RSV causes more severe respiratory illness than influenza in admitted children under 2-years-old.

INTRODUCTION: Both respiratory syncytial virus (RSV) and influenza-associated lower respiratory tract infections (LRTI) cause serious respiratory infections in infants and toddlers. We aimed to assess the frequency of complex hospital courses among patients admitted with influenza versus RSV LRTI.

METHODS: A retrospective cohort study was performed on admissions of children <2 years who were admitted for LRTI and tested positive for influenza or RSV from 2016 to 2019. The primary outcome, complex hospital course, was a composite including: intensive care unit admission, respiratory support, nasogastric tube feeds, prolonged length of stay, and death. Secondary outcomes included 7-day readmission and time to respiratory support. Differences between RSV and influenza groups were assessed and unadjusted and adjusted regression models and competing risks time to event models were developed.

RESULTS: There were 1094 (89%) RSV admissions and 134 (11%) influenza admissions. Children admitted for influenza were significantly older (336 vs. 165 days, p < 0.001), more likely to present with an abnormal heart rate for age (84.3% vs. 73.5%, p < 0.01) and a fever (27.6% vs. 18.9%, p = 0.02). Admissions with RSV were significantly more likely to experience a complex hospital course (ORadj  = 3.5, 95% CI: 2.2-5.6). In time to event analysis, RSV admissions had a significantly higher rate of respiratory support (HRadj  = 3.2, 95% CI: 2.0, 5.2). Readmission rates were similar.

CONCLUSIONS: RSV admissions were associated with a higher risk for a complex hospital course and required higher rates of respiratory support than influenza admissions. This information may help in evaluating hospital resources and admissions.