Influenza vaccine

The influenza vaccine elicits protective immunity against influenza virus, chiefly by generating antibodies against the surface glycoprotein hemagglutinin (HA), which the virus uses to bind host cells; neuraminidase contributes as a secondary target. Formulations differ by population and platform: standard-dose inactivated vaccines, high-dose or adjuvanted versions for older adults whose responses are weaker, live-attenuated vaccine given intranasally, and recombinant protein vaccines produced without eggs. Children aged 6–35 months receive a reduced dose (0.25 mL, 7.5 μg HA per strain) while older children and adults receive the full amount.

Annual reformulation is forced by the virus rather than chosen. Point mutations accumulate in the antibody-binding sites of HA — antigenic drift — so last season's antibodies bind this season's virus less well, and strain selection must be made months ahead of the season, with mismatch a recurring cause of low effectiveness. Effectiveness is therefore variable, typically moderate and lower in years of poor match or in older adults, and egg-based manufacture adds its own problem, since adaptation of the seed virus to grow in eggs can itself alter the antigenic surface. Reassortment between influenza A viruses of different hosts — antigenic shift — produces the abrupt novel subtypes behind pandemics, against which seasonal vaccines offer little.

vaccination matters most for those least able to withstand infection: older adults, young infants, pregnant women, and immunocompromised people, including those on immunosuppressive targeted therapies such as rituximab, which depletes the B cells the vaccine depends on. Patients receiving immune checkpoint inhibitors are also a group of interest, though these agents enhance rather than suppress immune responses. No vaccine is approved below six months of age, leaving maternal immunization as the means of protecting the youngest infants. That gap, together with the yearly reformulation cycle and the short duration of protection, motivates work toward universal vaccines — HA stem-targeting nanoparticle designs and adjuvanted platforms aimed at the conserved regions that drift cannot easily change.