Evolution of influenza B/Victoria/2/87-like viruses: occurrence of a genetically conserved virus under conditions of low epidemic activity.

Nucleotide sequence analysis of the gene region coding for the HA1 domain of the influenza B virus haemagglutinin was performed on seven field strains isolated during the 1989 to 1990 season and two field strains isolated in 1985 and 1988 in Finland. All isolates were antigenically and genetically related to B/Victoria/2/87 virus and distinct from B/Yamagata/16/88 virus. The three strains isolated at the beginning of the 1989 1990 season in Turku were almost identical to an American variant (B/Texas/37/88-B/Ohio/10/88) of the previous season, whereas the four strains isolated later in the 1989 to 1990 season in Helsinki formed a new group of heterogeneous viruses. The phylogenetic tree compiled suggests that the two branches had evolved from a common origin, probably in 1987.