-third (547 participants: risk ratio [RR] 1.02, 95% confidence interval [CI] 1.00-1.04), one-fifth (155 participants: RR 1.00, 95% CI 0.98-1.03), one-tenth (890 participants: RR 0.99, 95% CI 0.96-1.01), and one-fiftieth (661 participants: RR 0.97, 95% CI 0.92-1.02) of the standard dose. abstract, RESULTS Pooled across eight trials. The interval excludes a clinically meaningful loss at this fraction.
Seroconversion after a one-third fractional dose versus the standard dose severity: serological surrogate case definition: Seroconversion at four to five weeks after vaccination	trial populations, not separated here n: 547 participants All ages	WHO-prequalified yellow fever vaccines, one third of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 4-5 weeks vs standard dose	1.02 (1.00–1.04) RR, fractional versus standard dose	meta-analysis	Nnaji et al. 2020 Vaccine quote Seroconversion rates at four to five weeks following vaccination were similar between participants who received standard doses and participants who received fractional doses containing one-third (547 participants: risk ratio [RR] 1.02, 95% confidence interval [CI] 1.00-1.04), one-fifth (155 participants: RR 1.00, 95% CI 0.98-1.03), one-tenth (890 participants: RR 0.99, 95% CI 0.96-1.01), and one-fiftieth (661 participants: RR 0.97, 95% CI 0.92-1.02) of the standard dose. abstract, RESULTS Seroconversion rates similar to the standard dose at this fraction.
Seroconversion after a one-tenth fractional dose versus the standard dose severity: serological surrogate case definition: Seroconversion at four to five weeks after vaccination	trial populations, not separated here n: 890 participants All ages	WHO-prequalified yellow fever vaccines, one tenth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 4-5 weeks vs standard dose	0.99 (0.96–1.01) RR, fractional versus standard dose	meta-analysis	Nnaji et al. 2020 Vaccine quote Seroconversion rates at four to five weeks following vaccination were similar between participants who received standard doses and participants who received fractional doses containing one-third (547 participants: risk ratio [RR] 1.02, 95% confidence interval [CI] 1.00-1.04), one-fifth (155 participants: RR 1.00, 95% CI 0.98-1.03), one-tenth (890 participants: RR 0.99, 95% CI 0.96-1.01), and one-fiftieth (661 participants: RR 0.97, 95% CI 0.92-1.02) of the standard dose. abstract, RESULTS Still similar to the standard dose on the largest subgroup in the review.
Seroconversion after a one-fiftieth fractional dose versus the standard dose severity: serological surrogate case definition: Seroconversion at four to five weeks after vaccination	trial populations, not separated here n: 661 participants All ages	WHO-prequalified yellow fever vaccines, one fiftieth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 4-5 weeks vs standard dose	0.97 (0.92–1.02) RR, fractional versus standard dose	meta-analysis	Nnaji et al. 2020 Vaccine quote Seroconversion rates at four to five weeks following vaccination were similar between participants who received standard doses and participants who received fractional doses containing one-third (547 participants: risk ratio [RR] 1.02, 95% confidence interval [CI] 1.00-1.04), one-fifth (155 participants: RR 1.00, 95% CI 0.98-1.03), one-tenth (890 participants: RR 0.99, 95% CI 0.96-1.01), and one-fiftieth (661 participants: RR 0.97, 95% CI 0.92-1.02) of the standard dose. abstract, RESULTS The lowest fraction still similar to standard dose in this pooling. The review states that seroconversion was substantially lower at one-hundredth and below, and recommends fractional dosing specifically for the young, immunocompetent and well nourished.
Seroconversion after vaccination in people living with HIV severity: serological surrogate case definition: Seroconversion after yellow fever vaccine given at least once after HIV diagnosis	all ages including perinatally infected children immune status: people living with HIV All ages	yellow fever vaccine (17DD or 17D-204) platform: live attenuated doses: 1 or more vs none (single-arm pooled proportion)	97.6 (91.6–100) seroconversion rate (%)	meta-analysis	Martin et al. 2021 Clin Microbiol Infect quote A meta-analysis demonstrated that 97.6% (95% CI 91.6%-100%) of the included population seroconverted. abstract, RESULTS Pooled from studies the review graded mostly Poor quality. The review concludes the evidence base is too weak to design a vaccination schedule for this group.
Persistence of neutralising antibodies 1-10 years after vaccination in people living with HIV severity: serological surrogate case definition: Reported persistence of neutralising antibodies	all ages immune status: people living with HIV All ages	yellow fever vaccine platform: live attenuated doses: 1 or more time since vaccination: 1 to 10 years vs none (single-arm pooled proportion)	72 (53.6–91) antibody persistence (%)	meta-analysis	Martin et al. 2021 Clin Microbiol Infect quote Between 1 and 10 years after YF vaccine, reported persistence of neutralizing antibodies was 72% (95% CI 53.6%-91%), and it was 62% (95% CI 45.4%-78.6%) more than 10 years after YF vaccine. abstract, RESULTS The same sentence reports 62% (45.4-78.6) beyond 10 years, filed separately. Low study quality throughout.
Persistence of neutralising antibodies more than 10 years after vaccination in people living with HIV severity: serological surrogate case definition: Reported persistence of neutralising antibodies	all ages immune status: people living with HIV All ages	yellow fever vaccine platform: live attenuated doses: 1 or more time since vaccination: more than 10 years vs none (single-arm pooled proportion)	62 (45.4–78.6) antibody persistence (%)	meta-analysis	Martin et al. 2021 Clin Microbiol Infect quote Between 1 and 10 years after YF vaccine, reported persistence of neutralizing antibodies was 72% (95% CI 53.6%-91%), and it was 62% (95% CI 45.4%-78.6%) more than 10 years after YF vaccine. abstract, RESULTS Roughly four in ten without detectable neutralising antibodies beyond a decade. This is the evidence behind the ACIP exception that recommends a dose every 10 years for people with HIV who remain at risk.
Humoral vaccine response rate within three months of primary vaccination in children severity: serological surrogate case definition: Proportion testing positive for vaccine-induced yellow fever neutralising antibodies	children aged 12 years or younger country: endemic countries (all but one study) n: 14,028 infants and children in 33 articles Infant <1y1-4y5-11y	yellow fever vaccines including 17DD and 17D substrains platform: live attenuated doses: 1 time since vaccination: within three months vs none (single-arm pooled proportion)	91.9 (89.8–93.9) vaccine response rate (%)	meta-analysis	Ferrara et al. 2024 J Travel Med quote Within three months following vaccination, the pooled VRR was 91.9% (95% CI 89.8-93.9). abstract, RESULTS The review found a lower response with the Brazilian substrain on meta-regression, no significant difference by age, route, co-administration or fractional dosing, and a decline in response over time that becomes observable after the first year and a half.
Documented vaccine failures after primary vaccination severity: clinical disease case definition: Yellow fever disease in a person with a documented prior dose	not reported All ages	yellow fever vaccine platform: live attenuated doses: 1 or more vs none (case count against doses distributed)	23 vaccine failures identified (count)	expert body	cdc.gov quote A total of 23 vaccine failures were identified after the administration of >540 million doses of yellow fever vaccine ( 3 ). MMWR 2015, Vaccine effectiveness Against more than 540 million doses administered, per the same sentence. A count from passive case-finding, not an effectiveness estimate: unreported failures and the absence of a true denominator of vaccinated person-time both push the same way.
Timing of documented vaccine failures relative to vaccination severity: clinical disease case definition: Vaccine failures excluding the five that occurred within 10 days of vaccination	not reported n: 18 vaccine failures All ages	yellow fever vaccine platform: live attenuated doses: 1 or more vs none (descriptive distribution of cases)	89 proportion of failures occurring within 10 years of vaccination (%)	expert body	cdc.gov quote Of the remaining 18 cases, 16 (89%) occurred in persons who reported receiving a dose of the vaccine within the previous 10 years ( 3 ). MMWR 2015, Vaccine effectiveness Failures clustered soon after vaccination rather than accumulating with time since dose, which is the argument ACIP used against waning protection. One failure occurred two decades after vaccination and another later still.
