[Epidemiological analysis of influenza B virus belonging to B/Victoria/2/87 lineage isolated in off-season of 1998 and late epidemic season in Shizuoka Prefecture]

In Shizuoka Prefecture, influenza B viruses belonging to B/Victoria/2/87 lineage caused an outbreak among school children in the off-season of 1998, and the same B viruses were mainly isolated during the following epidemic. Low titer of HI antibody among children for influenza B virus belonging to the same lineage was a recognized factor in the causation of the distinctive epidemic. The herald virus strains seemed to be antigenically similar to the late epidemic virus strains, but the former strains were not genetically close to the latter. This indicates that the herald viruses are not always the parental viruses for the following influenza season.