Some regularities in variations of respiratory syncytial virus strains during their epidemiological circulation.

Among respiratory syncytial (RS) viruses circulating in the territory of U.S.S.R., strain variations were observed based on the differences in activity of virus reproduction in human embryo kidney cell cultures at 39 degrees C and in virus sensitivity to antibodies. Mathematical expression of the correlation between these two genetic markers made it possible to classify the new isolates of RS virus into three groups differing in their virulence, namely high, mild and low virulent strains. Populations of RS virus strains varied depending on the epidemic season: during the first period (1976-1977) predominated the high virulent (52-78%) while during the second period (1977-1978) the mild virulent (55-65%) strains. The third period (1978-1979) was characterized by the circulation of a mixture of mild and low virulent strains. In summer low virulent virus strains prevailed (56-89%).