Time to development of immunity after vaccination severity: serological surrogate case definition: Development of immunity after a single dose	not reported (all ages from 9 months) country: global All ages	yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 10 days vs none (expert-body statement)	80 proportion developing immunity within 10 days (%), lower bound of the reported range	expert body	who.int quote Immunity develops in for 80–100% of people within 10 days, and in over 99% within 30 days. Fact sheet, Prevention WHO states immunity develops in 80-100% of people within 10 days and in over 99% within 30 days, and that a single dose provides lifelong immunity with no booster needed. The durability records in this file, particularly those in children and people with HIV, are the ones to read against that last claim.
Seroconversion 28 days after a standard dose in African adults severity: serological surrogate case definition: Antibody titre at day 28 at least four times the pre-vaccination titre by PRNT	adults country: Uganda and Kenya n: 120 participants in the standard-dose group of 480 randomised 18-49y50-64y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, standard dose 13,803 IU platform: live attenuated doses: 1 time since vaccination: 28 days vs fractional doses of 1000, 500 and 250 IU in the other trial arms	98 (94–100) seroconversion rate (%)	RCT	Kimathi et al. 2025 N Engl J Med quote The incidence of seroconversion was 98% (95% confidence interval [CI], 94 to 100) with the standard dose. abstract, RESULTS The reference arm of the NIFTY trial. The fractional-dose comparisons from the same trial are separate records.
Difference in seroconversion, 500 IU versus standard dose, per-protocol severity: serological surrogate case definition: Seroconversion at day 28 by PRNT, per-protocol population	adults country: Uganda and Kenya n: 480 randomised, 120 per group 18-49y50-64y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, 500 IU platform: live attenuated doses: 1 fractional time since vaccination: 28 days vs standard dose of 13,803 IU	-1.8 (-6.7–3.2) difference in seroconversion (percentage points)	RCT	Kimathi et al. 2025 N Engl J Med quote The difference in the incidence of seroconversion between the 1000-IU dose and the standard dose was 0.01 percentage points (95% CI, -5.0 to 5.1) in the intention-to-treat population and -1.9 percentage points (95% CI, -7.0 to 3.2) in the per-protocol population; the corresponding differences between the 500-IU dose and the standard dose were 0.01 percentage points (95% CI, -5.0 to 5.1) and -1.8 percentage points (95% CI, -6.7 to 3.2), and those between the 250-IU dose and the standard dose were -4.4 percentage points (95% CI, -9.4 to 0.7) and -6.7 percentage points (95% CI, -11.7 to 1.6). abstract, RESULTS The non-inferiority margin was ten percentage points, which this dose met in adults. The trial authors conclude a dose as low as 500 IU was non-inferior in adults; the infant trial reached the opposite conclusion and is filed separately.
Difference in seroconversion, 250 IU versus standard dose, per-protocol severity: serological surrogate case definition: Seroconversion at day 28 by PRNT, per-protocol population	adults country: Uganda and Kenya n: 480 randomised, 120 per group 18-49y50-64y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, 250 IU platform: live attenuated doses: 1 fractional time since vaccination: 28 days vs standard dose of 13,803 IU	-6.7 (-11.7–1.6) difference in seroconversion (percentage points)	RCT	Kimathi et al. 2025 N Engl J Med quote The difference in the incidence of seroconversion between the 1000-IU dose and the standard dose was 0.01 percentage points (95% CI, -5.0 to 5.1) in the intention-to-treat population and -1.9 percentage points (95% CI, -7.0 to 3.2) in the per-protocol population; the corresponding differences between the 500-IU dose and the standard dose were 0.01 percentage points (95% CI, -5.0 to 5.1) and -1.8 percentage points (95% CI, -6.7 to 3.2), and those between the 250-IU dose and the standard dose were -4.4 percentage points (95% CI, -9.4 to 0.7) and -6.7 percentage points (95% CI, -11.7 to 1.6). abstract, RESULTS The lower bound crosses the non-inferiority margin, so this dose did not qualify. A negative estimate filed as it stands.
Seroconversion 28 days after a standard dose in infants severity: serological surrogate case definition: Fourfold or greater rise in antibody titre at day 28 by PRNT50, per-protocol population	infants aged 9-12 months country: Kenya and Uganda n: 179 of 210 infants in the standard-dose group Infant <1y1-4y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, standard dose above 13,000 IU platform: live attenuated doses: 1 schedule: co-administered with measles-rubella vaccine time since vaccination: 28 days vs 500 IU fractional dose in the other arm	99 (96–100) seroconversion rate (%)	RCT	Kimathi et al. 2026 Lancet quote The seroconversion rate at day 28 was 99% (95% CI 96-100; 177 of 179 infants) for the standard dose and 93% (88-96; 166 of 179 infants) for the 500 IU dose in the per-protocol population. abstract, FINDINGS The standard-dose reference arm in infants. The same sentence gives 93% (88-96) for the 500 IU arm.
Seroconversion 28 days after a 500 IU dose in infants severity: serological surrogate case definition: Fourfold or greater rise in antibody titre at day 28 by PRNT50, per-protocol population	infants aged 9-12 months country: Kenya and Uganda n: 179 of 210 infants in the 500 IU group Infant <1y1-4y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, 500 IU platform: live attenuated doses: 1 fractional schedule: co-administered with measles-rubella vaccine time since vaccination: 28 days vs standard dose above 13,000 IU	93 (88–96) seroconversion rate (%)	RCT	Kimathi et al. 2026 Lancet quote The seroconversion rate at day 28 was 99% (95% CI 96-100; 177 of 179 infants) for the standard dose and 93% (88-96; 166 of 179 infants) for the 500 IU dose in the per-protocol population. abstract, FINDINGS Lower than the standard dose in the same trial. The difference record carries the formal non-inferiority result.
Difference in seroconversion, 500 IU versus standard dose, in infants severity: serological surrogate case definition: Fourfold or greater rise at day 28 by PRNT50, per-protocol population	infants aged 9-12 months country: Kenya and Uganda n: 420 infants randomised Infant <1y1-4y	Institut Pasteur de Dakar 17D-204 yellow fever vaccine, 500 IU platform: live attenuated doses: 1 fractional schedule: co-administered with measles-rubella vaccine time since vaccination: 28 days vs standard dose above 13,000 IU	-6.15 (-10.27–-2.02) difference in seroconversion (percentage points)	RCT	Kimathi et al. 2026 Lancet quote The difference in seroconversion rate was -6·15 percentage points (95% CI -10·27 to -2·02); therefore, non-inferiority was not met for the 500 IU dose. abstract, FINDINGS Non-inferiority was not met: the lower bound crosses the margin. The trial concludes that adult minimum-dose requirements do not generalise to infants and that standard doses should be used in the routine programme. This is the record that contradicts fractional dosing for infants.
Seroconversion 28 days after a fractional dose in young children severity: serological surrogate case definition: Seroconversion at 28 days measured by PRNT50, per-protocol population	children aged 9-59 months country: Uganda n: 207 of 210 children in the fractional-dose group Infant <1y1-4y5-11y	17D-213 yellow fever vaccine, one fifth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 28 days vs standard dose, 100% (98-100) in the same trial	97 (95–99) seroconversion rate (%)	RCT	Juan-Giner et al. 2023 Lancet Infect Dis quote 28 days after vaccination, 202 (97%, 95% CI 95-99) of 207 participants in the fractional dose group and 191 (100%, 98-100) of 191 in the standard dose group seroconverted. abstract, FINDINGS Non-inferior at 28 days, with an absolute difference well inside the margin. Fewer children had seroconverted by day ten in the fractional arm, which matters for outbreak response where speed is the point.
Seroconversion 28 days after a fractional dose in adults living with HIV severity: serological surrogate case definition: Post-vaccination titre at least four times the pre-vaccination titre by PRNT50, per-protocol population	adults aged 18-59 years immune status: HIV-positive with baseline CD4 count of at least 200 cells per mL country: Kenya setting: community recruitment, Kilifi period: January-May 2019 n: 117 of 126 in the fractional-dose group 18-49y50-64y	17D-213 yellow fever vaccine, one fifth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 28 days vs standard dose, 98% (94-100) in the same trial	96 (90–99) seroconversion rate (%)	RCT	Kimathi et al. 2023 Lancet Infect Dis quote 28 days after vaccination, 112 (96%, 95% CI 90-99) of 117 participants in the fractional dose group and 115 (98%, 94-100) of 117 in the standard dose group seroconverted by PRNT50. abstract, FINDINGS Met non-inferiority, with a difference of -3 percentage points (95% CI -7 to 2), maintained for one year. Restricted to people with preserved CD4 counts, so it says nothing about more advanced immunosuppression.
