Influenza detection: a prospective comparison of surveillance methods and analysis of isolates.

Several prospective methods for surveillance of influenza were compared for the speed and efficacy with which they identified cases of influenza in metropolitan Atlanta during 1972-1975, a period in which two outbreaks of influenza occurred. In a large city-county hospital's outpatient clinics, systematic collection of pharyngeal cultures for virus isolation provided a rapid means for discovering the presence of influenzal illness and for monitoring the extent of the outbreak. Both obtaining cultures and determining the percentage of patients with respiratory complaints indicated an outbreak 3-5 weeks before epidemic disease was confirmed by a rise in the number of deaths due to pneumonia and influenza. Antigenic analysis of influenza A strains isolated during this three-year period showed that influenza A strains recovered in Atlanta in 1973-1974 (a non-epidemic year) were identical to the strains recovered in the 1974-1975 epidemic. Why no epidemic occurred in 1973-1974 in the presence of the epidemic strain of the virus is unknown, and the lack of an epidemic suggests that the role of virus persistence in the epidemiology of influenza requires further evaluation.