Torres, Ana RitaGaio, VâniaMelo, AryseLança, MiguelBarreto, MartaLagoa Gomes, Licínia IsabelAzevedo, InêsBandeira, TeresaLito, DavidGuiomar, RaquelRodrigues, Ana PaulaVigiRSV group2025-07-012025-07-012025-02-08J Med Virol. 2025 Feb;97(2):e70209. doi: 10.1002/jmv.702090146-6615http://hdl.handle.net/10400.18/10523During 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.engRSVAcute Respiratory InfectionChildrenCo‐infectionsEstados de Saúde e de DoençaInfecções Respiratóriasjournal article10.1002/jmv.7020939921395