Question
Antigenic variations of influenza virus
Answer
Antigenic variations of influenza virus: influenza’s segmented genome and error-prone RNA polymerase (lacking proofreading) allow two distinct mechanisms of antigenic change in its surface glycoproteins (haemagglutinin and neuraminidase):
1. Antigenic drift: minor, gradual changes arising from the accumulation of point mutations during viral replication. Occurs continuously in both influenza A and B. Results in new antigenic variants each season, sufficient to partially evade pre-existing population immunity — the basis of recurring seasonal influenza epidemics, and the reason vaccine composition must be reviewed/updated annually.
2. Antigenic shift: a major, abrupt change, arising from reassortment of gene segments between two different influenza A strains (e.g., human and avian/swine strains) co-infecting the same host cell — possible due to 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., the 1918, 1957, 1968, and 2009 influenza pandemics).
Clinical/public health implications: antigenic drift necessitates annual reformulation of the seasonal influenza vaccine based on WHO surveillance of circulating strains; antigenic shift underlies the unpredictable emergence of pandemic influenza strains, requiring global surveillance systems and rapid vaccine development capability to respond to novel subtypes.