Antigenic Relationship of 1961–1963 A2 Influenza Viruses to Prototype A2 1957 Strain

Summary Outbreaks of influenza in Europe, the Far East and New Zealand and widespread influenza in eastern and central United States in the winters of 1961–1962 and 1962–1963 were found to be caused by influenza type A2 viruses which had undergone considerable antigenic change from the prototype A2 virus recovered in 1957. This conclusion was based on evidence first obtained in virus neutralization tests employing the hemadsorption inhibition technique with contemporary and prototype strains of type A2 virus and serum obtained from mice and man following vaccination with influenza vaccine containing either A2/Japan/305/57 or A2/Japan/170/62 and later confirmed in virus neutralization tests performed in mice with the same reagents. These findings were made in time to incorporate a contemporary influenza type A2 virus into the influenza vaccine to be produced for use in the United States in the winter of 1963–1964.