The antigenic and genetic characteristics of the haemagglutinins of influenza type B viruses isolated since 1988 during periods of both widespread activity (1990/1991) and sporadic activity (1989/1990) were examined using microneutralization tests and direct RNA sequencing. During 1989/1990, influenza B viruses representative of two distinct lineages antigenically and genetically related to either B/Victoria/2/87 or B/Yamagata/16/88 were isolated, and a minor drift variant of B/Yamagata/16/88, B/Hong Kong/22/89, was identified. In 1990/1991, B/Hong Kong/22/89- or B/Yamagata/16/88-like viruses accounted for the majority of the influenza virus isolates in most countries. Sequence analysis of the HA1 domains of representative viruses confirmed the containued existence of two main lineages among recent strains of influenza B virus and identified unique amino acid changes that could account for the altered antigenic reactivity of some variants. Sequence analysis of the HA2 domains of some of the recent influenza B viruses allowed for a comparison of the evolutionary rates and patterns between the HA1 and HA2 domains.


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