Morbidity and mortality of respiratory syncytial virus versus influenza virus infections in hospitalized older adults: an adjusted comparative analysis.

Seasonal acute respiratory infections represent a growing public health challenge in older adults. Despite the historical relevance of influenza virus respiratory syncytial virus (RSV) infections are a growing concern due to their underestimated impact and historical limitations in epidemiological surveillance among adults. The objective of this retrospective observational study, conducted at the Hospital Universitario de Fuenlabrada (June 2023-June 2025), was to compare the morbidity and mortality of confirmed RSV and influenza infections in 1,073 hospitalized patients aged 65 years and older. Acute complications and length of hospital stay were analyzed using logistic and Poisson regression models, adjusting for age, chronic pulmonary disease, and cardiovascular history or atrial fibrillation. Multivariate analysis confirmed RSV as an independent risk factor for the development of pneumonia (aOR: 3.30; 95% CI: 2.30-4.75; p < 0.001) and acute respiratory failure (aOR: 1.58; 95% CI: 1.17-2.13; p = 0.003). Although the RSV group presented longer unadjusted hospital stays (6 vs. 5 days; p = 0.004), this difference was no significant after adjustment (IRR: 1.03; p = 0.668). There were no significant differences in infections associated mortality (3.9% in both groups). In conclusion, RSV presented a higher clinical severity than influenza. These findings support systematic RSV immunization in older adults, especially those with chronic cardiovascular profiles, to prevent the cascade of acute respiratory decompensations.