Impact of epidemic type A influenza in a defined adult population.

The authors studied the occurrence of excess morbidity and mortality among subsets of the adult population (persons over 14 years of age) of a large prepaid group practice during two epidemics of type A influenza (1968-1969 and 1972-1973). The experience from 1970-1971, a nonepidemic years, was used as a reference for estimating excesses. Ambulatory visits for influenza and influenza-related acute respiratory disease increased by 5-7 visits per 100 persons (30-50% excess) and pneumonia and influenza (P&I) hospitalization rates increased by 79-86 per 100,000 (140-150% excess) over the rates in the nonepidemic period. There were 11-13 excess deaths per 100,000 population, most of which involved persons over 65 years of age. Excess rates of ambulatory visits were similar for all adult age groups. Excess rates of persons hospitalized with P&I ranged from 23-25 per 100,000 among non-high risk persons aged 15-44 years to 150-172 per 100,000 among persons over age 65 years without underlying "high risk" conditions and 476-636 per 100,000 persons over 65 with underlying "high risk" conditions. These data provide a basis for increasing the precision of clinical and public decision-making regarding the prevention of serious consequences of epidemic influenza.