Influenza surveillance by age and target group.

Surveillance for influenza virus infections began in mid-1976, continued through three small influenza A outbreaks (mixed A/Victoria and A/Texas outbreaks in Spring, 1977, and Winter, 1977-1978 and an A/USSR outbreak peaking in March, 1978). Nasopharyngeal specimens were collected for virus isolation from febrile respiratory disease patients coming from four sources: 1) the Sunday walk-in clinic of the Group Health Cooperative of Puget Sound; 2) 200 families under continuous surveillance for respiratory viral infections; 3) patients referred by interested community physicians or self-referred persons in vaccine evaluation studies; and 4) students presenting at the University of Washington health center. Influenza virus isolation rates were as high in children as adults and peaked earlier in adults than in children in all three epidemics. Although referred patients and university students yielded the highest isolation rate, nearly 30%, all four community sources were important in detecting an impending epidemic.