Viral etiology and predictors of hospitalization in adults younger than 65 years with laboratory-confirmed respiratory viral infections in the pre-COVID era

Respiratory viral tract infections (VRTIs) are generally considered more severe in older adults, while their clinical impact in younger adults remains less well characterized. We conducted a multicenter retrospective cohort study of adults aged 18–64 years with RT-PCR-confirmed influenza, respiratory syncytial virus (RSV), human metapneumovirus (hMPV), or parainfluenza virus (PIV) infection during the 2016–2018 respiratory seasons. Among 172 patients, 110 had influenza, 31 RSV, 17 hMPV, and 14 PIV infections. Hospitalization occurred in 64.0% of patients overall, with rates of 53.6% for influenza, 77.4% for RSV, and 87.1% for hMPV/PIV infections. Chronic respiratory disease, neutrophilia, and hMPV/PIV infection were independently associated with hospitalization. Chronic respiratory disease and previous respiratory tract infection were independently associated with lower odds of short-term favorable outcome, whereas detectable eosinophils were associated with improved recovery. Ninety-day mortality was low across viral groups. Age was not independently associated with hospitalization or short-term recovery within this restricted age range. These findings highlight substantial morbidity associated with VRTIs in adults younger than 65 years and underscore the need for greater attention to this population, which remains less well characterized than older adults.