Antigenic and biochemical analysis of field isolates of influenza B virus: evidence for intra- and inter-epidemic variation.

Detailed antigenic analysis using a panel of monoclonal antibodies was carried out on the haemagglutinin antigen of 53 influenza B viruses isolated from an epidemic in a single school. Thirteen distinguishable antigenic groupings of influenza B viruses could be detected but 26 of the viruses were in two groups (III and IV) which co-existed during the entire epidemic. Antigenically distinguishable influenza B viruses were isolated from an epidemic in a second nearby school. Influenza B viruses isolated from the two schools could be further distinguished by different electrophoretic mobilities of NS1 polypeptides and of genes 1, 2, 3 and 6, whereas viruses from a single school epidemic were very closely related as regards these biochemical characteristics. The findings are consistent with the hypothesis that the outbreak was initiated by a single individual who excreted antigenic mutants of which predominantly two spread and co-existed during the epidemic, although the additional occurrence of random mutations during the evolution of the epidemic cannot be excluded.