Longitudinal studies of types A and B influenza among Seattle schoolchildren and families, 1968-74.

Longitudinal studies of influenza were conducted by postepidemic collection of sera and illness information among Seattle schoolchildren from 1968 to 1974 and in family groups from 1972 to 1974. The rate of infection with A/Hong Kong (H3N2) virus was lowest in the 1968-1969 epidemic; rates were twice as high in the epidemics of 1970 and 1972, and in the A/England (H3N2) epidemic of 1973. The introduction of a new strain of influenza type A or type B caused higher rates of infection in junior high than elementary schoolchildren. For influenza A, a shift in strain was associated with higher rates in the urban area than in the suburban area. The proportion of adolescents with serological evidence of infection who reported typical illness was 40%-42% in all three A/Hong Kong epidemics and 54% during the A/England epidemic. Reinfection with the A/Hong Kong strain was rare. Recent natural infection with the A/Hong Kong virus (1972) afforded 62% protection in the 1973 epidemic of A/England influenza.