RSV‐Bacterial Co‐Infection Is Associated With Increased Illness Severity in Hospitalized Children ‐ Results From a Prospective Sentinel Surveillance Study

During the autumn/winter respiratory syncytial virus (RSV) epidemics, bacterial co-infection is common and affects the disease severity. We aimed to understand the relationship between RSV-bacterial co-infections and clinical severity since the RSV seasonality change after COVID-19 pandemic. We conducted a prospective, sentinel surveillance study at 20 sites in Portugal in children under 2 years hospitalized with RSV, between April 21 and January 23. Effect of co-infection with potentially pathogenic bacteria (PPB) on the length of hospitalization and disease severity was investigated using multivariate linear and log-binomial regression models. Among 678 RSV hospitalizations, 67.4% occurred in children under 6 months and 15.3% in preterm; 20.4% tested positive for PPB; median length of hospitalization was 5 days (IQR: 3-7days). Children coinfected with PPB had a higher rate of ICU admission (29.7% vs. 3.5%, p < 0.001), resulting in more prolonged hospitalizations (7 vs. 5 days, p < 0.001) and a 13-fold risk of having severe disease (RR: 13.2, 95% CI:7.3-23.9). RSV-bacterial co-infection was associated with increased length of hospitalization and severe illness during off-season epidemics. This risk is probably overestimated, as laboratory testing for bacterial infections is usually higher in severely ill-appearing children. Measures to prevent outgrowth of pathogenic bacteria within the respiratory tract should be discussed.