[Differences in influenza epidemics in Osaka City--epidemiological surveillance of infectious disease]

Influenza viruses in outpatients with influenza symptoms in Osaka City were analyzed in an epidemiological surveillance of infectious disease between 1989 and 1993. During influenza epidemics a mixed prevalence of several types of influenza viruses existed. Three types of influenza viruses, AH1, AH3 and B, were isolated during the 1990/1991 season. Remarkably the three types of viruses were discovered in samplings collected on the same day and within a narrow area inside a radius of 800-1,000m from the surveyed hospitals. Different types of viruses were detected between brothers and among school children from same housing complexes. Influenza AH3 viruses detected in 1992/1993 season differed in antigenicity from those detected in the 1990/1991 and 1991/1992 seasons. Therefore it appears that mutation of the AH3 virus contributed to the large-scale influenza epidemic which occurred in the 1992/1993 season.