Antigenic Variation of Recently Isolated A2 Influenza Viruses

Abstract

Summary An antigenic variant of Type A₂ influenza virus appeared between 1960 and 1962. Although it was associated with human disease in the Far East in 1962, outbreaks were not noted in the United States until January 1963. This variant appears to lack one or more antigens present in the older A₂ virus, in that rooster antisera prepared to the 1957 strains readily inhibit current strains by HI test, whereas similarly prepared antisera to 1962–1963 viruses react weakly if at all with older strains. Antibody to the current strains in young adults' sera drawn in 1962 was found to be present at a significantly lower level than that to older A₂ viruses. No adequate evaluation can be made of the effect of the present shift upon disease in man. Controlled studies of vaccine prophylaxis hopefully will yield this information. The tissue culture neutralization test (hemadsorption-inhibition) was found to be a reliable method for measuring neutralizing antibody to influenza virus. This method is even more specific than the HI with rooster sera in demonstrating differences among strains. It has the further advantage of obviating the need for preliminary chemical or enzymatic treatment of human sera, and gives a better measure of antibody than other serologic tests.