Antigenic shift

Antigenic shift is a major change in influenza A viruses caused by reassortment of genome segments. It can produce a virus with surface proteins unfamiliar to human immunity.

Influenza A virus: An influenza virus with a segmented RNA genome that infects humans and many animal species. Antigenic shift occurs in this virus type, whose segmented genome makes reassortment possible.
Influenza pandemic: A worldwide epidemic caused by an influenza virus spreading across populations. Antigenic shift can generate a strain capable of sustained spread in people with little prior immunity.
Avian influenza: Influenza viruses maintained in bird populations, including many influenza A subtypes. Birds provide diverse gene segments that can enter reassortant viruses.
Influenza surveillance: The systematic monitoring of influenza viruses in humans and animals. Surveillance can detect unusual subtypes and reassortant viruses before or during their spread.
Viral reassortment: The exchange of genome segments between viruses coinfecting the same cell. Reassortment is the direct genetic mechanism that produces an antigenic shift.
1918 influenza pandemic: A devastating influenza pandemic that caused tens of millions of deaths worldwide. It illustrates the severe consequences associated with a novel pandemic influenza A virus.
Swine influenza: Influenza viruses that circulate in pigs, which can be infected by multiple influenza A lineages. Pigs can host coinfections that permit gene segments from different influenza viruses to reassort.
Zoonotic influenza: Influenza infection in humans acquired from animals, often with limited onward transmission. A shifted virus must overcome barriers to move from animal hosts into people.
Influenza hemagglutinin: A surface protein that lets influenza viruses attach to and enter host cells. A reassorted virus may acquire a hemagglutinin subtype unfamiliar to human immunity.
1957 influenza pandemic: An influenza pandemic caused by an H2N2 virus that spread globally beginning in 1957. The H2N2 virus emerged through reassortment involving human and avian influenza viruses.

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