TECUMSEH STUDY OF ILLNESS. XIII. INFLUENZA INFECTION AND DISEASE, 1976–1981

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Abstract
The occurrence of influenza was followed in Tecumseh, Michigan during the five year period 1976–1981 by identifying onset of acute respiratory illness and by virus isolation and serology. Type B outbreaks were observed in 1976–1977 and 1979–1980, type A (H3N2) in 1977–1978 and 1980–1981, and type A (H1N1) viruses in 1977–1978, 1978–1979, and 1980–1981. Evidence of low level circulation of viruses in the year preceding an outbreak was not obtained. Age-specific isolation rates from specimens collected by the community physicians differed from age-specific isolation rates from specimens collected from the surveillance, suggesting the operation of a selection mechanism in the former. Symptoms associated with virus isolation were strongly influenced by age. Within age groups, several variables, especially median duration, indicated type A (H3N2) had produced the most severe illnesses, type A (H1N1) the mildest, with type B intermediate. Age-specific infection rates determined by serology for the 1976–1977 and 1977–1978 influenza seasons confirmed the consistently high rates for type A (H3N2) in children with some fall-off with increasing age. Type A (H1N1) rates peaked in children aged 5–19 years and type B in children aged 5–14 years. This may be related in part to insensitivity of the hemagglutinatlon inhibition test in those under age 5 years. Infection with type A (H1N1) was detected at low frequency in adults. Pathogenicity was caiculated based on the serologic data. It was estimated for all ages combined that, at a minimum, typo A (H3N2) infection produced febrile illness in 25% of cases and type B infection produced respiratory illness in 34% of cases.