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Influenza virus generates most new antigenic variants in two ways, either by antigenic drift of the Haemagglutinin (HA) and Neuraminidase (NA) or by antigenic shift involving genomic reassortment. Influenza pandemics like those that occurred in 1957 and 1968 are thought to have involved reassortment of HA and NA genes between human and avian or swine viruses. The factors controlling reassortment are...
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