Seroconversion three months after vaccination at 6 versus 9 months of age severity: serological surrogate case definition: Yellow fever antibody titred on Vero cells against the vaccine virus, three months after immunisation	infants immunised at 6 months of age country: Ghana n: 420 infants randomised Infant <1y	17D yellow fever vaccine platform: live attenuated doses: 1 schedule: given after BCG, OPV and DPT, at 6 months of age time since vaccination: 3 months vs infants immunised at 9 months, 98% seroconversion in the same trial	98.6 seroconversion rate (%)	RCT	Osei-Kwasi et al. 2001 Bull World Health Organ quote Infants immunized at 6 months recorded seroconversion of 98.6% and those immunized at 9 months recorded 98% seroconversion. abstract, FINDINGS No pre-immunisation sera had detectable antibodies in either group. The authors suggest the vaccine could be used at 6 months in outbreaks or high-risk endemic areas. Short follow-up only.
Seroconversion after vaccination with two 17D substrains in young children severity: serological surrogate case definition: Yellow fever antibodies titred before and 30 or more days after vaccination	children aged nine to 23 months country: Brazil (Mato Grosso do Sul, Minas Gerais, Sao Paulo) n: 1,966 children randomised Infant <1y1-4y	yellow fever vaccine prepared with substrain 17D-213/77 platform: live attenuated doses: 1 vs 17DD substrain, 85.8% in the same trial	84.8 seroconversion rate (%)	RCT	Collaborative 2015 Mem Inst Oswaldo Cruz quote After vaccination, seropositivity rates of 81.9% and 83.2%, seroconversion rates of 84.8% and 85.8% and rates of a four-fold increase over the pre-vaccination titre of 77.6% and 81.8% were observed in the 17D-213/77 and 17DD subgroups, respectively. abstract Lower than the seroconversion seen in adults, in the age group that is the main target in endemic regions. The authors state that the failures in seroconversion support a booster dose within a decade of the first dose.
Seroconversion when yellow fever vaccine is co-administered with MMR severity: serological surrogate case definition: Seroconversion in children aged 12 months or older given MMR concomitantly	children aged 12 months or older country: Brazil n: subgroup of the 1,966 randomised children 1-4y	yellow fever vaccine (17DD or 17D-213/77) platform: live attenuated doses: 1 schedule: concomitant with measles, mumps and rubella vaccine vs children not co-vaccinated with MMR in the same trial	69 seroconversion rate (%)	RCT	Collaborative 2015 Mem Inst Oswaldo Cruz quote Among children aged 12 months or older, the seroconversion rates of 69% were associated with concomitant vaccination against measles, mumps and rubella. abstract A substantially lower response with MMR co-administration. The CDC Yellow Book makes the same point and advises spacing yellow fever and MMR about a month apart where possible. Interference from other vaccines, not maternal immunity, is what this trial found.
Seroconversion after vaccination with WHO-17D and Brazilian 17DD vaccines in adults severity: serological surrogate case definition: Neutralising yellow fever antibody in pre- and post-immunisation serum samples	adults country: Brazil (Rio de Janeiro) n: 1,087 adults 18-49y50-64y	yellow fever vaccines from WHO-17D and Brazilian 17DD substrains platform: live attenuated doses: 1 vs placebo group included; the three vaccine seed-lots compared with each other	98 seroconversion rate (%)	RCT	Camacho et al. 2004 Rev Saude Publica quote RESULTS: Seroconversion rates were 98% or higher among subjects previously seronegative, and 90% or more of the total cohort of vaccinees, including those previously seropositive. abstract, RESULTS A placebo-controlled double-blind equivalence trial in which the vaccines explained almost all of the seroconversion observed. Differences between the three products were a few percentage points at most, and transient viraemia was detected in a small minority of vaccinees in the first week.
Seroprotection one month after a single dose, two manufacturers compared severity: serological surrogate case definition: Neutralising antibody titre of 1/10 or greater one month after vaccination	healthy adults n: 211 healthy adults 18-49y50-64y	PM-YF (Stamaril, Pasteur Merieux Connaught) platform: live attenuated doses: 1 time since vaccination: 1 month vs Arilvax (Evans Medical-Wellcome), 99% in the same trial	100 seroprotection rate (%)	RCT	Lang et al. 1999 Am J Trop Med Hyg quote While the geometric mean titer values at days 10-14 and day 28 after vaccination were higher in the PM-YF group, the vaccines provided equivalent seroprotection (titers > or = 1/10) one month after a single vaccine dose (100% PM-YF versus 99% W-YF; P = 0.001, by one-sided equivalence test). abstract Equivalent seroprotection from two European manufacturers. Geometric mean titres were higher in the PM-YF group at days 10-14 and day 28. No serious local or systemic reactions were reported in either arm.
Seroprotection after a single dose, three 17D products compared severity: serological surrogate case definition: Neutralising antibody titre of 1:10 or greater	healthy adult volunteers n: 304 healthy adult volunteers 18-49y50-64y	BERNA-YF (Flavimun), RKI-YF and Stamaril (AP-YF) platform: live attenuated doses: 1 vs the three products compared with each other	100 seroprotection rate (%)	RCT	Pfister et al. 2005 Am J Trop Med Hyg quote All three vaccines elicited an effective immune response with seroprotection achieved in 100% of individuals in each vaccine group at a neutralizing antibody titer > or = 1:10. abstract All three products seroprotected every volunteer, though Stamaril produced significantly lower antibody titres than BERNA-YF. Males had a higher antibody response than females to two of the three products. Seroprotection proportion, not disease protection.
Seroconversion after vaccination during pregnancy severity: serological surrogate case definition: Seroconversion to yellow fever virus 2-4 weeks after vaccination	pregnant women, mostly third trimester pregnancy: vaccinated primarily in the third trimester n: 83 pregnant women 18-49y	yellow fever vaccine platform: live attenuated doses: 1 vs 94% (89 of 95) in the general population	39 seroconversion rate (%)	cohort	cdc.gov quote Among pregnant women who received yellow fever vaccine primarily in their third trimester, 39% (32 of 83) had evidence of seroconversion to yellow fever virus at 2–4 weeks post-vaccination, compared with 94% (89 of 95) in the general population ( 12 ). MMWR 2015, Other relevant evidence Markedly impaired response compared with the non-pregnant comparator in the same sentence. This is why ACIP recommends an additional dose for women vaccinated while pregnant. The first-trimester evidence points the other way and is a separate record.
Seroconversion after inadvertent vaccination in early pregnancy severity: serological surrogate case definition: IgG positivity at least six weeks after vaccination	pregnant women, mean 5.7 weeks of gestation pregnancy: vaccinated in the first trimester during a mass campaign country: Brazil setting: mass vaccination campaign, Campinas region period: February-March 2000 n: 480 pregnant immunised women identified 18-49y	17DD yellow fever vaccine platform: live attenuated doses: 1 vs none (single-arm proportion)	98.2 IgG seropositivity (%)	cohort	Suzano et al. 2006 Vaccine quote After a minimum 6-week interval, 98.2% pregnant women were IgG positive. abstract, FINDINGS Contrary to the third-trimester finding, maternal seroconversion was very high when the vaccine was given in early pregnancy. The authors found malformation, miscarriage and stillbirth frequencies similar to the general population, but the study was not powered as a safety study.
