Unusual features of the 1971 influenza epidemic in Hungary and the antigen analysis of the isolates by a serum adsorption test.

In Hungary the 1971 influenza epidemic, unlike earlier influenza A2 epidemics, started unusually early and in 2 foci. Average morbidity (19.3%) was in the usual range but the geographical distribution of the cases was irregular. When 179 isolates were analysed antigenically with monospecific sera prepared by serum adsorption, it was found that they represented 2 clearly distinguishable variants of influenzavirus A (H3N2); 126 strains were closely related to the prototype strain A/Hong Kong/1/68, and 53 strains to the strain A/England/878/69. Adsorbed sera monospecific for closely related variants, like those employed in the present study, are recommended for use in the rapid and accurate identification of influenzavirus isolates.