A Retrospective Unicenter Study of Clinical and Inflammatory Features in Hospitalized Adults with Respiratory Syncytial Virus Infection Across Two Epidemic Waves in Catalonia, Spain

Background: Respiratory syncytial virus (RSV) is a serious disease in older adults with comorbidities; however, comparative data across epidemic waves, both clinically and in terms of inflammatory profiles and their diagnostic and prognostic utility, remain limited. Methods: We conducted a retrospective study of adults hospitalized with RSV infection across two epidemic waves (2022–2023 and 2024–2025). Clinical characteristics, comorbidities, severity scores, and outcomes were collected. Serum interleukin-6 (IL-6), C-reactive protein (CRP), and hematological parameters were analyzed and compared with healthy controls. Results: A total of 152 patients were included (81 in wave 1 and 71 in wave 2). Patients in wave 2 were older and had a higher burden of comorbidities, although ICU admission and in-hospital mortality were similar across waves. RSV induced a consistent systemic inflammatory response in both waves, characterized by elevated IL-6 and CRP levels, neutrophilia, lymphopenia, and increased neutrophil-to-lymphocyte ratio, without relevant inter-wave differences. All biomarkers demonstrated good diagnostic performance. The neutrophil-to-lymphocyte ratio, showed the highest accuracy, while IL-6 exhibited excellent rule-in capacity due to perfect specificity. However, none of the evaluated biomarkers were associated with disease severity (McCabe index) or in-hospital mortality. Conclusion: RSV infection in older adults is associated with a stable inflammatory signature across epidemic waves. Although biomarkers showed strong diagnostic utility, they lacked clinical prognostic value. We suggest that disease severity is mainly driven by host-related factors, particularly comorbidities, rather than differences in inflammatory response, highlighting the need for improved preventive and risk stratification strategies in this population.