Seroconversion one month after a fractional dose during an outbreak campaign severity: serological surrogate case definition: PRNT50 titre of 10 or higher in participants seronegative at baseline	four age strata from 2 years upward country: Democratic Republic of the Congo setting: preemptive outbreak campaign in Kinshasa period: 2016 n: 493 participants seronegative at baseline 1-4y5-11y12-17y18-49y50-64y65+	17DD yellow fever vaccine, one fifth of the standard dose (0.1 mL) platform: live attenuated doses: 1 fractional time since vaccination: 1 month vs none (single-arm; no standard-dose arm in this campaign study)	98 (96–99) seroconversion rate (%)	cohort	Casey et al. 2019 N Engl J Med quote Among 493 participants who were seronegative at baseline, 482 (98%; 95% CI, 96 to 99) underwent seroconversion. abstract, RESULTS The study that underpinned WHO's acceptance of fractional dosing for outbreak response. No concurrent standard-dose comparator, so the non-inferiority evidence comes from the randomised trials filed separately.
Seropositivity one year after a fractional dose severity: serological surrogate case definition: Seropositivity for yellow fever antibody at 1-year follow-up	four age strata from 2 years upward country: Democratic Republic of the Congo setting: outbreak campaign, Kinshasa n: 684 participants completing 1-year follow-up 1-4y5-11y12-17y18-49y50-64y65+	17DD yellow fever vaccine, one fifth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 1 year vs none (single-arm proportion)	97 (96–98) seropositivity rate (%)	cohort	Casey et al. 2019 N Engl J Med quote Among 684 participants who completed the 1-year follow-up, 666 (97%; 95% CI, 96 to 98) were seropositive for yellow fever antibody. abstract, RESULTS Titres stayed above the seropositivity threshold at one year in nearly all who were seropositive at one month. One year is not evidence about a decade.
Immune response to a fractional dose in those already seropositive severity: serological surrogate case definition: Fourfold or greater rise in PRNT50 titre in participants seropositive at baseline	four age strata from 2 years upward prior infection: seropositive at baseline country: Democratic Republic of the Congo n: 223 participants seropositive at baseline 1-4y5-11y12-17y18-49y50-64y65+	17DD yellow fever vaccine, one fifth of the standard dose platform: live attenuated doses: 1 fractional time since vaccination: 1 month vs none (within-group, pre versus post titre)	66 (60–72) proportion with a fourfold titre rise (%)	cohort	Casey et al. 2019 N Engl J Med quote Among 223 participants who were seropositive at baseline, 148 (66%; 95% CI, 60 to 72) had an immune response. abstract, RESULTS A boosting outcome, not seroconversion, and not comparable with the seroconversion records. Lower baseline titres predicted a higher chance of responding.
Seroconversion 30 days after vaccination in people with HIV severity: serological surrogate case definition: Yellow fever neutralisation titres measured at day 30	adults aged 18-59 years immune status: people with HIV on antiretroviral therapy, median CD4 630 cells per microlitre country: Brazil n: 218 people with HIV and 82 controls 18-49y50-64y	17DD yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 30 days vs HIV-uninfected controls, 100% (93.9-100) in the same study	98.6 (95.6–99.6) seroconversion rate (%)	cohort	Motta et al. 2023 AIDS quote At Day 30, seroconversion was observed in 98.6% of PWH [95% confidence interval (CI): 95.6-99.6] and in 100% of controls (95% CI: 93.9-100); at Year 1, 94.0% of PWH (95% CI: 89.6-96.7) and 98.4% of controls (95% CI 90.3-99.9) were seropositive. abstract, RESULTS Seroconversion was near-universal, but the same study found lower geometric mean titres in people with HIV at both day 30 and year 1, and titres decayed over time. High viral load, low CD4 and low CD4/CD8 ratio predicted lower titres.
Seropositivity one year after vaccination in people with HIV severity: serological surrogate case definition: Yellow fever neutralisation titres at year 1	adults aged 18-59 years immune status: people with HIV on antiretroviral therapy country: Brazil n: 218 people with HIV 18-49y50-64y	17DD yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 1 year vs HIV-uninfected controls, 98.4% (90.3-99.9) at year 1	94.0 (89.6–96.7) seropositivity rate (%)	cohort	Motta et al. 2023 AIDS quote At Day 30, seroconversion was observed in 98.6% of PWH [95% confidence interval (CI): 95.6-99.6] and in 100% of controls (95% CI: 93.9-100); at Year 1, 94.0% of PWH (95% CI: 89.6-96.7) and 98.4% of controls (95% CI 90.3-99.9) were seropositive. abstract, RESULTS A fall from day 30 within the first year, and faster than in controls.
Seropositivity 10-11 years after a single dose in adults severity: serological surrogate case definition: Yellow fever neutralising antibody seropositivity, time since vaccination from immunisation cards	adults aged 18-83 years country: Brazil (Rio de Janeiro and Alfenas, non-endemic southeast) period: recruited 2011-2012 n: 691 subjects 18-49y50-64y65+	yellow fever vaccine (17DD, routine immunisation services) platform: live attenuated doses: 1 time since vaccination: 10-11 years vs none (single-arm, stratified by time since vaccination)	76 (68–83) seropositivity rate (%)	cohort	Collaborative 2014 Vaccine quote Seropositivity rates (95%C.I.) and GMT (International Units/mL; 95%C.I.) decreased with time since vaccination: 93% (88-96%), 8.8 (7.0-10.9) IU/mL for newly vaccinated; 94% (88-97), 3.0 (2.5-3.6) IU/mL after 1-4 years; 83% (74-90), 2.2 (1.7-2.8) IU/mL after 5-9 years; 76% (68-83), 1.7 (1.4-2.0) IU/mL after 10-11 years; and 85% (80-90), 2.1 (1.7-2.5) IU/mL after 12 years or more. abstract, RESULTS The lowest stratum in a sentence that reports seropositivity falling from 93% in the newly vaccinated. The authors argue immunogenicity under routine service conditions is likely lower than in controlled studies and that a booster may be needed sooner than currently recommended.
Seropositivity 5-9 years after a single dose in adults severity: serological surrogate case definition: Yellow fever neutralising antibody seropositivity	adults aged 18-83 years country: Brazil, non-endemic southeast period: recruited 2011-2012 n: 691 subjects 18-49y50-64y65+	yellow fever vaccine (17DD, routine immunisation services) platform: live attenuated doses: 1 time since vaccination: 5-9 years vs none (single-arm, stratified by time since vaccination)	83 (74–90) seropositivity rate (%)	cohort	Collaborative 2014 Vaccine quote Seropositivity rates (95%C.I.) and GMT (International Units/mL; 95%C.I.) decreased with time since vaccination: 93% (88-96%), 8.8 (7.0-10.9) IU/mL for newly vaccinated; 94% (88-97), 3.0 (2.5-3.6) IU/mL after 1-4 years; 83% (74-90), 2.2 (1.7-2.8) IU/mL after 5-9 years; 76% (68-83), 1.7 (1.4-2.0) IU/mL after 10-11 years; and 85% (80-90), 2.1 (1.7-2.5) IU/mL after 12 years or more. abstract, RESULTS Already below the 93-94% seen within the first four years. Seropositivity was not affected by previous dengue infection.
Seropositivity 73-100 months after vaccination in childhood severity: serological surrogate case definition: Neutralising antibody seropositivity by plaque reduction neutralisation test, children with accessible vaccination records	children aged 9 months to 12 years, vaccinated in the first two years of life country: Brazil (metropolitan southeast) setting: primary care units period: recruited August 2010 to July 2011 n: 824 children tested Infant <1y1-4y5-11y	yellow fever vaccine (17DD) platform: live attenuated doses: 1 time since vaccination: 73-100 months (about 6-8 years) vs none (single-arm, stratified by time since vaccination)	42.2 (33.8–51.0) seropositivity rate (%)	cohort	de et al. 2019 Vaccine quote The proportion of seropositivity (95% C.I.) and GMT (95% C.I.) dropped markedly across time periods: from 86.7% (80.5-91.4%), GMT 47.9 (38.3-59.9) in newly vaccinated to 59.0% (49.7-67.8%), GMT 14.8 (11.6-19.1) and 42.2% (33.8-51.0), GMT 8.6 (7.1-12.1), respectively in the subgroups vaccinated 31-72 months and 73-100 months before. abstract, RESULTS Under half seropositive six to eight years after vaccination in infancy, against 86.7% in the newly vaccinated in the same sentence. Supports the Brazilian programme's booster at 4 years and cuts against single-dose-for-life in this age group.
