We isolated two strains (A/Ishikawa/42/98 and 43/98) of influenza A (H1N1) virus, which are antigenically different from A/Beijing/262/95, from school children who had an influenza like illness in November, 1998 in Ishikawa Prefecture. Although the HI antibody prevalence rate against A/Ishikawa/42/98 was quite low in all age groups tested, this virus did not cause an outbreak until the end of 1999.
In our country, A/Ishikawa/42/98 like virus caused an outbreak after January, 2000, and was interestingly shown to possess similar HA antigenicity to that of A (H1N1) that had caused an outbreak in New Caledonia in May, 1999. Based on these observations, we speculate that A/Ishikawa/42/98 might so change in some unknown viral property during seasons from 1998/99 to 2000 as to cause an outbreak. The analysis of the viral property involved in the occurrence of the outbreak, therefore, seems to be important to prevent an occurrence of influenza outbreak