Post-COVID-19 Seasonality of Influenza, Respiratory Syncytial Virus, and SARS-CoV-2 Among Hospitalized Children in Western Iran: A Molecular Surveillance Study (2023–2024)

Background This study aimed to characterize the incidence, seasonality, and co-infection patterns of respiratory syncytial virus (RSV), influenza A and B, and SARS-CoV-2 among hospitalized children aged 0–5 years in Hamedan Province, a semi-arid region in western Iran, from April 2023 to March 2024. Key research questions included assessing post-pandemic shifts in viral seasonality, evaluating the extent of RSV circulation, and determining the frequency of co-infections in a resource-limited pediatric setting where regional data remain scarce. Methods A total of 586 nasopharyngeal/oropharyngeal samples were collected from children aged 0–5 years hospitalized with acute respiratory symptoms (≥ 2 of: fever ≥ 38 °C, cough, dyspnea, oxygen saturation < 95%). Multiplex real-time PCR (sensitivity 95%, specificity 98%) was used to detect RSV, SARS-CoV-2, and influenza A (H1N1, H3N2) and B. Statistical analysis included chi-square and Fisher’s exact tests, and generalized linear models (binomial distribution, logit link). Results Among 586 inpatients (mean age: 2.8 years; 62.5% male), 27.0% tested positive for influenza (60% influenza A [35% H1N1, 25% H3N2], 40% influenza B), 6.0% for RSV, and 6.3% for SARS-CoV-2. Influenza peaked in autumn (41.3%, p < 0.001), RSV in winter (18.2%, p < 0.001), and SARS-CoV-2 in spring (15.3%, p = 0.005). Co-infections were rare (0.9%). Conclusions Findings reveal altered post-pandemic seasonality, reduced RSV activity, and low co-infection rates, suggesting potential ecological and immunological shifts. These trends highlight the need for sustained virus-specific surveillance and recalibrated vaccination strategies—particularly influenza vaccination in autumn and RSV prophylaxis in winter—in resource-limited pediatric settings.