Question
Antigenic drift.
Answer
Antigenic drift is the gradual, minor accumulation of point mutations in the haemagglutinin (HA) and neuraminidase (NA) genes of influenza virus, occurring in both influenza A and B, due to the intrinsically error-prone viral RNA-dependent RNA polymerase, which lacks proofreading capability.
Mechanism: as the virus replicates within an infected host and spreads through a population that has partial immunity from prior infection/vaccination, minor mutations that allow the virus to partially evade existing antibody are selectively favoured, gradually accumulating over successive infections/seasons.
Consequence: since only minor antigenic changes occur, pre-existing partial cross-reactive immunity from previous infection/vaccination is not entirely lost, but is progressively eroded — this is responsible for recurring seasonal influenza epidemics and necessitates the annual reformulation of the influenza vaccine to match the currently circulating drifted strains.
Contrast with antigenic shift: antigenic shift is a major, abrupt change in HA/NA through genetic reassortment (exchange of whole gene segments between two co-infecting strains), occurring only in influenza A (requiring an animal reservoir), and capable of causing a pandemic due to the near-complete absence of pre-existing population immunity to the novel combination — a fundamentally different mechanism from the gradual mutation-driven process of drift.