Ten-Year Surveillance of Respiratory Syncytial Virus Hospitalizations in Adults: Incidence Rates and Case Definition Implications

BACKGROUND: The impact of respiratory syncytial virus (RSV) in older adults is underrecognized, and the limited existing studies on the incidence of hospitalizations show great variability. This study aims to estimate the seasonal incidence rates (IRs) of RSV hospitalizations among adults aged ≥60 years and evaluate how different case definitions influence these estimates. METHODS: A prospective, multicenter observational study with active monitoring was conducted over 10 seasons (2010-2011 to 2019-2020) in 4-10 hospitals (depending on the season) and covered 21%-46% of the region's total population (about 5 million people). RSV hospitalization IRs per 100 000 person-years and 95% confidence intervals were calculated with the exact Poisson method and were stratified by age group (≥60, ≥70, or ≥80 years), RSV season, sex, and the entire study period. Two case definitions were compared: influenzalike illness (ILI) and the combined use of ILI and extended severe acute respiratory infection (ILI/SARI). RESULTS: A total of 40 600 hospitalizations of individuals aged ≥60 years were included. The RSV hospitalization IRs ranged from 21 to 402 per 100 000 person-years, varying by season, age group, and case definition. The highest IRs were observed in those aged ≥80 years. The ILI case definition underestimated RSV hospitalizations by 13%-40% when compared with the ILI/SARI case definition. CONCLUSIONS: On average, approximately 1 in every 1000 adults aged ≥60 years is hospitalized due to RSV. The risk of a severe RSV infection increases with age and varies significantly between seasons. These are key results for the estimation of the potential impact of the new available RSV vaccines.