Determinants of Severity in Pediatric Viral Lower Respiratory Tract Diseases From Emergency Department Presentation to Hospital Course

BACKGROUND: Predicting severity in pediatric lower respiratory tract diseases (LRTDs) remains challenging, and the role of viral coinfection is debated. We hypothesized that determinants of severity differ between emergency department (ED) presentation and in-hospital course, with viral coinfection acting as a virus-specific effect modifier rather than a uniform risk factor. METHODS: This was a retrospective cohort of 1242 children <6 years hospitalized for viral LRTDs during 2022-2023 and 2023-2024. Severity at ED presentation was defined as a Pediatric Respiratory Severity Score (PRESS ≥4), and in-hospital deterioration as a Clinical Severity Score (CSS >3). Multivariable logistic regression assessed independent predictors, including virus × coinfection interactions. RESULTS: Severe ED presentation occurred in 12.0% (n = 150), and severe hospitalization in 6.6% (n = 82). Infants 0-6 months accounted for 76.8% of severe hospitalizations (P = 0.003), while those 7-12 months had higher severe ED rates (20.0% vs. 9.5%, P < 0.001). Presentation ≥5 days after symptom onset was associated with lower odds of both outcomes (ED: adjusted odds ratio [aOR] 0.60, 95% confidence interval [CI]: 0.30-0.98; hospitalization: aOR 0.36, 95% CI: 0.16-0.76). Influenza predicted lower odds of severe ED presentation (aOR 0.34, 95% CI: 0.12-0.97) but higher odds of severe hospitalization (aOR 2.96, 95% CI: 1.37-6.40), amplified by coinfection (aOR 4.44, 95% CI: 1.91-10.36). Rhinovirus coinfection increased severe ED risk (aOR 1.59, 95% CI: 1.06-2.39). CONCLUSIONS: Severity determinants in pediatric viral LRTDs are dynamic and stage-specific. Viral coinfection acts as a context-dependent effect modifier rather than a uniform predictor. Recognizing these patterns enables precise, time-sensitive risk stratification.