Seropositivity 31-72 months after vaccination in childhood severity: serological surrogate case definition: Neutralising antibody seropositivity by plaque reduction neutralisation test	children vaccinated in the first two years of life country: Brazil period: recruited 2010-2011 n: 824 children tested Infant <1y1-4y5-11y	yellow fever vaccine (17DD) platform: live attenuated doses: 1 time since vaccination: 31-72 months vs none (single-arm, stratified by time since vaccination)	59.0 (49.7–67.8) seropositivity rate (%)	cohort	de et al. 2019 Vaccine quote The proportion of seropositivity (95% C.I.) and GMT (95% C.I.) dropped markedly across time periods: from 86.7% (80.5-91.4%), GMT 47.9 (38.3-59.9) in newly vaccinated to 59.0% (49.7-67.8%), GMT 14.8 (11.6-19.1) and 42.2% (33.8-51.0), GMT 8.6 (7.1-12.1), respectively in the subgroups vaccinated 31-72 months and 73-100 months before. abstract, RESULTS The intermediate stratum between 86.7% newly vaccinated and 42.2% at 73-100 months.
Seropositivity after a single dose in children, and its decline severity: serological surrogate case definition: Neutralising antibodies by PRNT in children vaccinated at 9-12 months, measured across a 10-year span	children vaccinated at 9-12 months of age country: Brazil (area with yellow fever virus circulation) period: 10-year cross-sectional time span Infant <1y1-4y5-11y12-17y	17DD yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 30-45 days vs none (single-arm, time-stratified)	85 seropositivity rate (%)	cohort	Campi-Azevedo et al. 2019 Front Immunol quote The results revealed that a single dose induced 85% of seropositivity at 30-45 days and a progressive time-dependent decrease was observed as early as 2 years and declines toward critical values (below 60%) at time-spans of ≥4-years. abstract The same sentence reports a progressive time-dependent fall beginning as early as 2 years and declining below 60% at 4 years or more. The study also found short-lived memory T and B cell responses after 4 years.
Seropositivity eight years after vaccination in a dose-response trial cohort severity: serological surrogate case definition: Yellow fever seropositivity in non-revaccinated individuals	adults country: Brazil period: vaccinated 2009, tested eight years later n: 318 participants 18-49y50-64y	17DD yellow fever vaccine at full and tapered doses platform: live attenuated doses: 1 time since vaccination: 8 years vs none (pooled across dose groups, which were similar)	85 seropositivity rate (%)	cohort	de et al. 2018 Vaccine quote According to the current study's results, seropositivity was maintained in 85% of 318 participants and was similar across groups. abstract Similar across all the tapered dose groups. The authors conclude each fractional dose should contain at least the second-lowest dose tested, and support the WHO minimum potency.
Seroconversion 30-45 days after a single dose, adults versus infants severity: serological surrogate case definition: Yellow fever seroconversion measured by micro focus reduction neutralisation test	children aged 9 months to 4 years and adults aged 18-50 country: Brazil (northeastern municipalities without wild-type yellow fever circulation) setting: area with high incidence of other orthoflaviviruses Infant <1y1-4y18-49y	17DD yellow fever vaccine platform: live attenuated doses: 1 time since vaccination: 30-45 days vs infants in the same cohort, 10% of whom had no detectable antibodies	99 seroconversion rate in adults (%)	cohort	Saucha et al. 2025 PLoS Negl Trop Dis quote YF seroconversion rates 30-45 days post-vaccination increased with age, reaching 99% in adults, while 10% of infants remained without detectable antibodies. abstract Seroconversion rose with age. One year after vaccination titres fell considerably in all age groups. Dengue seropositivity was inversely associated with post-vaccination yellow fever titres. The authors say single-dose lifelong protection does not appear sufficient in high-risk areas where infants are the main target.
Absence of yellow fever neutralising antibodies in a population at risk severity: serological surrogate case definition: Absence of neutralising antibodies by PRNT in residents of a yellow fever risk area	not reported vaccination status: mixed country: Brazil (southeast risk area with outbreaks since 2016) setting: community serosurvey in a risk area n: 581 individuals All ages	yellow fever vaccine as delivered by the national programme platform: live attenuated doses: as received in routine practice, not verified for most participants vs none (cross-sectional prevalence of seronegativity)	25.8 proportion without yellow fever neutralising antibodies (%)	cohort	Stoffella-Dutra et al. 2020 Vaccine quote Through, a PRNT assay, we found 25.8% of individuals lacking YF-neutralizing antibodies. abstract A quarter of people in an active risk area had no neutralising antibodies, and neutralising antibodies were absent even in a handful of individuals with proven vaccination. Mixed vaccination status, so this measures population susceptibility rather than vaccine failure alone.

Vaccine safety and adverse events

Harms measured after vaccination against an unvaccinated or baseline comparator. Kept separate from post-acute sequelae, which follow the disease, not the vaccine.

Outcome (O)	Population (P)	Intervention / comparator	Estimate	Design (S)	Source
Adverse events after yellow fever vaccination in immunocompromised recipients severity: any adverse event case definition: Adverse events after yellow fever vaccination in people with HIV, transplants, cancer or on immunosuppressive therapy	not separated immune status: immunocompromised from any cause n: 25 studies included, 13 with usable data, 6 cohorts pooled All ages	yellow fever vaccine platform: live attenuated doses: not separated vs non-immunocompromised vaccine recipients	1.00 (0.78–1.29) RR versus non-immunocompromised recipients	meta-analysis	Wigg et al. 2023 J Travel Med quote Six cohort studies were combined, with an RR of 1.00 (95% CI 0.78-1.29). abstract, RESULTS No increased risk detected, but most included studies were at critical risk of bias and subgroup analysis by cause of immunosuppression also found nothing. The review concludes only that a higher risk cannot be affirmed, which is not the same as establishing safety; immunosuppression remains a contraindication in CDC guidance.
Mild systemic symptoms after yellow fever vaccination severity: mild case definition: Headache, low-grade fever and myalgia beginning within days of vaccination and lasting 5-10 days	not reported (all ages) All ages	yellow fever vaccine platform: live attenuated doses: not specified vs none (uncontrolled proportion, no placebo arm)	10 proportion of vaccinees reporting mild systemic symptoms (%), lower bound of the reported range per vaccinee	expert body	cdc.gov quote Adverse events Common adverse reactions Reactions to YF vaccine are generally mild; 10–30% of vaccinees report mild systemic symptoms, including headache, low-grade fever, and myalgia, that begin within days after vaccination and last 5–10 days. Yellow Book chapter, Common adverse reactions The source gives a range of 10-30%; only the lower bound is filed. No comparator group, so some of this is background symptom reporting rather than vaccine-attributable.
Significant neurologic and multisystem adverse events by age in a general practice database severity: significant neurologic and multisystem adverse event case definition: Significant neurologic and multisystem adverse events recorded in a UK general practice database	65-74 years country: United Kingdom setting: retrospective general practice database n: 43,555 persons receiving YF 17D 65+	YF 17D vaccine platform: live attenuated doses: not separated vs the 25-44 year-old reference group	2.82 (0.81–9.81) age-specific reporting rate ratio	cohort	Monath et al. 2005 Hum Vaccin quote The age-specific reporting rate ratio (RRR) was approximately twice that in the 25-44 year-old reference group for subjects in the 45-64 year age group (RRR 1.82; 95% CI 0.88,3.77) and 3-fold higher for the 65-74 year-old age group (RRR 2.82; 95% CI 0.81, 9.81). abstract The interval crosses one, so this is not a demonstrated increase on its own; it is consistent in direction with the VAERS and Brazilian findings. The same sentence gives a lower, also non-significant ratio for ages 45-64. Notably, in the two clinical trials reported in the same paper there were no neurological or viscerotropic events at all and common reactions were less frequent in the elderly, while neutralising antibody responses were equivalent across ages.
