Comparison of clinical outcomes associated with RSV and influenza infections in adults: a multicenter study from Türkiye (for AP-TR)

Objectives Respiratory syncytial virus (RSV) is an increasingly recognized cause of serious respiratory illness in adults, yet data from middle-income countries remain scarce. This study aimed to compare clinical outcomes of RSV and influenza virus infections in hospitalized and outpatient adults in Türkiye. Methods This retrospective multicenter study included 3299 adult patients (≥18 years) with polymerase chain reaction-confirmed RSV (n = 628) or influenza (n = 2671) from 21 centers across Türkiye between January 2022 and March 2024. The primary outcome was viral infection–related hospitalization. The secondary composite outcome was intensive care unit (ICU) admission and/or 30-day mortality among hospitalized patients. Propensity score–based overlap weighting was used to adjust for baseline differences including age, risk group, clinical severity, co-infection status, and study center. Results After overlap weighting, RSV infection was not significantly associated with increased hospitalization risk compared to influenza (odds ratio [OR]: 0.96, 95% confidence interval [CI]: 0.77-1.20, P = 0.705). Similarly, no significant difference was observed in the composite outcome of ICU admission or 30-day mortality (OR: 1.40, 95% CI: 0.95-2.05, P = 0.088). Age ≥60 years, high-risk comorbidities, and severe acute respiratory infection presentation were the primary independent predictors of both hospitalization and worse outcomes. Bacterial or fungal co-infections and secondary infections were observed in 12.9% and 7.3% of hospitalized patients, respectively, with no significant difference between RSV and influenza groups. Conclusion After adjustment for baseline characteristics, RSV and influenza were associated with comparable risks of hospitalization and severe clinical outcomes in adults. Host-related factors, particularly older age and underlying comorbidities, were the dominant determinants of adverse outcomes. These findings from a largely unvaccinated, middle-income country cohort highlight the need to expand RSV and influenza vaccination programs targeting high-risk adult populations. © 2026 The Author(s).