Respiratory syncytial virus as a major driver of SARS in Brazilian children under five years of age: a national surveillance study, 2022-2023.

Objective: To describe the burden and characteristics of respiratory syncytial virus (RSV) infections among children under five years of age in Brazil during the post-COVID-19 period by using national surveillance data. Methods: This was a retrospective study of reported SARS cases in the Brazilian National Epidemiological Surveillance Information System for the 2022-2023 period. RSV cases were confirmed by antigen or polymerase chain reaction testing and compared with SARS cases of other defined or undefined etiologies. Data were analyzed by age, sex, region of the country, yearly quarter, hospitalization status, and ICU admission. The proportions of RSV-positive cases among total SARS cases and among those with a confirmed etiology were calculated, along with notification rates by age group per 100,000 population. Results: Of a total of 244,345 SARS cases in children under five years of age, 16.9% were RSV-positive, whereas 60.5% had an undefined etiology. Among SARS cases with a confirmed etiology, RSV accounted for 42.8%, with the highest proportions being observed in infants under six months of age. ICU admission was more common in younger infants (47% in those < 1 month of age). Mortality was low (≈1%), being highest among infants under six months of age. Seasonal peaks occurred mainly between April and May, varying across regions. Conclusions: RSV remains a major cause of severe respiratory infection and hospitalization among young children in Brazil, particularly infants under six months of age. Strengthening RSV surveillance and expanding preventive strategies-including maternal vaccination and monoclonal antibody use-are key to reducing its impact.