Adults hospitalized with respiratory syncytial virus (RSV) face mortality risks comparable to or higher than those with influenza A or B. However, studies on the impact of bacterial co-infections on mortality are inconsistent.

This multicenter cohort study included adults hospitalized with RSV, influenza A, or B over 3 years at two tertiary care hospitals. Microbiological testing, bacterial co-infections, antibiotic use, and their association with clinical outcomes were analyzed using adjusted linear and logistic regression models.

Of 986 patients, 352 (36%) had RSV, 347 (35%) influenza A, and 287 (29%) influenza B. The median age was 74 years, 54% were women, and 76% had at least one comorbidity. Overall, 32% had pneumonia. The prevalence of bacterial co-infections was comparable across patients with RSV (23%), influenza A (25%), and B (28%). Among patients without bacterial co-infection, antibiotic use within 48 hours remained common across all virus groups (77%, 71%, and 75%, respectively). In adjusted analyses, bacterial co-infection in patients with RSV was not associated with mortality at 14, 30, or 90 days, high-flow oxygen therapy, mechanical ventilation, or length of stay (LOS). Early antibiotic treatment was associated with prolonged LOS but not improved survival.

Bacterial co-infections were identified in approximately one-quarter of patients with RSV, influenza A, and B. Among patients with RSV, bacterial co-infection was not associated with adverse clinical outcomes, and early antibiotic treatment did not appear to improve clinical outcomes.