Cocirculation of two distinct evolutionary lineages of influenza type B virus since 1983.

Abstract During 1988–1989 two highly distinct antigenic variants of influenza type B were recognized in hemagglutination-inhibition tests with postinfection ferret serum. These viruses were antigenically related to either BNictoria/2/87, the most recent reference strain, or BNamagata/16/88, a variant that was isolated in Japan in May 1988. All influenza B viruses isolated in the United States during an epidemic in the winter of 1988–1989 were antigenically related to B/ Victoria/2/87. However, in several countries in Asia, both BNictoria/2/87-like viruses and BNamagata/16/88-like viruses were isolated. Sequence analysis of the hemagglutinin (HA) genes of several influenza B isolates from 1987 to 1988 indicated that the HA1 domains of the BNamagata/16/88-like viruses and B/VI/87-like viruses isolated in 1988 differed by 27 amino acids. Evolutionary relationships based on this sequence data indicated that the BNamagata/16/ 88-like viruses were more closely related to epidemic viruses from 1983 (B/USSR/100/83-like viruses) than to more recent reference strains such as BNictoria/2/87. All other Asian strains, as well as selected isolates from the United States in 1988, were confirmed by sequence analysis as being genetically related to BNictoria/2/87. These data provide clear evidence that two parallel evolutionary pathways of influenza type B have existed since at least 1983 and that viruses from each of the separate lineages were isolated from cases of influenza B in 1988. This finding is similar to earlier observations for type A H 1 N1 and H3N2 influenza viruses.