Antigenic relationships among influenza B viruses isolated over the 21-year period from 1940 through 1961 were studied with the use of both infected ferret sera and sera obtained from persons of three age groups after vaccination with monovalent vaccines. Three antigenic clusterings having successive periods of prevalence could be distinguished but were not sufficiently different from each other to warrant classifying them as distinct antigenic families in analogy to the influenza A viruses.

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