Incidence of respiratory syncytial virus and influenza: A Danish nationwide cohort study.

Respiratory syncytial virus (RSV) is, like influenza, one of the most common causes of severe acute respiratory infections (ARIs) in adults; however, its incidence across different risk and age groups is unknown. We estimated the incidence of registered ARIs in Danish adults, focusing on RSV and contextualizing with influenza. We conducted a nationwide cohort study in Denmark, including all adults aged ≥18 y with a registered ARI, during the 2011/12 to 2022/23 seasons. We estimated the incidence (per 100,000 person-years) of ARI overall and ARI attributable to RSV or influenza, overall and stratified by sex, age, and comorbidity. We identified 962,858 ARIs, of which 10,437 (1.1%) were attributed to RSV and 62,869 (6.5%) to influenza. The ARI incidence increased over time (range: 1,272-2,144), including a rise in RSV incidence (range: 0.2-91.6), while influenza incidence fluctuated (range: 6.7-322.0). In the 2022/23 season, RSV incidence was higher among females, older adults (198.7 in ≥60-y-olds, and 303.2 in ≥75-y-olds), and those with comorbidities, especially hematologic disease (868.6) and chronic obstructive pulmonary disease (679.5). Although influenza incidence was generally higher than RSV, the incidences were comparable for older adults and those with comorbidities. In conclusion, RSV is an important cause of ARIs among adults, comparable to influenza, particularly in older adults and those with comorbidities. These findings underscore the substantial burden of RSV that is relevant for public health planning and clinical decision-making, particularly given the recent approval of three highly effective RSV vaccines.