Medical practice-based influenza surveillance: viral prevalence and assessment of morbidity.

Regular surveillance of influenza virus activity and associated illness is necessary to monitor changes in circulating strains. As part of a demonstration project, medical practices in a seven-county area in southern lower Michigan were recruited as sentinel surveillance sites. The practices were asked to collect specimens for virus isolation and/or data on age-specific frequency of respiratory illnesses that met a case definition. Data were used to establish periods of influenza virus activity for vaccine effectiveness determinations. Over three study periods, 1989-1992, there were documented outbreaks of influenza type A(H3N2), type A(H1N1), and type B. The incidence of influenza-like illness increased during these outbreaks, with variation in age-specific frequency from year to year. Recovery of virus was not affected during the winter season by long delays in specimen receipt by the laboratory. Results indicated that medical practices can be successfully recruited as surveillance sites to provide information on the timing, etiology, and intensity of acute respiratory illness.

Influenza vaccine effectiveness in preventing hospitalization for pneumonia in the elderly.

During the winter of 1989-1990, influenza type A(H3N2) circulated widely, causing excess morbidity and mortality nationwide. From November through April, 1989-1990, hospitalized cases of pneumonia and influenza occurring among noninstitutionalized individuals 65 or more years of age were identified by 20 acute care hospitals in southern lower Michigan. These cases were group matched on age, sex, race, and zip code to randomly sampled, community-based controls from a comprehensive listing of Medicare beneficiaries residing in the study area. Self-reported data were collected from cases and controls on influenza vaccine status for the 1989-1990 season and on a number of other factors which could have influenced vaccination status or outcome. Questionnaires were completed by 1,907 individuals, 449 of whom were cases, resulting in an overall response rate of 76%. A community-based influenza surveillance system was implemented to determine the timing and intensity of viral activity and influenza-like illness. Vaccine effectiveness in preventing overall pneumonia and influenza hospitalizations was estimated by logistic regression. During the 3-month period of surveillance-confirmed peak influenza type A(H3N2) circulation, vaccine effectiveness was 45% (95% confidence interval 14-64, p = 0.009). However, during the 3-month period of low or absent virus activity, identical methodology and model specification resulted in an effectiveness estimate of 21% that was not statistically different from zero (p = 0.36). The effectiveness determined during the peak period of virus circulation is felt to be a conservative estimate, since agents other than influenza are responsible for pneumonia and influenza hospitalizations, even during times of peak influenza activity.