Antigenic and Biochemical Analysis of Field Isolates of Influenza B Virus: Evidence for Intra- and Inter-epidemic Variation

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Abstract
Detailed antigenic analysis using a panel of monoclonal antibodies was carried out on the hemagglutinin antigen of 53 influenza B viruses isolated from an epidemic in a single school. Thirteen distinguishable antigenic groupings of influenza B viruses were detected; 26 of the viruses were in 2 groups (III and IV) which co-existed during the entire epidemic. Antigenically distinguishable influenza B viruses were isolated from an epidemic in a 2nd nearby school. Influenza B viruses isolated from the 2 schools could be further distinguished by different electrophoretic mobilities of NS1 polypeptides and of genes 1, 2, 3 and 4; viruses from a single school epidemic were very closely related as regards these biochemical characteristics. Evidently, the outbreak was initiated by a single individual who excreted antigenic mutants of which predominantly 2 spread and co-existed during the epidemic, although the additional occurrence of random mutations during the evolution of the epidemic cannot be excluded.