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Respiratory-syncytial (RS) virus strains circulating during several years appeared polymorphic in respect of two genetic markers: the regression coefficient of infectious activity (RCIA39) characterizing the isolates by their reproduction in tissue cultures at supraoptimal temperature (39 degrees C) and the regression coefficient of neutralization indices (RCNI) characterizing the degree of sensitivity of the strains to antibodies. High-yield RS viruses were more often isolated from children frequently afflicted by the disease, moderate-yield viruses from moderately sick children, while low-yield or none-yield (at 39 degrees C) strains were isolated from rarely afflicted children. On the other hand, RS strains of low reactivity with prototype antibodies were mainly found in often or moderately sick children and the high-reactive ones in rarely sick children. The variability of the RS virus population was continuous, which is consistent with the uninterrupted course of the epidemic process in the nursery-school community. A change of the RCIA39 marker was observed in nearly 50% of strains already after 1 1/2 to 2 months, but most frequently within 5-6 months from the end of disease. The changes of growth intensity at 39 degrees C followed the pattern: high----moderate----low----none, however, in the next epidemic season these properties showed reversion in an opposite direction.

Biological properties of 188 strains of respiratory syncytial (RS) virus isolated from 93 institutionalized children during 4 years of observations were correlated with the form and stage of the infectious process, frequency of ARD, and blood group of the children for the evaluation of the epidemiological hazard of sources of RS infection. The degree of virulence of a strain was evaluated by its yields in human embryo lung culture and sensitivity to antibodies (avidity). In clinically manifest forms of RS infection more virulent strains were isolated than in inapparent infections and transitory asymptomatic virus-carrier state. Among the strains derived from children with frequent ARD, highly and moderately virulent strains were prevalent (in 72.7% high and moderate yields in cell culture, and 94.2% low avidity to antibodies) whereas children with rare ARD yielded mainly low virulent strains (70% none or low yields, and 62.5%--highly sensitive to antibodies). The RS virus strains isolated from children with A (II) blood group were found to be less sensitive to antibodies.

Respiratory syncytial (RS) virus strains isolated in different years varied by their antigenic and biological properties. The lowest degree of relatedness was found between the "street" virus and the prototype Long strain; the highest occured among the isolates from a given isolation period. Based on the mean indices of efficiency of the virus reproduction in human embryo lung (HEL) cells at 37 degrees C and 39 degrees C as well as on the degree of virus sensitivity to reference antibodies, the isolates from various years could be divided into three groups, namely high, mild and low virulent strains. The incidence of RS virus infections in children depended on the strain characteristic of virus population circulating in a community of children during the long-term observation period of 1976-1979. Cyclic variation was found in isolation rates of RS viruses; the duration of each cycle in different years ranged from 21 to 41 days. The variability of isolation cycles and the frequency of RS virus reinfections were closely related to the biological characteristics of circulating virus strains.

The manifestations of the epidemic process in respiratory syncytial (RS) virus infection induced by the strains of the infective agent, differing in their capacity for reproduction at 39 degrees and 37 degrees C and in their sensitivity to antibodies, were compared. The observation of children in a group (about 80 children simultaneously) with the systematic serological and virological examination of sick and healthy children was the main method in this investigation. The circulation of RS viruses with greater capacity for reproduction at 39 degrees and 37 degrees C and lesser sensitivity to antibodies, i.e. viruses with greater virulence, was accompanied by the increased intensity of manifestations of the epidemic process. An increase in the heterogeneity of RS virus populations isolated at the same period of observation was accompanied by the intensification of the epidemic process, which was manifested by increased morbidity rate and a higher level of contamination in children, an increase in the incidence of outbreaks and in the frequency of RS virus reinfection.