Molecular epidemiology of influenza.

The genome of the influenza A viruses comprises eight single-stranded RNA segments, and this property makes genetic reassortment after double infection of a host with two different influenza A strains possible. Nature takes advantage of genetic reassortment during antigenic shift creating new pandemic strains. After concurrent infection of a host with both avian and human strains, the hemagglutinin gene of the human virus may be replaced by the allelic gene of the avian virus. This reassortment leads to a human virus strain that has avian hemagglutinin molecules on its surface, significant because the human population lacks neutralizing antibodies to this new glycoprotein. The Hong Kong pandemic of 1968 resulted from just such an event.