Influenza viruses are single-stranded RNA viruses of the orthomyxovirus family. There are three types—A B, and C—as determined by their genetic material. Type C does not cause significant human disease and does not warrant further discussion. The virions have surface glycoproteins that contain either hemagglutinin or neuraminidase activity. Different subtypes of influenza (i.e., influenza A-H1N1 versus influenza A-H2N2) have large differences between the hemagglutinin (H) or neuraminidase (N) molecules, whereas different strains of the same subtype have minor differences in the H or N molecules.
Antigenic drift is minor mutations in the RNA genome of either the H or N molecule causing a change in antigenicity. Influenza A has much more frequent drift than B, and H more than N, so that influenza A and H molecules show virtually annual changes accounting for decreased antigenicity and facilitating annual epidemics.
Antigenic shift occurs by genetic reassortment within a host infected by two different influenzaviruses, producing a new virus with little or no antigenic similarity to the old viruses. The population lacks immunity against the new virus, but has high levels of immunity against the old virus, giving the new virus a competitive advantage. This type of shift has occurred three times with hemagglutinin and twice with neuraminidase in this century (thus, we have H1, 2, and 3, and N1 and 2). Antigenic shifts are responsible for the flu pandemics, such as in 1918.
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