An outbreak of influenza B in an elderly population.

An outbreak of influenza B occurred in an elderly population residing in a Minnesota nursing home between April 24 and May 21, 1979 and involved 129 (35.9%) of the 359 residents. Throat swabs from 11 and 19 acutely ill residents yielded influenza B virus similar to the B/Singapore/79 strain. Fourfold or greater increases in titer of complement-fixing or hemagglutination-inhibiting antibodies were detected in paired sera from 18 of the 19 persons affected and from three of 16 unaffected residents. Three hundred thirty-three (93%) of the 359 residents had received trivalent influenza virus vaccine in November 1978. The attack rate was age-specific and increased with age (z = 2.69; P = 0.007). Increasing susceptibility to influenza B with age in the elderly has not been previously demonstrated in this infrequently studied population. Increased risk for becoming ill was also found to be statistically associated with decreasing levels of care required and residence in closed wards and temporally associated with taking meals in the facility's dining areas.