Question
Antigenic variations of influenza virus
Answer
Antigenic variation of influenza virus occurs through two distinct mechanisms affecting haemagglutinin (HA) and neuraminidase (NA):
Antigenic drift: gradual, minor accumulation of point mutations in the HA/NA genes (error-prone viral RNA polymerase lacking proofreading), occurring in both influenza A and B; responsible for seasonal epidemics and the need for annual vaccine reformulation.
Antigenic shift: a major, abrupt change in HA/NA through genetic reassortment — exchange of whole gene segments between two different strains co-infecting the same cell (e.g., in a pig “mixing vessel”) — occurring only in influenza A, since it requires an animal reservoir; produces a completely novel HA/NA combination with little/no pre-existing population immunity, capable of causing a pandemic (e.g., 1918 H1N1, 1957 H2N2, 1968 H3N2, 2009 H1N1).
Basis: influenza A’s segmented RNA genome (8 segments) permits reassortment (shift); the error-prone polymerase drives ongoing point mutations (drift).