Seasonal dynamics and burden of respiratory syncytial virus, influenza, and COVID-19 in Brazil: a national surveillance study (2019-2023).

BACKGROUND: The burden and seasonal patterns of respiratory syncytial virus (RSV), influenza, and COVID-19 remain poorly characterised in Brazil and other Latin American countries. Understanding their epidemiological patterns is essential to inform public health interventions and preventive strategies. We aimed to assess respiratory virus seasonality using national surveillance data to guide vaccination timing and strengthen preparedness strategies in Brazil. METHODS: We conducted a retrospective, observational, descriptive population-based study using national surveillance data collected through the SIVEP system between 2019 and 2023. We analysed confirmed cases, hospitalisations, and deaths attributed to RSV, influenza, and COVID-19. We performed time-series decomposition and seasonal trend analysis using the multiple seasonal-trend decomposition via locally estimated scatterplot smoothing method. We calculated monthly mean values for each aetiology component to assess their seasonal burden. RESULTS: There were 69,389 RSV, 45,881 influenza, and 2,299,712 COVID-19 cases among the laboratory-confirmed severe acute respiratory infection (SARI) cases reported in SIVEP. Each year, more than 90% of RSV diagnoses were reported in children aged 0-10 years, while influenza and COVID-19 were more common among older adults. The proportion of deaths in children related to RSV increased from 19.2% in 2020 to 62.0% in 2023. Seasonal peaks occurred from March to June for RSV, influenza cases peaked in January, while COVID-19 showed less consistent seasonality. Geographic variation was observed, with higher case concentrations in the Southeast and South regions of Brazil. CONCLUSIONS: The burden of respiratory viruses among SARI cases reported in the national surveillance system in Brazil differed by pathogen, age group, region, and season, and may be influenced by variations in testing strategies. Strengthening diagnostic capacity and surveillance is critical, particularly for adult RSV cases, which are likely underreported due to limited testing and reduced diagnostic sensitivity. Seasonal vaccination programmes, such as those for influenza, remain essential and may be further enhanced by integrating new preventive measures, such as RSV vaccination for older adults and maternal immunisation to protect infants.