Serious adverse events following yellow fever vaccination during an outbreak campaign severity: serious adverse event case definition: Serious adverse events recorded in the National Immunization Program Information System, with non-serious events as controls	median 28 years, range 9-49 country: Brazil setting: mass vaccination during the 2016-2017 outbreak period: 2016-2017 n: 135 serious adverse event cases and 1,058 controls 5-11y12-17y18-49y	17DD yellow fever vaccine platform: live attenuated doses: primary vaccination associated with higher odds (OR 1.65, 95%CI 1.01;2.71) vs non-serious adverse events as controls	1.3 incidence per 100,000 doses administered per 100,000 vaccine doses administered	case-control	Lucena et al. 2020 Epidemiol Serv Saude quote RESULTS: we analyzed 135 SAE cases and 1,058 controls; of the 135 SAE, 79 (58.5%) were males and median age was 28 years [09-49]; incidence in January 2017 reached 1.3 case per 100,000 vaccine doses administered; there was statistical association with males (Odds Ratio [OR]=1.73 - 95%CI 1.20;2.48), primary vaccination (OR=1.65 - 95%CI 1.01;2.71), and being 60 years of age or older taking as reference those aged under 5 (OR=4.4; p-value <0.02). abstract, RESULTS A case-control design within reported events, so the odds ratios compare serious with non-serious events rather than vaccinated with unvaccinated people. Serious events were associated with male sex (OR 1.73, 95%CI 1.20;2.48), primary vaccination, and age 60 or over against under-5s (OR 4.4).
Serious adverse event reported after yellow fever vaccination severity: serious adverse event case definition: Reports to VAERS classified as serious adverse events	all ages reported to VAERS country: United States setting: passive national surveillance (VAERS) period: 2007-2013 n: 938 adverse event reports All ages	yellow fever vaccine (YF-VAX) platform: live attenuated doses: not separated by primary or booster vs none (reporting rate against doses distributed, no unvaccinated comparator)	3.8 reporting rate per 100,000 doses distributed per 100,000 doses distributed	surveillance	Lindsey et al. 2016 J Travel Med quote Of these, 84 (9%) were classified as SAEs for a rate of 3.8 per 100 000 doses distributed. abstract, RESULTS A passive reporting rate, not an incidence: under-reporting pulls it down and a distributed-dose denominator is not a vaccinated-person denominator. The age-stratified rates from the same study are separate records.
Serious adverse event reported after yellow fever vaccination, ages 60-69 severity: serious adverse event case definition: Reports to VAERS classified as serious adverse events	60-69 years country: United States setting: passive national surveillance (VAERS) period: 2007-2013 50-64y65+	yellow fever vaccine (YF-VAX) platform: live attenuated doses: not separated by primary or booster vs the overall rate of 3.8 per 100,000 across all ages	6.5 reporting rate per 100,000 doses distributed per 100,000 doses distributed	surveillance	Lindsey et al. 2016 J Travel Med quote Reporting rates of SAEs increased with increasing age with a rate of 6.5 per 100 000 in persons aged 60-69 years and 10.3 for ≥70 years. abstract, RESULTS The rate for the oldest age band is filed as its own record. Reporting rates rose with age across all serious-event categories in this system.
Serious adverse event reported after yellow fever vaccination, ages 70 and over severity: serious adverse event case definition: Reports to VAERS classified as serious adverse events	70 years and over country: United States setting: passive national surveillance (VAERS) period: 2007-2013 65+	yellow fever vaccine (YF-VAX) platform: live attenuated doses: not separated by primary or booster vs the overall rate of 3.8 per 100,000 across all ages	10.3 reporting rate per 100,000 doses distributed per 100,000 doses distributed	surveillance	Lindsey et al. 2016 J Travel Med quote Reporting rates of SAEs increased with increasing age with a rate of 6.5 per 100 000 in persons aged 60-69 years and 10.3 for ≥70 years. abstract, RESULTS Roughly three times the all-age rate. This age gradient is the reason vaccination at 60 and over is a precaution requiring a risk-benefit judgement rather than a routine recommendation.
Anaphylaxis after yellow fever vaccination severity: anaphylaxis case definition: Anaphylaxis reported to VAERS	all ages, highest in those aged 18 and under country: United States setting: passive national surveillance (VAERS) period: 2007-2013 All ages	yellow fever vaccine (YF-VAX) platform: live attenuated doses: not separated by primary or booster vs none (reporting rate against doses distributed)	1.3 reporting rate per 100,000 doses distributed per 100,000 doses distributed	surveillance	Lindsey et al. 2016 J Travel Med quote The reporting rate for anaphylaxis was 1.3 per 100 000 doses distributed and was highest in persons ≤18 years (2.7 per 100 000). abstract, RESULTS The same sentence gives a roughly twofold higher rate in those aged eighteen and under, the one serious outcome in this study that was highest in the young rather than the old.
Vaccine-associated neurologic disease (YEL-AND) after yellow fever vaccination severity: vaccine-associated neurologic disease case definition: YEL-AND reported to VAERS	all ages, rates increasing with age country: United States setting: passive national surveillance (VAERS) period: 2007-2013 All ages	yellow fever vaccine (YF-VAX) platform: live attenuated doses: not separated by primary or booster vs none (reporting rate against doses distributed)	0.8 reporting rate per 100,000 doses distributed per 100,000 doses distributed	surveillance	Lindsey et al. 2016 J Travel Med quote Reporting rates of YEL-AND and YEL-AVD were 0.8 and 0.3 per 100 000 doses distributed, respectively; both rates increased with increasing age. abstract, RESULTS The same sentence gives a lower rate for YEL-AVD, filed separately. Both rose with increasing age.
Vaccine-associated viscerotropic disease (YEL-AVD) after yellow fever vaccination severity: vaccine-associated viscerotropic disease case definition: YEL-AVD associated with YF-VAX administration in the United States	all ages country: United States setting: national surveillance All ages	yellow fever vaccine (YF-VAX) platform: live attenuated doses: first dose only (CDC states YEL-AVD has been reported only after the first dose) vs none (rate against doses administered)	0.3 incidence per 100,000 doses administered per 100,000 doses administered	surveillance	cdc.gov quote The case-fatality ratio is approximately 43% and the incidence is 0.3 cases per 100,000 doses of vaccine administered. Yellow Book chapter, Yellow fever vaccine-associated viscerotropic disease The same sentence gives a case-fatality ratio of approximately 43% among YEL-AVD cases. CDC states no laboratory-confirmed YEL-AVD has been reported after a booster dose, and median onset is 4 days after vaccination.
Vaccine-associated viscerotropic disease (YEL-AVD) in older recipients severity: vaccine-associated viscerotropic disease case definition: YEL-AVD associated with YF-VAX administration	60 years and over country: United States setting: national surveillance 65+50-64y	yellow fever vaccine (YF-VAX) platform: live attenuated doses: first dose vs 0.3 per 100,000 doses across all ages	1.2 incidence per 100,000 doses administered per 100,000 doses administered	surveillance	cdc.gov quote The incidence of YEL-AVD is greater for people aged ≥60 years (1.2 per 100,000 doses) and greater still for people aged ≥70 years. Yellow Book chapter, Yellow fever vaccine-associated viscerotropic disease Four times the all-age rate, and CDC states the rate is greater still at 70 and over without giving that figure. This is the central number in the risk-benefit judgement for an older traveller, and it should be read against the traveller risk-of-illness records.
YEL-AVD under enhanced safety monitoring of an imported product severity: vaccine-associated viscerotropic disease case definition: YEL-AVD during enhanced safety monitoring for Stamaril in the United States	all recipients, and separately those aged 60 and over country: United States setting: enhanced safety monitoring during a YF-VAX shortage period: 2015-2021 All ages	Stamaril (Sanofi Pasteur, France), imported under an expanded-access protocol platform: live attenuated doses: first dose vs 0.3 per 100,000 for YF-VAX under routine passive surveillance	0.5 incidence per 100,000 doses administered per 100,000 doses administered	surveillance	cdc.gov quote The incidence of YEL-AVD was slightly higher when enhanced safety monitoring was performed for Stamaril in the United States, with 0.5 YEL-AVD cases per 100,000 doses among all recipients and 2.1 per 100,000 doses in people aged ≥60 years. Yellow Book chapter, Yellow fever vaccine-associated viscerotropic disease Slightly higher than the YF-VAX figure, and 2.1 per 100,000 in those aged 60 and over per the same sentence. CDC attributes the difference to enhanced monitoring rather than to the product, which is exactly the comparison a reader should be warned about: active and passive surveillance do not produce comparable rates.
