Co-circulation of two influenza A (H3N2) antigenic variants detected by virus surveillance in individual communities.

From March through June 1977 a total of 31 influenza A (H3N2) viruses were isolated from students with respiratory disease who were seen at the student health service on the Berkeley campus of the University of California, and 32 influenza A (H3N2) viruses were isolated from persons who participated in a city-wide febrile respiratory disease surveillance program in Seattle. The antigenic specificity of the hemagglutinin was determined for each isolate by hemagglutination inhibition testing with sera from ferrets infected with prototype strains A/Victoria/3/75 and A/Texas/1/77. In each of the three months, April, May and June, A/Victoria/3/75-like and A/Texas/1/77-like viruses were identified among isolates from both communities, and the numbers of isolates of the two antigenic variants from patients seen with influenza-like illnesses were similar. The findings emphasize the need to examine multiple isolates even from within single communities to determine the antigenic specificity of current strains of influenza virus.