1. Basic Information

Disease Protected Against

Haemophilus influenzae type b (Hib) is a bacterial pathogen that primarily affects children <5 years. Before vaccination, Hib was the leading cause of bacterial meningitis in U.S. children, as well as epiglottitis, septic arthritis, pneumonia, and bacteremia. Hib meningitis has a case-fatality rate of ~3–6% and causes permanent neurological sequelae (hearing loss, developmental delay) in 15–30% of survivors. Nontypeable H. influenzae (NTHi) strains, which are not covered by the Hib vaccine, cause otitis media and respiratory infections in children and adults but not invasive disease at the same rates.

CDC Recommended Schedule (U.S., 2025)

Dose	Recommended Age	Notes
Dose 1	2 months	Minimum age 6 weeks
Dose 2	4 months
Dose 3 (if needed)	6 months	Depends on product (PedvaxHIB = 2-dose series; ActHIB/Hiberix/Pentacel/VAXELIS = 3-dose series)
Booster	12–15 months	All products require a booster dose at 12–15 months

Source: CDC ACIP, 2025 Child & Adolescent Immunization Schedule. Note: PedvaxHIB (Merck, PRP-OMP conjugate) requires only a 2-dose primary series at 2 and 4 months plus a booster at 12–15 months.

Licensed Products (U.S.)

ActHIB^® (Sanofi) — PRP-T conjugate (tetanus toxoid carrier). Licensed 1993.
Hiberix^® (GSK) — PRP-T conjugate. Licensed 2009, primarily for booster dose.
PedvaxHIB^® (Merck) — PRP-OMP conjugate (outer membrane protein carrier). Licensed 1989. 2-dose primary series.
Combination vaccines: Pentacel (DTaP-IPV-Hib), VAXELIS (DTaP-IPV-Hib-HepB). Both use PRP-T conjugate.

2. Pre-Licensure Clinical Trial Data

The first Hib polysaccharide vaccine (unconjugated) was licensed in 1985 but was poorly immunogenic in infants <18 months. The first conjugate vaccine (PRP-D, ProHIBIT) was licensed in 1987. Current PRP-T and PRP-OMP conjugate vaccines were licensed between 1989–1993. Pivotal trials enrolled several thousand infants each.

Metric	Data	Evidence Strength
Combined pre-licensure safety database	~10,000+ infants across all conjugate products	Moderate
Efficacy (invasive Hib disease)	~95–100% in clinical trials	Strong
Seroprotection (anti-PRP ≥0.15 µg/mL)	~90–100% after primary series	Strong

Most Common Adverse Reactions

Reaction	Frequency (Approx.)
Injection site pain/tenderness	~10–25%
Injection site redness/swelling	~5–10%
Fever >38°C	~5–15%
Irritability	~15–30%
Drowsiness	~10–20%

Sources: Respective prescribing information; FDA review documents. Adverse reactions are generally mild and self-limited (1–3 days). Local reaction rates are higher when Hib is given as part of combination vaccines (Pentacel, VAXELIS).

Key Limitations

No true placebo: Most trials used other vaccines as comparators due to ethical constraints, limiting absolute risk attribution.
Combination products: Modern Hib vaccination is predominantly via combination vaccines (Pentacel, VAXELIS), making it difficult to isolate Hib-specific adverse events from those attributable to co-administered antigens (DTaP, IPV).
Carrier protein effects: Different Hib conjugates use different carrier proteins (tetanus toxoid, OMP). The long-term safety implications of repeated carrier protein exposure in combination schedules are not fully characterized.

3. Post-Licensure Safety Data

VSD and VAERS

Hib vaccines have an extensive post-licensure safety record with >30 years of U.S. data. VSD and VAERS surveillance have not identified unexpected safety signals. Key findings:

No association between Hib vaccination and serious adverse events, SIDS, or neurodevelopmental outcomes has been identified in VSD studies.
VAERS data are consistent with clinical trial reactogenicity profiles. No confirmed safety signals beyond those identified in pre-licensure trials.
IOM (2012) reviewed Hib vaccine and found evidence inadequate to accept or reject causality for most rare outcomes, reflecting the low frequency of adverse event reports and limited targeted post-licensure studies (relative to more controversial vaccines).

⚠ Critical Caveat

VAERS data represent unverified reports temporally associated with vaccination. A report to VAERS does not mean the vaccine caused the event.

4. Documented Adverse Events

▶ Strong Evidence

Injection site reactions: Pain (10–25%), redness/swelling (5–10%). Mild, self-limited. Strong
Fever: 5–15% (low-grade). Strong
Irritability/drowsiness: 15–30%. Self-limited (1–2 days). Strong
Anaphylaxis: ~1 per million doses (rare). Strong

▶ Published Evidence Does Not Support a Causal Association

SIDS: No association. No Association
Type 1 diabetes: Studied; no association found. No Association

5. Disease Prevention Benefits

Metric	Pre-Vaccine Era	Post-Vaccine Era
Invasive Hib disease (annual, children <5)	~20,000 cases (~40–100 per 100,000)	<50 cases (<1 per 100,000) — >99% reduction
Hib meningitis (annual)	~12,000 cases; leading cause of acquired intellectual disability	<30 cases/year
Hib deaths (annual, children <5)	~600–800	<5/year

Source: CDC Pink Book; MMWR. The Hib vaccine has near-eliminated invasive Hib disease in the U.S. Vaccination also reduces nasopharyngeal carriage, providing herd protection to unvaccinated children.

6. Evidence Summary

Hib conjugate vaccines have been in U.S. use for >35 years with a well-established safety and effectiveness profile. Pre-licensure trials were moderate in size but post-licensure data from national surveillance and VSD are extensive. The Hib vaccine is among the most effective in the pediatric schedule, with >99% reduction in invasive disease. No significant post-licensure safety signals have been identified.

Domain	Evidence Grade
Invasive Hib disease prevention	Strong
Common adverse events	Strong
Rare adverse events	Moderate

7. Key References

IOM. Adverse Effects of Vaccines: Evidence and Causality. National Academies Press; 2012. nationalacademies.org
CDC. Pink Book — Hib chapter. cdc.gov/pinkbook
Peltola H. Worldwide Haemophilus influenzae type b disease at the beginning of the 21st century. Clin Microbiol Rev. 2000;13(2):302–317.
CDC. VSD. cdc.gov/vaccine-safety/about/vsd.html
CDC/FDA. VAERS. vaers.hhs.gov