Restriction pattern variability of respiratory syncytial virus during three consecutive epidemics in Denmark.

A PCR-based assay was used to distinguish between respiratory syncytial virus (RSV) group A and B in order to analyze their prevalence in Denmark in three consecutive epidemics during the winters of 1992/93, 1993/94 and 1994/95. A total of 96 RSV strains isolated from hospitalized children were examined, showing alternation of group prevalence. Furthermore, the genetic variability of the RSV isolates was illustrated by restriction enzyme analysis of PCR products originating from a part of the F and G proteins that has been reported to be highly variable. We found that, in general, different genome types predominated each year, some types being present in consecutive epidemics, indicating a contribution of strains circulating unattended between outbreak seasons, while others seemed to disappear or became undetectable, being replaced by emerging types. Some of the genome types found seemed related to strains isolated up to more than two decades ago in other parts of the world. This indicates that the temporal fluctuation in predominance of genome types presumably caused by selective pressure exerted by host immunity is due to the favoring of strains from a pool of globally circulating, genetically relatively stable genome types, rather than a molecular evolution in strains induced or directed by immunoselective pressure.