EPIDEMIOLOGIC STUDIES ON INFLUENZA IN FAMILIAL AND GENERAL POPULATION GROUPS, 1951–1956

Abstract

This study of influenza and influenza vaccines involved population groups of some 3,000 in 1951 to nearly 19,000 by 1956. Study persons resided in many local areas centering about elementary schools in 3 cities. They were observed biweekly during the fall-winter-spring months of each year and 17,344 acute febrile respriatory illnesses (AFRI) occurred. Of these 3,797 (22%) were visited by medical personnel who obtained throat specimens from which 209 type A and 173 type B strains of influenza virus were isolated. The chief findings were; (1) AFRI outbreaks identified as type B influenza occurred during 7 weeks of 1952 and 1955 and type A during 9 weeks of 1953. Although testing of AFRI was fairly uniform throughout the study, influenza virus was isolated only during outbreaks. (2) Influenza B was spotty in distribution as evidenced by considerable variability in AFRI attack rates from one local area to another. Influenza A attack rates were uniformly high throughout all areas. Influenza B attack rates were particularly pronounced among school-age children. Influenza A attack rates were high in all age groups, but especially so among preschool and school-age children. (3) In households where cases of influenza A or B were confirmed, illnesses among associates aggregated within 10 days of onset of the initial case. During this time influenza virus was recovered from most AFRI, from a few afebrile illnesses, and rarely from a non-ill contact. (4) One area was observed during both type B outbreaks. There was no evidence that residents who had influenza or were exposed to influenza in the household in 1952 were protected against influenza in 1955. (5) Perhaps the most significant finding of this study was the high proportion of initial household cases of influenza among school children. This suggests that the school group is a most important means through which influenza virus is disseminated from the community to the household and indicates a vital link in the chain of infection at which control of outbreaks by prophylaxis might effectively be directed.