Large sequential outbreaks caused by influenza A (H3N2) and B viruses in an institution for the mentally handicapped.

During the mixed epidemic caused by influenza A (H3N2) and B in the 1992-1993 season in Japan, large sequential outbreaks occurred in an institution for mentally handicapped people where none of the residents or staff members had been immunized. During the influenza A outbreak (A/ Beijing/32/92-like strain) in January, 37.0% of the residents (85/230) and 31.4% of the staff (75/239) had an influenza-like illness. During the influenza B outbreak (B/Panama/45/90- and B/Beijing/184/ 93-like strain) in late February, 59.0% of the residents and 24.3% of the staff had an influenza-like illness. As many as 25.2% of the residents had two episodes of influenza-like illness during the season, as opposed to only 5.4% of the staff members. Mixed epidemics probably have a severe impact on institutionalized high-risk people, adversely affecting them almost twice as much as influenza epidemics caused by a single virus.