Vaccine-associated neurologic disease (YEL-AND) after yellow fever vaccination severity: vaccine-associated neurologic disease case definition: YEL-AND after YF-VAX administration, including ADEM, Guillain-Barre syndrome, meningoencephalitis and cranial nerve palsies	all ages, and separately 60 and over country: United States setting: national surveillance All ages	yellow fever vaccine (YF-VAX) platform: live attenuated doses: almost all in first-time recipients vs none (rate against doses administered)	0.8 incidence per 100,000 doses administered per 100,000 doses administered	surveillance	cdc.gov quote The incidence of YEL-AND following the administration of YF-VAX in the United States is 0.8 per 100,000 doses administered but is greater (2.2 per 100,000 doses) in people aged ≥60 years. Yellow Book chapter, Yellow fever vaccine-associated neurologic disease The same sentence gives 2.2 per 100,000 in people aged 60 and over. CDC states YEL-AND is rarely fatal, that almost all cases globally occur in first-time recipients, and that onset ranged from a few days to about eight weeks after vaccination in documented US cases.
YEL-AND under enhanced safety monitoring of an imported product severity: vaccine-associated neurologic disease case definition: YEL-AND during enhanced safety monitoring for Stamaril in the United States	all recipients, and separately those aged 60 and over country: United States setting: enhanced safety monitoring during a YF-VAX shortage period: 2015-2021 All ages	Stamaril (Sanofi Pasteur, France), imported under an expanded-access protocol platform: live attenuated doses: first dose vs 0.8 per 100,000 for YF-VAX under routine passive surveillance	1.0 incidence per 100,000 doses administered per 100,000 doses administered	surveillance	cdc.gov quote When enhanced safety monitoring was implemented for Stamaril, YF vaccine imported from France during a stockout of YF-VAX in the United States, the incidence was found to be 1.0 per 100,000 doses administered for all people and 6.2 per 100,000 doses in people aged ≥60 years. Yellow Book chapter, Yellow fever vaccine-associated neurologic disease 6.2 per 100,000 in those aged 60 and over per the same sentence, the highest age-specific serious-event rate in this file. Enhanced versus passive surveillance again explains part of the gap, so the comparison with YF-VAX is not clean.
YEL-AND in infants under 6 months of age severity: vaccine-associated neurologic disease case definition: YEL-AND (historically encephalitis) in infants vaccinated under 6 months	infants under 6 months NeonateInfant <1y	yellow fever vaccine platform: live attenuated doses: 1 vs 0.8 per 100,000 doses in the general US population	50 cases per 100,000 infants vaccinated, range as reported per 100,000 infants vaccinated	surveillance	cdc.gov quote Age younger than 6 months YF vaccine is contraindicated in infants aged <6 months because the rate of YEL-AND is high, 50–400 cases per 100,000 infants vaccinated. Yellow Book chapter, Age younger than 6 months This is the historical basis of an absolute contraindication, not a rate observed in current practice: the vaccine is not given under 6 months, and under 6 months is a contraindication. The source reports a range of 50-400 per 100,000; only the lower bound is filed as the value. It must always be labelled as the reason for the contraindication rather than presented as a current risk.
Serious adverse events in older versus younger vaccinees severity: serious adverse event case definition: Serious adverse events reported to VAERS, age-adjusted reporting rates	60 years and over, compared with 19-29 years country: United States setting: passive national surveillance (VAERS) period: 1990-2002 n: 722 adverse event reports 50-64y65+	yellow fever vaccine platform: live attenuated doses: not separated vs vaccinees aged 19-29 years	5.9 (1.6–22.2) reporting rate ratio	surveillance	Khromava et al. 2005 Vaccine quote We found that the reporting rates of serious adverse events were significantly higher among vaccinees aged > or =60 years than among those 19-29 years of age (reporting rate ratio = 5.9, 95% CI 1.6-22.2). abstract A nearly sixfold higher reporting rate in those 60 and over, though the interval is very wide. Passive reporting, so differential reporting by age cannot be excluded. The authors frame it as a risk to be balanced against the risk of yellow fever itself for elderly travellers.
Neurological adverse events after 17DD vaccination in Brazil severity: neurological adverse event case definition: YEL-AND, neurological autoimmune disease and neurotropic disease in first-time vaccinees under national passive and intensified passive surveillance	5 to 9 years country: Brazil setting: national passive and intensified passive surveillance period: 2007-2012 n: 67 neurological cases 5-11y	17DD yellow fever vaccine platform: live attenuated doses: first-time vaccinees vs none (rate against doses administered)	2.66 rate per 100,000 vaccine doses per 100,000 vaccine doses, Rio Grande do Sul state	surveillance	Martins et al. 2014 Vaccine quote Sixty-seven neurological cases were found, with the highest rate of neurological adverse events in the age group from 5 to 9 years (2.66 per 100,000 vaccine doses in Rio Grande do Sul state, and 0.83 per 100,000 doses in national analysis). abstract The same sentence gives 0.83 per 100,000 doses in the national analysis. The authors describe this as the largest YEL-AND series analysed and note that, unusually, the 5-9 year age group carried the highest risk here rather than the elderly. They conclude neurological events generally have a good prognosis and should not contraindicate vaccination where yellow fever risk exists.
YEL-AND and YEL-AVD reported to a national arbovirus reference centre severity: vaccine-associated neurologic and viscerotropic disease combined case definition: Cases of YEL-AND and YEL-AVD with samples sent for YFV serology or RT-qPCR	mean 31 years in previously healthy cases, mean 56 years in those with cardiovascular comorbidity country: France setting: national reference centre, retrospective period: June 2012 to June 2022 n: 10 YEL-AND and 2 YEL-AVD cases 18-49y50-64y65+	17D live-attenuated yellow fever vaccine platform: live attenuated doses: not separated vs none (rate against doses)	0.6 incidence per 100,000 doses per 100,000 doses	surveillance	Le et al. 2024 J Travel Med quote RESULTS: There were 10 cases of YEL-AND and 2 cases of YEL-AVD reported to the NRCA in the past 10 years, which represented an overall incidence of 0.6 for 100 000 doses. abstract, RESULTS Twelve cases in ten years. Half occurred in previously healthy people and four of twelve had cardiovascular comorbidities. Most YEL-AND cases recovered by six months; one YEL-AVD patient died. The authors found anti-type-1-alpha-interferon antibodies in 3 of 10 tested, suggesting an innate immunity defect in some cases.

Gaps

No vaccine efficacy or effectiveness estimate from a controlled field study exists in this file, because none was found. The 17D vaccine never underwent a phase III efficacy trial (PMID 39577331 states this), and repeated searches for case-control, test-negative or cohort effectiveness studies returned none. Every "efficacy" record here is a seroconversion or seroprotection proportion, i.e. an immunological surrogate with no established correlate of protection. This is the single most important limitation of the file.
No post_acute records are compiled. Searches surfaced one late-relapsing-hepatitis study after yellow fever (PMID 37099356) but it was not fetched or read, so nothing is filed. Convalescence after severe yellow fever is described by ECDC as long and often with serious sequelae, with no quantification found. Renal outcomes after dialysis-requiring yellow fever, and neurological outcomes after yellow fever encephalopathy, are uncompiled.
No record quantifies the ratio of inapparent to symptomatic infection with a denominator from a serosurvey. The WHO and CDC progression proportions (15% and 12%) are expert-body statements without intervals or stated denominators, and the two disagree.
No lot-to-lot or manufacturer-stratified safety comparison is compiled, and no record addresses whether YEL-AVD risk differs between the 17D-204, 17DD and 17D-213 substrains.
No burden figure is compiled for the 2015-16 Angola/DRC epidemic with a confirmed-case denominator: the available surveillance numbers are suspected cases, which in that setting include substantial malaria, viral hepatitis and typhoid.
