Variability of influenza A2 viruses in nature

: A comparative study of variability of the properties of influenza A2 viruses isolated in different epidemic years (1964-1965, 1968-1969, 1972-1973) showed that in the course of influenza virus evolution changes occurred not only in the antigenic properties of surface proteins (hemagglutinins and neuraminidase) but in a number of other properties as well. In contrast to previously circulating strains, influenza A2 virus strains isolated in the epidemic of 1971-1972 were found to have higher reproducing and neuraminidase activities, greatly increased thermostability and the capacity to induce viremia regularly in white mice.