Flu vaccine 2026–27: strains, side effects (VAERS and trials) and lot lookup

Evidence state

For each outcome: the specific question, what controlled studies found, and how certain that evidence is on the GRADE scale (high, moderate, low, very low). Ratings are editorial judgments by the site applying GRADE to the studies in the table below — observational evidence starts at low and is rated up for large, consistent effects or a dose/age gradient, and down for inconsistency or imprecision (Methods). Where an effect is confined to a subgroup (for example young males), the question says so. "Not searched" means no controlled study has been compiled here yet; it is not a finding that the outcome is rare or caused by the vaccine.

1. Basic Information

Disease Protected Against

Seasonal influenza is a respiratory viral infection causing annual epidemics. Disease burden varies substantially by season, circulating strains, and population immunity. In the U.S., influenza causes an estimated 9–41 million illnesses, 140,000–710,000 hospitalizations, and 12,000–52,000 deaths annually (2010–2023 average). Children <5 years (especially <2) and children with underlying medical conditions are at highest risk for severe complications. The 2009 H1N1 pandemic was a reminder of pandemic influenza risk, though seasonal and pandemic vaccines are distinct products.

CDC Schedule (U.S., 2025)

Source: CDC ACIP. Multiple products available: egg-based IIV, cell-culture-based IIV (Flucelvax), recombinant HA vaccine (Flublok), and LAIV (FluMist, intranasal). For the 2024–25 season and beyond, all U.S. influenza vaccines are trivalent (H1N1, H3N2, and one B lineage, following the global disappearance of the Yamagata B lineage).

Notable Historical Safety Events

• 1976 Swine flu vaccine and GBS: The 1976 H1N1 swine flu vaccine was associated with an estimated ~1 excess case of Guillain-Barré Syndrome per 100,000 doses. This vaccine was withdrawn. The 1976 GBS association is the most significant safety event in influenza vaccine history and is the reference point against which all subsequent influenza vaccine GBS surveillance is compared.
• 2009 H1N1 pandemic vaccines (monovalent): Some European countries identified a small increased risk of narcolepsy following Pandemrix (AS03-adjuvanted H1N1 vaccine used in Europe; not used in the U.S.), estimated at ~3–7 excess cases per 100,000 vaccinated children. The mechanism is thought to involve molecular mimicry between the H1N1 nucleoprotein and the hypocretin receptor in genetically susceptible individuals (HLA-DQB1*0602). This adjuvant and formulation were not used in U.S. seasonal vaccines.
• 2014–2015 LAIV reduced effectiveness: LAIV (FluMist) showed low effectiveness against H1N1pdm09 during the 2013–14 and 2015–16 seasons. ACIP recommended against LAIV use in 2016–17 and 2017–18; LAIV was reformulated and re-recommended starting 2018–19.

Licensed Flu (Influenza) vaccine products

Each product has its own page with package-insert ingredients, side-effect data, lot lookup and comparisons with the other brands.

• Fluzone Quadrivalent — Sanofi Pasteur
  inactivated; intramuscular; no adjuvant. Superseded by trivalent Fluzone from the 2024–25 season.

Ingredients (Package Insert)

Structured composition for 1 branded product covered on this page, taken from FDA-approved package inserts (DailyMed / manufacturer prescribing information). Lists are per product — formulations differ by manufacturer and presentation. Click an ingredient name to open its safety-context page when available.

Causality assessment & potential mechanisms

Conditions below combine WHO-style causality levels with potential biological mechanisms from the site mechanism catalog (ae_mechanisms_catalog.json). HRSA VICP table listing (where shown) indicates a compensable temporal association under U.S. program rules — not automatic proof of causation for every case. Mechanisms are hypothesis-level pathways with graded evidence.

Framework: WHO causality + HRSA VICP (where applicable) + AE mechanism catalog. Last updated: 2026-07-18. Schema: schemas/vaccine_injury_table.schema.json · Mechanisms: schemas/ae_mechanism.schema.json. Not medical or legal advice.

2. Pre-Licensure Clinical Trial Data

Because influenza vaccines are reformulated annually, the typical pre-licensure paradigm does not apply in the same way as for other vaccines. New products undergo initial licensure with full clinical trials; subsequent annual strain changes are licensed based on manufacturing process consistency and immunogenicity data in smaller bridging studies (per FDA guidance).

Most Common Adverse Reactions (Seasonal IIV)

Key Limitations

• Variable effectiveness: Influenza vaccine effectiveness ranges from ~10–60% depending on season, strain match, age, and prior vaccination. This is fundamentally different from other childhood vaccines with >85% effectiveness. The need for annual reformulation is a unique limitation.
• Prior vaccination effects: Repeated annual vaccination may reduce vaccine effectiveness in some seasons (antigenic distance hypothesis), though data are inconsistent. This is actively studied.
• Annual safety data limited: Strain-change bridging studies are small (hundreds of participants). Most safety evidence is post-licensure.
• Febrile seizure risk with PCV13 combination: In the 2010–11 season, co-administration of IIV and PCV13 was associated with an increased febrile seizure risk (~1 per 2,000 doses). This was season- and product-specific. (This is also noted on the PCV page.)

