Respiratory viral infections in children under five during two consecutive autumn–winter seasons

Background: Respiratory viral infections are a leading cause of hospitalization in children < 5 years. Understanding viral circulation, co-infections, and disease severity is essential for prevention and clinical management. Methods: This retrospective, single-center observational study was conducted between September 2023 and April 2025, and was based on multiplex PCR viral panel results obtained from respiratory samples of children under 5 years of age referred to IRCCS Policlinico. Repeated respiratory samples from the same patient yielding identical results were considered a single episode. When multiple samples collected < 7 days apart showed alternating positive and negative results, only the first positive sample was considered. Descriptive statistics and Fisher’s exact test evaluated episode characteristics, virus circulation, and co-infections. Logistic regression analysis identified factors associated with pediatric high-dependency ward or intensive care unit admission. Results: A total of 2,512 respiratory episodes in 2,239 patients were analyzed; 1,405 (55.9%) occurred in male patients, with a median(IQR) age of 1.03 (0.26–2.17) years. Pediatric high-dependency ward admission and intensive care unit admissions were reported in 432 (21.7%) and 103 (5.2%) of 1990 episodes with available clinical data, respectively. Human rhinovirus (HRV, 31.4%) was the most frequently detected pathogen, followed by RSV-A/B (13.9%). Co-infections were detected in 598 (37.6%) samples, most commonly HRV plus enterovirus (21.4%). Compared with 2023/2024, the 2024/2025 season showed reduced circulation of RSV (18.4% vs. 13.9%, P = 0.006) and human metapneumovirus (MPV; 8.9% vs. 5.5%, P = 0.002), alongside increased influenza A/B (7.4% vs. 11.8%, P = 0.0005). Logistic regression analysis revealed that pediatric high-dependency ward or intensive care unit admission was positively associated with lower respiratory tract symptoms and with the detection of RSV A/B, HRV, MPV, human enterovirus, human parainfluenza virus, and adenovirus (P < 0.05). Conclusion: Respiratory viruses remained prevalent among symptomatic children < 5 years, with HRV predominating across seasons. A reduction in RSV circulation was observed in 2024/2025, temporally coinciding with nirsevimab introduction, even though no direct attribution can be made in the absence of individual-level immunization data. Overall, these findings support continued hospital-based virological surveillance in young children to monitor changes in viral circulation and clinical burden.