Influenzavirus infections in Seattle families, 1975-1979. I. Study design, methods and the occurrence of infections by time and age.

Intensive surveillance of Seattle, Washington, families with school-age children for influenzavirus infections during 1975-1979 encompassed 639 family- and 2732 person-seasons of observation, covering four influenzavirus epidemic seasons: type B (1975-1979), type A/H3N2 (1975-1976 and 1977-1978) and type A/H1N1 (1978-1979). Late spring "herald" waves of infection occurred in 1977 (A/H3N2), 1978 (A/H1N1) and 1979 (type B), the latter presaging an epidemic in 1979-1980. Out-of-season infections, recognized by serology only, included type B and A/H3N2 viruses in each summer and A/H1N1 virus in 1978. In epidemic seasons, infection rates were highest in children aged 5-9 years (A/H3N2) or in teenagers (A/H1N1 and type B). A/H1N1 virus caused the sharpest epidemic, with 31% of the population (but only 2% of adults) infected and 72% of households invaded in 1978-1979. These compare with infection rates of 17-24% overall and 6-13% of adults and the invasion of 38-53% of households observed in the type B and two A/H3N2 epidemics. Extended observation (largely serologic) of a cohort of 1965-1969 Virus Watch families for up to 14 years (including one three-year gap) indicated overall infection rates of 13.7 and 16.4 per 100 person-years with types B and A/H3N2 viruses, respectively, and rates of first and second reinfections of about 3 and 1 per 100 person-years, respectively, with each virus. Close surveillance in 1975-1979 revealed second family episodes of infection with each prevalent virus, 37 with A/H3N2, 15 with type B and 13 with A/H1N1 virus. Risk of infection in these episodes was related more to current hemagglutination-inhibiting titers than to experience (infected or not) in the initial episodes, with 67-100% reinfection when titers were low. Among younger (less than 20 years old) members, related illness was as frequent with reinfection as with initial infection.