No estimate of yellow fever burden in pregnancy, or of outcomes for the fetus after maternal yellow fever, was found.
The WHO 2013 position paper (Weekly Epidemiological Record 88(27)) could not be snapshotted as full text; only its landing page was retrievable. The SAGE single-dose conclusion is therefore cited here from the ACIP 2015 MMWR, which summarises it, rather than from the primary WHO document.

Flags for review

Every efficacy record in this file is a serological surrogate (seroconversion or seroprotection), not measured protection against yellow fever disease. There is no established immunological correlate of protection for yellow fever, and the 17D vaccine has never been tested in a phase III efficacy trial. The records must not be rendered or read as vaccine efficacy against disease. A reviewer should decide whether "efficacy" is even the right kind for them, or whether the schema needs an "immunogenicity" kind.
The burden records mix three incompatible denominators: per infection, per severe/toxic-phase case, and per hospitalised or ICU-admitted case. The ICU series figures (67% in-hospital fatality, 80% with diabetes) describe the most severely ill patients who reached a tertiary hospital and must never be presented as the risk of death from yellow fever infection. Likewise the 15% progression and 50% toxic-phase fatality figures must not be multiplied together without saying so explicitly.
WHO and CDC disagree on the proportion of infections progressing to severe disease (15% vs approximately 12%), and three expert bodies give three different CFR ranges for severe disease (20-50% ACIP, 30-60% CDC Yellow Book, 39% (31-47) meta-analysed). None of the expert-body ranges carries an interval. A human should choose which figure the site leads with.
The African country CFR range of 10% to 86% (PMID 34688249) almost certainly reflects how completely each surveillance system detects mild cases rather than real differences in the risk of death. The 86% figure for Nigeria should not be quoted without that caveat.
Two of the four burden records I filed as "deaths among suspected cases" use suspected-case denominators (DRC Kongo Central, Angola/DRC epidemic). These are not yellow fever infections and the resulting proportions are not case-fatality ratios. They are kept only to document the scale of the 2016 epidemic response.
The long-term seroprotection evidence conflicts by setting and age in a way that bears directly on the single-dose-for-life recommendation. Pooled seroprotection 10+ years after one dose is 94% (86-99) in travellers from non-endemic countries but 76% (65-85) in endemic (Brazilian) settings, 47% (35-60) in those vaccinated as children under 2, and 61% (38-82) in people living with HIV. Brazilian paediatric studies find seropositivity falling below 60% within 4 years. The review attributes part of the endemic-setting difference to a higher seroprotection cutoff used in Brazil, which is a methodological explanation a reviewer should weigh before the site states that one dose protects for life.
Safety records in this file are reporting rates from passive surveillance (VAERS, Brazilian and French national systems) with distributed- or administered-dose denominators. Passive reporting undercounts, and a distributed-dose denominator is not the same as a vaccinated-person denominator, so these are not incidence rates. The age gradient they show is nonetheless consistent across four independent systems.
The 50-400 YEL-AND cases per 100,000 figure for infants under 6 months is the basis of a contraindication, not an observed rate in current practice; the vaccine is not given to that age group. It must be labelled as such wherever it appears.
I could not reach the WHO 2013 position paper full text (iris.who.int returned an empty DSpace page; three PDF URLs 404ed). Separately, several PubMed searches failed with DNS errors mid-run and were retried; two never succeeded. Nothing was filled in from memory, but the search log is thinner than intended for traveller risk and inapparent-infection ratio.

Search log (28 searches)

yellow fever burden Africa cases deaths estimate Garske · PubMed → Found PMID 24800812 (Garske 2014, modelled African burden); compiled as the 2013 case and death records.
yellow fever case fatality systematic review toxic phase · PubMed → No hit on that phrasing; the broader "yellow fever case fatality" search found PMID 34399718 (Servadio 2021 meta-analysis, CFR 39%).
yellow fever case fatality · PubMed → Found PMID 34399718 (CFR meta-analysis), 34688249 (African incidence/mortality review), 41786198 (editorial, no abstract, not used).
yellow fever vaccine single dose lifelong immunity systematic review seropositivity · PubMed → Found PMID 41071104 (revaccination review, not compiled); the duration evidence came from PMIDs 38272044 and 35856638 via a later search.
yellow fever vaccine duration of immunity seroprotection 10 years · PubMed → Found PMID 35856638 (Kling 2022) and 32655896 (knowledge-gaps review).
yellow fever vaccine immunogenicity meta-analysis · PubMed → Found PMIDs 31859201 (fractional dose), 38438165 (infants and children), 38272044 (long-term immunity), 33813107 (HIV).
fractional dose yellow fever vaccine Kinshasa seroconversion · PubMed → Found PMID 29443626 (Casey 2019, NEJM final report on the Kinshasa campaign).
yellow fever vaccine effectiveness · PubMed → Returned reviews and vaccine-safety papers, no controlled field-effectiveness study; recorded as a gap.
yellow fever vaccine effectiveness outbreak case-control Africa · PubMed → No case-control or test-negative effectiveness study found; recorded as a gap.
yellow fever vaccine failure cases vaccinated · PubMed → Found PMIDs 39577331 (breakthrough review across 8 decades), 33146313 (vaccine failures), 36841725 (factors associated with failure).
yellow fever vaccine HIV seroconversion · PubMed → Found PMIDs 37650759, 37127045, 35285913, 33813107, 24453061 (Cochrane).
yellow fever vaccine viscerotropic disease incidence per 100,000 doses age · PubMed → Found PMID 38123499 (French 10-year series); the denominator-bearing rates came from PMID 27378369 and the CDC Yellow Book.
yellow fever vaccine adverse events VAERS reporting rate per 100,000 doses elderly · PubMed → Found PMID 27378369 (Lindsey 2016, VAERS 2007-13).
yellow fever vaccine-associated viscerotropic disease age 60 risk reporting rate · PubMed → Found PMIDs 24079979 (elderly systematic review) and 27378369.
Khromava yellow fever vaccine adverse events age · PubMed → Found PMID 15837230 (Khromava 2005, reporting rate ratio by age).
yellow fever travellers risk estimate per month · PubMed → Found PMIDs 37341307 (Steffen 2023) and 16967591; the per-stay traveller risks used here are from the CDC Yellow Book and ACIP instead.
yellow fever vaccine 17D vaccine randomized trial seroconversion infants 9 months · PubMed → Found PMIDs 41544642, 37127047, 26517656, 17316925, 11731813, 26553692.
yellow fever vaccine booster doses ACIP recommendations 2015 · PubMed → Found PMID 26086636; the full MMWR text was snapshotted as web_id cdc-gov-cdffdc2acd for the GRADE figures.
yellow fever Brazil 2017 2018 outbreak case fatality · PubMed → Found PMIDs 29565840 (fatal travellers) and 36108073 (Minas Gerais reproduction number).
yellow fever Brazil 2018 clinical cohort · PubMed → Found PMIDs 31150098 (ICU series), 31104909 (predictors of mortality), 32004691 (Minas Gerais ICU).
yellow fever outbreak Angola 2016 cases deaths · PubMed → Found PMIDs 38444843, 28017559, 28358796, 29338001.
yellow fever vaccine adverse events Brazil surveillance per 100,000 doses · PubMed → Found PMIDs 24837504 (67 neurological cases) and 32215531 (severity of AEFI, 2016-17).
yellow fever vaccine 10 years children seropositivity Brazil · PubMed → Found PMIDs 31590934, 31616412, 40202994.
yellow fever vaccine immunogenicity randomized adults 17D · PubMed → Found PMIDs 39970397, 15499438, 10403341, 15772332, 17012867.
yellow fever asymptomatic proportion symptomatic infection severe Monath · PubMed → No usable denominator-bearing source; the progression proportions used here are WHO and CDC expert-body figures.
yellow fever inapparent infection ratio · PubMed → No result (network failure on the retry as well); recorded as a gap.
yellow fever vaccine pregnancy seroconversion · PubMed → Found PMID 16236402 (inadvertent vaccination in early pregnancy, Campinas).
yellow fever vaccine immunocompromised safety systematic review · PubMed → Found PMID 35947986 (meta-analysis of adverse events in immunocompromised recipients).