a-sudden change in the antigenicity of a virus resulting from the recombination associated with the genomes of two viral strains. Antigenic change is observed only with influenza A viruses. It results typically from replacement of this hemagglutinin (the viral accessory protein that also mediates the entry of the virus to the cell) with a novel subtype which has had maybe not already been present in real human influenzaviruses for quite some time. The source of these brand new genetics may be the big reservoir of influenzaviruses in waterfowl. The effects of the introduction of a unique hemagglutinin into individual viruses is usually a pandemic, or an internationally epidemic.