Antigenic variations of the two virus-coded surface proteins of influenza virus—hemagglutinin and neuraminidase—were examined in seven strains of influenza A2 virus (including the Hong Kong/68 strain) isolated from 1957 to 1968. Changes in the two antigens were found to occur independently in nature, resulting in new viruses which differ from older strains more with respect to one antigen than the other. The Hong Kong/68 strain is markedly different from previous A2 strains in its hemagglutinin antigen but possesses neuraminidase indistinguishable antigenically from that of recent strains. Immunization experiments in mice provided evidence that only the neuraminidase component of an earlier A2 strain provided protection against Hong Kong virus challenge
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