Pattern of respiratory syncytial virus epidemics in Finland: two-year cycles with alternating prevalence of groups A and B.

Time-resolved fluoroimmunoassay with monoclonal antibodies distinguishing between respiratory syncytial virus (RSV) group A and B strains was used to analyze their prevalence in Finland during 1981-1990 among 3285 patients with laboratory diagnosis of RSV, most of them hospitalized. The group typing of antigens in 608 RSV-positive nasopharyngeal aspirates showed a regular alternation of group prevalence, following the cyclic occurrence of the virus. Group A predominated in 73%-90% of specimens from 1981-1982, 1985-1986, and 1989-1990, whereas group B predominated in 70%-100% of specimens from 1983-1984 and 1987-1988. The epidemiologic occurrence of verified reinfections in hospitalized children and the group typing results indicated that children greater than 6 months of age during the first infection were more resistant to severe reinfection with the homologous than with the heterologous group of virus. The study shows that group antigenic variation of RSV has a significant effect on the epidemiology of the virus.