[Reversion in the natural variability of influenza A virus].

In mid-November, 1977, local outbreaks of acute respiratory diseases (ARD) in institutionalized communities began to be recorded in a number of geographical zones of the USSR, and by the end of the month a general increase in the incidence was observed in some areas of the country. The epidemic outbreaks extended gradually and were characterized by moderate development involving mainly young subjects. The strains causing the epidemic had no antigenic relationship with reference A (H3N2) virus and the H1 test were neutralized with antisera to influenza A virus with the antigenic formula H1N1 to 1 1/4 titer. Their neuraminidase was inhibited by the antiserum to the recombinants containing neuraminidase of the subtype N1, especially A/New Jersey 8/76. These results permit to classify the strains causing the epidemic outbreaks in November--December, 1977, as influenza A virus with the antigenic formula H1N1.