Antigenic shift describes the sudden replacement of surface antigens in a virus through genetic reassortment, producing a new strain with novel antigenic properties. In influenza A, it occurs when two different viruses co‑infect a host and exchange gene segments encoding haemagglutinin or neuraminidase, leading to abrupt changes that can bypass existing immunity.
Mechanism and conditions
Influenza A viruses have a segmented RNA genome consisting of eight separate segments, which allows reassortment to occur when a single cell is infected by two distinct strains. During replication, gene segments are packaged into progeny virions randomly; if segments from each parent virus are combined, the resulting virus may possess a novel haemagglutinin and/or neuraminidase combination. This antigenic shift is distinct from antigenic drift, which involves gradual accumulation of point mutations. Shifts typically require a mixing host such as a pig or a human where avian and human influenza strains can co‑infect the same cell. Because humans often have little or no pre‑existing immunity to the new HA or NA subtype generated by reassortment, the shifted virus can spread rapidly through the population. Other segmented viruses, like rotaviruses and bunyaviruses, can also undergo reassortment, but influenza A remains the prototypical example.
Pandemic examples and impact
Historic influenza pandemics illustrate the consequences of antigenic shift. The 1918 H1N1 “Spanish flu,” the 1957 H2N2 “Asian flu,” and the 1968 H3N2 “Hong Kong flu” each arose when avian influenza viruses donated new haemagglutinin and neuraminidase genes to human‑adapted viruses, resulting in high morbidity and mortality worldwide. The 2009 H1N1 pandemic virus contained gene segments from avian, swine and human lineages. Because antigenic shift produces viruses with antigenic profiles distinct from contemporary strains, seasonal vaccines offer little protection, necessitating rapid development of pandemic vaccines. Public health surveillance of animal reservoirs, particularly birds and pigs, is crucial for early detection of reassortant viruses. Understanding the mechanisms and risks of antigenic shift informs pandemic preparedness and underscores the importance of global influenza monitoring.
Antigenic shift is a rare but significant evolutionary event in segmented viruses that results in abrupt changes in surface antigens. In influenza A, such shifts can produce new subtypes against which human populations lack immunity, leading to pandemics.
Related Terms: Antigenic drift, Reassortment, Pandemic influenza, Hemagglutinin, Neuraminidase