Immunity to Influenza A

Nature of Influenza A Immunity

Antibody responses to influenza A are primarily strain-specific, meaning immunity to one influenza virus subtype confers limited or no protection against another subtype, according to the Centers for Disease Control and Prevention 1, 2
Historical data demonstrates that immunity can persist for years when viral strains remain stable—for example, 67% effectiveness was observed 3 years after vaccination with the 1968 pandemic vaccine against the same A/Hong Kong/68 virus, as reported by the CDC 1

T-cell responses develop after infection and target more conserved viral proteins, potentially providing broader heterosubtypic immunity across different influenza A subtypes, as noted by the CDC 1
Live attenuated influenza vaccine (LAIV) induces stronger cellular immune responses than inactivated vaccines, particularly in children, with significant increases in influenza A-specific interferon-γ producing T cells, according to the CDC 1

Protection from vaccination typically lasts 6-8 months in healthy non-elderly populations, which is why annual vaccination is necessary, as recommended by the CDC 1
In young adults, vaccine efficacy has been documented at 92-100% during the first year, declining to 68% in the second year and 61% in the third year without revaccination, as reported by the CDC 1
Immunity declines in the year following vaccination, necessitating annual revaccination even when vaccine strains remain unchanged, according to the CDC 3

Influenza A viruses undergo continuous antigenic drift through point mutations during replication, creating new variants that can evade existing immunity, as noted by the CDC 2
The vaccine composition changes most seasons to match anticipated circulating strains, as previous immunity may not protect against newly emerged variants, according to the CDC 1, 3
Even when vaccine strains are unchanged, declining antibody levels over time necessitate annual boosting, as recommended by the CDC 3
The strain-specific nature of antibody responses means that immunity from one infection provides limited protection against future infections with antigenically distinct strains, as reported by the CDC 2
Do not assume that prior influenza A infection provides broad, long-lasting protection—the highly mutable nature of influenza A and strain-specific immunity means that individuals remain susceptible to new variants and require annual vaccination regardless of infection history, according to the CDC 1, 2

The influenza vaccine cannot cause influenza infection because it contains only noninfectious viruses that have been rendered inactive, according to the Centers for Disease Control and Prevention (CDC) 4, 5, 6, 7, 8

Influenza vaccines are made from highly purified, egg-grown viruses that have been rendered noninfectious through inactivation processes, as recommended by the CDC 5, 9
Each year's influenza vaccine contains three virus strains (typically two type A and one type B) representing influenza viruses likely to circulate in the upcoming season, as determined by the World Health Organization (WHO) 5, 10

The most frequent side effect is soreness at the vaccination site lasting up to 2 days, reported by less than one-third of vaccine recipients, with a strength of evidence rated as high by the CDC 4, 6, 8
Systemic reactions that may occur include fever, malaise, myalgia, and other systemic symptoms that occur infrequently, most often affecting persons with no prior exposure to influenza virus antigens, such as young children, with a strength of evidence rated as moderate by the CDC 4, 6, 8

Persons with severe egg allergies should consult a physician before receiving influenza vaccination, as vaccines contain small amounts of egg protein, as advised by the American Academy of Allergy, Asthma, and Immunology (AAAAI) 4, 6, 7
Influenza vaccination can be safely administered simultaneously with other vaccines, including pneumococcal vaccine, at different sites on the body, as recommended by the CDC 4, 11

Annual vaccination is necessary because immunity declines in the year following vaccination, with a strength of evidence rated as high by the CDC 5, 10
Vaccination remains the most effective strategy to prevent influenza and its complications, especially in high-risk populations, such as the elderly and young children, as recommended by the CDC and the WHO 5, 10, 8