Seasonal Incidence of Medically Attended Respiratory Syncytial Virus Infection From 2015 to 2019 in a Cohort of Adults With High-risk Conditions.

Background: Adults with high-risk conditions (underlying health conditions that increase risk of severe outcomes after respiratory infection) may have a substantial incidence of respiratory syncytial virus (RSV), but existing information on this topic is limited. We assessed the seasonal incidence of RSV in adults with high-risk conditions in a Wisconsin community.

Methods: We conducted a retrospective study using data and respiratory specimens from participants with medically attended acute respiratory illness (MAARI) in a test-negative study of influenza vaccine effectiveness. We included individuals ≥18 years old in 2015-16 through 2019-20 seasons, with ≥1 high-risk condition. Residual respiratory specimens were retested for RSV using a multiplex viral panel. We calculated seasonal incidence using Poisson regression and population weighting, with the sum of observed and extrapolated RSV cases in the study cohort divided by the number of adults with high-risk conditions in the underlying source population.

Results: Of 3601 respiratory samples tested, 97% were White and 66% were female. The mean (standard deviation) age of participants was 53 (19) years. We identified 303 RSV infections; 40% were RSV A. Estimated incidence of RSV-related MAARI was 94.1 (79.5-111.5) per 10 000 high-risk adults across all seasons and varied by season. Age-specific incidence per 10 000 was 69.3 (95% confidence interval [CI], 52.4-91.7) for those 18-49 years; 131.6 (95% CI, 92.3-187.6) for those 50-59 years; 109.9 (95% CI, 80.2-150.6) for those 60-74 years; and 150.5 (95% CI, 100.8-224.6) for those ≥75 years.

Conclusions: Overall, these findings suggest a substantial incidence of RSV-related MAARI in adults with high-risk conditions.