3. Post-Licensure Safety Data

VSD Findings

• Guillain-Barré Syndrome (GBS): The VSD and other surveillance systems have estimated the GBS risk following seasonal influenza vaccine at ~1–2 excess cases per million doses, substantially lower than the 1976 swine flu vaccine risk (~1 per 100,000). Some seasons show no statistically significant increase. IOM (2012) concluded evidence favors acceptance of a causal relationship for influenza vaccine and GBS (based primarily on the 1976 vaccine data; evidence for seasonal vaccines is less robust).
• Febrile seizures (IIV + PCV13, 2010–11): Signal identified and resolved. See PCV page.
• No increase in adverse pregnancy outcomes with IIV during pregnancy. Extensive VSD and registry data are consistent.
• Anaphylaxis: ~1–2 per million doses.

VAERS reports per million doses

U.S. VAERS reports for vaccinations given 2006–2024, divided by the 2,714 million doses distributed in the same years (HRSA, from CDC data).

These are reporting rates: they mix real reactions, coincidental events and reports of conditions that would have happened anyway, and they depend on how actively each vaccine is watched. Doses are distributed, not administered. Combination vaccines count toward each component in both numerator and denominator. Compare with the observed-vs-expected analysis and the controlled studies above. Data: vaers_reporting_rates.json.

What controlled safety studies found

Unlike VAERS counts, these studies compare vaccinated people with a control group or control time window, so they can estimate whether a risk is raised and by how much. 24 results for influenza vaccines: 13 found an increased risk of a specific outcome, 10 found no increased risk.

Benefit and risk on the same scale (per million)

Bars are drawn to scale within each comparison. Disease risks are per case of the disease; vaccine risks are the excess above background per dose, from controlled studies. How likely someone is to catch the disease, without vaccination, decides how these weigh up for an individual.

Side-by-side pictures are shown only where one study measured both risks in the same population. Other rows below come from different studies and should not be divided into each other.

Compiled from study abstracts and FDA/CDC pages, verified 2026-10-09; full table with every source on vaccine safety studies. Data: active_surveillance_results.json, disease_risks.json.

Trials, timing and other data sources

Randomized placebo-controlled trials: serious adverse events

Trials on ClinicalTrials.gov with posted results and a placebo, saline or no-vaccine control arm (2,344 vaccinated vs 2,304 controls), pooled with a random-effects model:

• Any serious adverse event: RR 0.87 (95% CI 0.53–1.41), 1 trial — no significant difference
• Death (any cause, including from the disease itself): RR 0.98 (95% CI 0.06–15.71), 1 trial — no significant difference

High heterogeneity (I² above 50%) means the trials disagree; the pooled figure is an average of different populations and products.

Only blinded, placebo-controlled periods are used (open-label, crossover, booster and extension periods excluded) and vaccine and placebo arms are matched within the same trial part. Risk ratios use participant counts, not person-time: where placebo recipients crossed over to vaccine, the placebo arm is followed for less time, which pushes count-based ratios up. Serious adverse events are sponsor-coded and include events unrelated to vaccination; trials were powered for efficacy, not rare harms; most childhood-vaccine trials use active comparators and therefore do not appear here. Code: analysis/vaccine_trial_meta.py; data: vaccine_trial_meta.json.

When do reported events start? (time to onset)

Days from vaccination to onset in U.S. VAERS reports (0–180 days). A causal reaction usually starts inside a characteristic window; but people also report what happens soon after a shot, so each event is compared with all other reports for this vaccine. The Weibull shape measures bunching (below 1 = most onsets early).

Timing alone cannot establish causation; it can make it more or less plausible. ^*Fitted on a random sample of 5,000 of these reports. Data: vaers_onset.json.

VAERS Reporting Data — Halma & Varon (2025), DARE-SAFE

The DARE-SAFE paper (Halma & Varon, Pharmacoepidemiology 2025, CC BY 4.0) analyzed VAERS reports for vaccines administered in the United States from 2006–2022. The following data are extracted from Table 1 of that paper for this vaccine (Seasonal influenza (all formulations combined); ~2.4 billion doses is far larger than any other denominator):

Source: Halma, M.; Varon, J. DARE-SAFE. Pharmacoepidemiology. 2025. DOI: 10.3390/pharma4020007. CC BY 4.0. Data from Table 1.

