Antigenic subgroups of influenza A (Hsw1 N1) virus differentiated by hemagglutination inhibition.

Eleven influenza A/swine virus strains could be arranged in three subgroups by hemagglutination inhibition (HI) titrations. The antisera used include post-infection ferret sera (native as well as after adsorption with heterologous Hsw1 N1 virus) and paired human sera from recent influenza A patients, showing antibody titre rises against A/swine virus. For serological detection of infections with an unknown A/swine virus, virus strains of the subgroup with the broadest reactivity are more suitable than other strains. This subgroup comprises A/swine/1976/31, A/New Jersey/8/76 and A/X-53. The same will hold for vaccine strains.