Flu Vaccine Mechanism: How Antibodies Protect Against Influenza

Influenza vaccines induce strain-specific antibodies targeting hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins.

Vaccine Types & Antigen

• Inactivated (IIV): Killed virus, IM.
• Recombinant (RIV): HA protein only (baculovirus), egg-free.
• Live attenuated (LAIV): Intranasal, replicates in nasopharynx.
• High-dose / Adjuvanted: For ≥65 years (↑ immunogenicity).

Antibody Response

1. HA head antibodies: Neutralize by blocking receptor binding (strain-specific).
2. HA stalk antibodies: Broader, cross-reactive (universal vaccine target).
3. NA antibodies: Reduce viral release, severity.
4. Mucosal IgA (LAIV): Local neutralization.

Timing & Effectiveness

• Peak antibodies: 2 weeks post-vaccination.
• Effectiveness: 40–60% when matched; reduces severity/hospitalization even if mismatched.
• Wanes over 6–8 months; annual vaccination needed (antigenic drift/shift).

Special Populations

• Pregnancy: IIV/RIV any trimester (protects infant via transplacental IgG).
• Egg allergy: Any vaccine (no longer contraindicated).
• Immunocompromised: IIV/RIV only (no LAIV).

Annual vaccination remains the best population-level defense against influenza.