INFLUENZAVIRUS INFECTIONS IN SEATTLE FAMILIES, 1975–1979

Abstract

Influenzavirus infections in Seattle, Washington, families during 1975–1979 were analyzed, and It was found that, in family episodes, the relation of infection rates to age and preepisode hemagglutination-inhibiting titer varied with the invading virus. In influenza A/H3N2 episodes, rates for both children and adults decreased with hemaggiutinatlon-inhibiting titers; with equal titers, rates for children were double those for adults; and, for all ages, appreciable protection required a titer of 1:80. In A/H1N1 episodes, the few children with titers of 1:>20 and adults, regardless of titer, generally escaped infection. In type B episodes, no titer was protective for children whereas, for adults, a titer of 1:>20 was 57% protective. Illness frequency among those infected varied little with infecting virus and was 85% in susceptible persons in virus-positive episodes. Both frequency and severity varied modestly (and inversely) with age and hemagglutination-inhibiting titer in A/H3N2 infections, only with age with type B infectIons, and with neither in A/H1N1 virus. Only with A/H3N2 infections were iliness frequency and severity inversely related to magnitude of complement-fixing response. Preseason levels of antineuraminidase (studied only in relation to A/H3N2 infections) were inversely but weakly related to illness severity, suggesting that vaccine-induced antineuraminidase would confer little benefit. Age-specific infection and introduction rates indicated that young school children (5–9 years) were more often introducers in A/H3N2 epidemics, whereas teenagers were more likely to serve as introduc era with type B (probably) and A/H1N1 (definitely) viruses. Apparent secondary attack rates varied with virus type only among adults (highest with A/H3N2 virus and lowest with A/H1N1). Additional variation was found with age (in verse) and with how episodes were recognized (higher when virus was isolated). While parents and preschool children were effective introducers, the most abundant apparent exchange within families in all epidemics was among those 5–19 years of age. However, secondary attack rates for these ages probably are inflated by misclassification as secondary of many infections acquired from the community.