Antigenic relationships between strains of influenza B virus.

An immunological relationship between strains of influenza B virus, considerably differing from one another in haemagglutination inhibition (HI) and virus neutralization (VN) tests, was established. The relationships were also evaluated based on the ability of influenza B viruses to replicate in the lungs of mice immunized with strains possessing antigenically distinct haemagglutinin. There was no substantial difference in the protection of animals immunized with homologous or heterologous strains. Studies on the character of the immunological response of men convalescent after influenza B infection or after vaccination showed an antibody increase to both the epidemic virus and chronologically remote viruses considerably differing in antigenic properties. The data obtained suggest that influenza B viruses isolated from 1940 to 1975 belong to one antigenic subtype.