Clinical Outcomes of Respiratory Syncytial Virus Versus Influenza Pneumonia in Critically Ill Adults

Background: Respiratory syncytial virus (RSV) and influenza can both cause severe viral pneumonia requiring intensive care. Influenza is supported by established vaccination programs and antiviral therapy, whereas outcomes of RSV infection among critically ill adults remain less well characterized. This study compared clinical presentation, treatment, and outcomes between ICU admissions classified as RSV pneumonia and those classified as influenza pneumonia. Methods: A retrospective cohort was assembled from MIMIC-IV data covering 2008–2019. The analytical cohort comprised 201 infection-specific ICU observations classified as RSV pneumonia (n = 40) or influenza pneumonia (n = 161). Demographic characteristics, clinical measurements, treatments, and hospital outcomes were compared. Multivariable logistic regression was used to examine the association between infection type and in-hospital mortality, with adjustment for age, sex, and serum lactate. Internal model validation used 300 bootstrap resamples. Results: The RSV group was older than the influenza group (66.7 vs. 59.1 years; p = 0.009), had a longer ICU stay (median 3.0 [IQR 2.0–7.0] vs. 2.0 [IQR 1.0–4.0] days; p = 0.003), and had a higher admission heart rate (p = 0.009). Oseltamivir was recorded in 89.4% of influenza observations and 2.5% of RSV observations (p < 0.001). In-hospital mortality was 30.0% in the RSV group and 11.8% in the influenza group (p = 0.0091). In the primary adjusted model, RSV pneumonia was associated with higher odds of in-hospital death (adjusted OR 6.03, 95% CI 2.08–17.49; p = 0.001), as was increasing serum lactate (adjusted OR 1.46, 95% CI 1.24–1.74; p < 0.001). The apparent AUC was 0.864, and the optimism-corrected AUC after bootstrap validation was 0.845. Sensitivity analyses indicated that the magnitude of the RSV association varied with covariate adjustment. Conclusions: RSV pneumonia was associated with higher in-hospital mortality and a longer ICU stay than influenza pneumonia in this ICU cohort. The mortality association remained after adjustment for the variables included in the primary model, although its magnitude was sensitive to model specification. Given the observational design, these findings should not be interpreted as causal. They underscore the clinical burden of severe RSV infection in adults and support further investigation in larger cohorts with standardized virological testing and measures of acute illness severity.