Antigenic drift in the haemagglutinin (HA) molecule of influenza B viruses was studied with monoclonal antibodies. Antigenic drift occurred in each of the 12 different epitopes studied and there was evidence that at least two antigenically distinguishable influenza B virus strains can co-circulate during an epidemic. The frequency of antigenic variation in the HA of influenza B viruses was frequently less than 1 in 108 and was approx. 1000-fold below that found in influenza A strains. Haemagglutination inhibition (HI) tests on antigenic variants selected with 12 different monoclonal antibodies suggested that the antigenic determinants could be subdivided into three partially overlapping groups. Many of the antigenic variants selected with monoclonal antibodies were distinguishable from the parental virus with post-infection ferret sera, suggesting that the majority of the variants that do occur could have epidemiological potential.
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