Characteristics of antigenic and biological properties of influenza A viruses isolated different epidemics of the same cycle.

The antigenic character of the haemagglutinin and of the neuraminidase from Myxovirus influenzae strains isolated in the USSR during the 1968, 1970, 1971-1972 and 1972-1973 epidemics, are studied. The haemagglutinins were, at first, relatively, homogenous and constant, but subsequently changed gradually newer isolates became less and less reactive against a specific serum prepared with A/Hong Kong/68. However, this druft up to 1972-1973, did go beyond the limits of the H3 subtype. There were also some differences in the biological properties between the earlier and later haemagglutinins, especially with respect to their sensitivity to gamma-inhibitor in animal sera, and to their pattern of thermoinactivation at 56 degrees C. There was a similar antigenic drift in the case of the neuraminidase, but here it was more pronounced than with the haemagglutinin, for the 1972-1973 isolates had lost virtually all cross reactivity against the 1957-1969 strains. In this manner, three neuraminidase variants could be distinguished: N2-1 from H2 sub-type isolates since 1957, N2-2 from 1964-1971 isolates and N2-3 from 1972, and beyond, strains. Central strains of virus, such as England/42/72 and some of the Leningrad/72, had neuraminidases intermediary between N2-2 and N2-3. The in vitro thermostability of these neuraminidases was variable, according to the strains. The explosive nature of the 1972-1973 epidemic was no doubt linked to the changes in the virus neuraminidase, which were more pronounced than the changes in the haemagglutinin.