Question
Antigenic variations in influenza virus
Answer
Antigenic variations of influenza virus:
1. Antigenic drift: minor, gradual changes arising from accumulation of point mutations in the surface glycoproteins (haemagglutinin, neuraminidase), occurring continuously in both influenza A and B. Results in new antigenic variants each season, sufficient to partially evade pre-existing immunity — the basis of recurring seasonal influenza epidemics, requiring annual vaccine reformulation.
2. Antigenic shift: a major, abrupt change, arising from reassortment of gene segments between two different influenza A strains co-infecting the same host cell (possible given the segmented genome). Occurs only in influenza A. Produces a novel viral subtype to which the population has little/no pre-existing immunity, creating pandemic potential (e.g., 1918, 1957, 1968, and 2009 pandemics).
Clinical/public health implications: drift necessitates annual reformulation of the seasonal influenza vaccine; shift underlies the unpredictable emergence of pandemic influenza strains, requiring global surveillance systems and rapid vaccine development capability.