Age-Related Differences in the Clinical Presentation of Respiratory Syncytial Virus Infection in Hospitalized Children: A Three-Year Retrospective Study

Background: Respiratory syncytial virus (RSV) is a major cause of hospitalization in infants and young children, with the highest risk of severe disease occurring during early infancy. Age may influence not only disease severity but also the clinical phenotype of RSV infection. This study aimed to evaluate age-related differences in clinical presentation and respiratory severity among children hospitalized with RSV infection. Methods: We conducted a retrospective observational cohort study including 241 children aged 0–24 months hospitalized with laboratory-confirmed RSV infection between January 2022 and December 2024. Clinical manifestations were compared across four age groups (0–3, 4–6, 7–12, and 13–24 months). Multivariable logistic regression was used to assess associations between exact age in months and individual clinical manifestations, adjusting for sex, prematurity, documented viral co-infection and feeding type. The Holm procedure was applied to account for multiple comparisons in the exact-age analyses. Supplemental oxygen requirement was evaluated as a marker of respiratory severity. Results: Fever increased significantly with age, from 48.8% among children aged 0–3 months to 81.7% among those aged 13–24 months (p < 0.001). In adjusted analyses, each additional month of age was associated with higher odds of fever (aOR 1.090, 95% CI 1.045–1.137; Holm-adjusted p < 0.001) and lower odds of thoracoabdominal asynchrony (aOR 0.951, 95% CI 0.917–0.986; Holm-adjusted p = 0.044). Associations with tachypnea/polypnea and intercostal/subcostal retractions did not remain significant after correction for multiple comparisons. Supplemental oxygen was required in 29.0% of patients. Infants aged 0–3 months required oxygen more frequently than older children (46.5% vs. 25.3%) and had significantly higher adjusted odds of oxygen requirement (aOR 2.54, 95% CI 1.26–5.0; p = 0.009). Conclusions: The clinical phenotype and respiratory severity of RSV infection may vary s with age during the first two years of life. Younger infants exhibit more prominent signs of respiratory distress despite a lower frequency of fever, and infants aged 0–3 months have a particularly high likelihood of requiring supplemental oxygen. These findings highlight the importance of age-specific clinical assessment and reinforce the vulnerability of infants during the earliest months of life.