Question
Antigenic shift in influenza virus.
Answer
Antigenic shift is a major, abrupt structural change in the surface antigens (haemagglutinin, HA, and/or neuraminidase, NA) of influenza A virus, occurring through genetic reassortment — an entirely different mechanism from the gradual point-mutation-driven “antigenic drift” seen in both influenza A and B.
Mechanism: influenza A virus has a segmented RNA genome (8 separate gene segments), which allows two different influenza A strains — for example, a human strain and an avian or swine strain — to co-infect the same host cell simultaneously (a mixing vessel, classically pigs, which are susceptible to both avian and human influenza strains) and exchange whole gene segments during viral assembly, producing a novel reassortant virus carrying an entirely new combination of HA and/or NA surface antigens.
Consequence: because this new HA/NA combination is completely novel to the human population, there is little or no pre-existing population immunity against it (unlike antigenic drift, where partial cross-reactive immunity from prior exposure/vaccination often persists) — this can result in a strain capable of causing a pandemic, with rapid, widespread spread and often higher morbidity/mortality due to the near-universal population susceptibility.
Restriction to Influenza A only: antigenic shift occurs only in influenza A virus, since it requires an animal reservoir (avian/swine) with which reassortment can occur — influenza B and C viruses lack a significant animal reservoir and therefore undergo only antigenic drift, not shift.
Historical examples: the 1918 “Spanish flu” (H1N1), 1957 “Asian flu” (H2N2), 1968 “Hong Kong flu” (H3N2), and the 2009 “swine flu” pandemic (H1N1) all arose through antigenic shift events.
Contrast with antigenic drift: antigenic drift is a gradual accumulation of point mutations in the HA/NA genes over time (due to the error-prone viral RNA polymerase lacking proofreading), producing minor antigenic changes responsible for the need for annual influenza vaccine reformulation and seasonal epidemics, rather than the major population-wide susceptibility associated with shift-driven pandemics.