Emergence of an influenza B virus with antigenic change.

Influenza B virus Yamagata group strains, isolated in the 2000 to 2001 influenza epidemic season, reacted poorly to the polyclonal ferret sera prepared against strains isolated earlier. The results of genetic analysis clarified that a point mutation of the nucleotide at position 126 in the HA1 region and the corresponding one-amino-acid substitution altered viral antigenicity.