CIRCULATION PATTERNS OF GENETICALLY DISTINCT GROUP-A AND GROUP-B STRAINS OF HUMAN RESPIRATORY SYNCYTIAL VIRUS IN A COMMUNITY

Human respiratory syncytial virus (HRSV) is classified into two major groups, A and B, each of which contains multiple variants. To characte rize the molecular epidemiology of HRSV strains over time, sequencing studies of a variable region of the attachment protein gene from a sin gle community in the United States during 5 successive years were perf ormed. Phylogenetic analysis revealed distinct clades (genotypes) that were further classified in subtypes based on greater than or equal to 96% nucleotide similarity. Five genotypes and 22 subtypes among 123 g roup A HRSV isolates, and four distinct genotypes and six subtypes amo ng 81 group B HRSV isolates were identified. One to two genotypes or s ubtypes accounted for greater than or equal to 50% of isolates from a given year, a shift in the predominant genotype or subtype occurred ea ch year such that no genotype or subtype predominated for more than 1 of the 5 study years. The consistency in the displacement of the predo minant strain suggests that a shift, even within the same group, is ad vantageous to the virus. It was hypothesized that the 'novel' strain i s better able to evade previously induced immunity in the population a nd consequently either circulates more efficiently or is more pathogen ic. The yearly shift in HRSV strains may contribute to the ability of HRSV to consistently cause yearly outbreaks of HRSV disease. These res ults also suggest that isolates may need to be characterized as to bot h group and genotype to fully understand protective immunity after nat ural infection and efficacy studies of candidate vaccines.