Passive Surveillance: AE Type Breakdown (Multi-System)

Side-by-side view of U.S. VAERS, Health Canada Canada Vigilance, Japan JADER (PMDA), EU EudraVigilance, and live-scraped international systems via SurVigilance (VigiAccess, Lareb, DAEN, DMA, Medsafe). SurVigilance panels show MedDRA PT mention totals (not individual-case counts). Category assignment uses keyword matching — approximate, not official SOC coding. VAERS files on this site are built from the public VAERS download. FAERS figures use the public quarterly FAERS files. Counts are mentions or reports, not incidence.

SurVigilance: VigiAccess · Lareb · DAEN · DMA · Medsafe

Compare AE patterns across all vaccines →

4. Documented Adverse Events

▶ Strong Evidence

• Injection site reactions: 40–65% (pain). Strong
• Systemic reactions: Fever (5–15% in young children), fatigue, headache, myalgia. Strong
• Oculorespiratory syndrome (ORS): Transient red eyes, cough, wheezing within 24 hours of IIV. Reported primarily in Canada with one specific product in 2000–2001. Self-limited. Strong
• Anaphylaxis: ~1–2 per million doses. Strong
• Wheezing (LAIV in young children with asthma): Contraindication in children <2 and those with asthma. Strong

▶ Moderate Evidence

• GBS: ~1–2 excess cases per million doses with seasonal IIV (vs. ~1 per 100,000 with 1976 swine flu vaccine). Evidence is stronger for the 1976 vaccine specifically. Moderate

▶ No Causal Association

• Autism: No association. IOM (2012) rejected causality. No Association
• Narcolepsy (U.S. seasonal vaccines): The narcolepsy signal was specific to Pandemrix (AS03-adjuvanted, used in Europe). No signal with U.S. seasonal vaccines. No Association

5. Disease Prevention Benefits

Source: CDC MMWR, FluView; CDC Pink Book. Effectiveness varies by season. In the 2023–24 season, pediatric VE against hospitalization was ~52–61%, and >80% of influenza-associated pediatric deaths occurred in unvaccinated children.

Disease Burden Over Time

Reported U.S. disease burden by year. The dashed vertical line marks vaccine introduction. Hover or tap data points for values; use arrow keys when a chart has focus.

7. Evidence Summary

Influenza vaccines differ fundamentally from other childhood vaccines in their variable effectiveness (10–60%), annual reformulation, and the resulting constant need for new post-licensure safety and effectiveness data each season. The safety profile for seasonal IIV is well-established after decades of use in hundreds of millions of annual doses. The primary historical safety concern — the 1976 GBS signal — has not been replicated at comparable magnitude with seasonal vaccines. The narcolepsy signal was adjuvant- and product-specific (Pandemrix). The main evidence limitation is the inherent inability to fully evaluate very rare adverse events for each new seasonal formulation, which is mitigated by the consistency of the manufacturing platform and the extensive passive and active surveillance infrastructure.

8. International Surveillance & Global Data

Quick links to public pharmacovigilance databases and trial registries relevant to Influenza Vaccine. Reporting counts do not establish causality.

9. Curated Adverse Event Literature

Curated peer-reviewed literature linking specific adverse events to Influenza Vaccine. Each entry is a case report, case series, or related safety publication identified via PubMed. Expand Search PubMed for additional literature below to run custom queries.

Common questions

How long do Influenza vaccine side effects last?

Most reported Influenza vaccine reactions are short-lived: injection-site soreness, tiredness, and low-grade fever that typically begin within a day or two and resolve within a few days. Longer-lasting or more serious events are rarer and are the ones tracked in the post-licensure surveillance and documented adverse-event sections of this page, with report counts that are not incidence rates. Anything severe or persisting beyond a week warrants medical review.

Is the Influenza vaccine given to adults, and are side effects different in adults?

Whether the Influenza vaccine is recommended for adults depends on the vaccine and the person's risk factors; the schedule and product sections above describe the licensed indications and age groups. Reporting patterns in adults can differ from children, so this page's VAERS and international database breakdowns should be read with the reporting population in mind. Check the current CDC/ACIP adult schedule and the product's prescribing information for adult dosing.

10. Key References

1. IOM. Adverse Effects of Vaccines: Evidence and Causality. National Academies Press; 2012.
2. CDC. Pink Book — Influenza chapter. cdc.gov/pinkbook
3. Schonberger LB, et al. Guillain-Barré syndrome following vaccination in the National Influenza Immunization Program, United States, 1976–1977. Am J Epidemiol. 1979;110(2):105–123.
4. Nohynek H, et al. AS03 adjuvanted AH1N1 vaccine associated with an abrupt increase in the incidence of childhood narcolepsy in Finland. PLoS One. 2012;7(3):e33536.
5. CDC. FluView — Weekly U.S. Influenza Surveillance Report. cdc.gov/flu/weekly
6. CDC. VSD. cdc.gov/vaccine-safety/about/vsd.html
7. CDC/FDA. VAERS. vaers.hhs.gov