Abstract
During the 1998–1999 influenza season, two distinct influenza B virus Yamagata group strains were isolated from the patients of a private clinic. Each responded differently to monoclonal antibodies (Mabs) 5H4 and 8B3 on staining, and hemagglutination inhibition and neutralizing tests. When the analysis of nucleotide sequences was undertaken, the identity of deduced amino acid sequences of the HA1 region was 94%, which suggested that they derived from different strains. They were termed 5H4‐responding strains and 5H4‐nonresponding strains, respectively. The analysis of laboratory‐induced antigenic variants suggested that the amino acid at position 149 is important to the reactivity to 5H4. This residue was “Arg” in 5H4‐responding strains and “Lys” in nonresponding strains. During the 1998–1999 season, a total of 100 influenza B virus strains were isolated and 5H4‐responding strains were the major type (94 strains). In the 1999–2000 influenza season, only two influenza B virus strains were isolated. Neither responded to 5H4. However, analysis of the deduced amino acid sequences of the HA1 region suggested that one of the two strains was derived from the 5H4‐responding strains of the previous season. The amino acid residue at position 149 was “Lys” in place of “Arg.” These observations suggested that 5H4‐nonresponding strains will increase in coming seasons. J. Med. Virol. 65:745–750, 2001. © 2001 Wiley‐Liss, Inc.