| Abstract: | Three epidemics of influenza A (H1N1) occurring in 1977, 1979 and 1981 were studied. These epidemics were found to be gradually dying down, which was manifested by progressively decreasing morbidity rate, the frequency and intensity of seroconversions, as well as by a decrease in the duration of the epidemic period. Changes in the biological properties of influenza A (H1N1) virus were accompanied by changes in its antigenic properties. The drift of neuraminidase in the influenza A (H1N1) virus of 1981 towards increased relationship with neuraminidase in the virus of 1952 was observed, while hemagglutinin in the strains of each of these two groups retained its individual character. |
