From the Pandemic to the Return of Seasonal Viruses: Evolution of Severe Respiratory Infections Caused by Coronavirus, Influenza, and Respiratory Syncytial Virus in Brazil (2019–2024)

Severe acute respiratory infections (SARI) caused by respiratory syncytial virus (RSV), Influenza, and SARS-CoV-2 represent important causes of morbidity and mortality in Brazil. The dynamics among these agents were profoundly impacted by the COVID-19 pandemic, requiring constant reassessment of surveillance and prevention strategies. This study aims to compare the epidemiological behavior of SARI caused by RSV, Influenza, and COVID-19 in Brazil between 2019 and 2024, focusing on temporal trends, most affected age groups, and lethality. A retrospective and descriptive analysis of secondary data obtained from InfoGripe/Fiocruz bulletins and records from the Brazilian Ministry of Health, based on DATASUS databases. Cases of SARI with laboratory confirmation by RT-PCR or direct immunofluorescence were included. Absolute and relative frequencies of cases, hospitalizations, and deaths were evaluated, with temporal and age-group analyses. In 2022, COVID-19 reached its peak with approximately 19.4 million cases and a high number of deaths, followed by a marked decline in 2023 and 2024. In 2024, it accounted for approximately 7% of SARI cases, with a significant reduction in lethality. RSV became the main agent among children under 2 years of age from 2023 onward, with positivity rates above 50% during peak weeks and an incidence close to 21% in 2024. RSV lethality among children under 1 year reached 1.28%. Influenza accounted for about 9% of positive cases in 2024 and was associated with up to 30% of deaths from respiratory viruses, mainly among older adults. From 2023 to 2024, RSV was concentrated in young children, while Influenza affected older age groups more frequently, in contrast to the sustained decline in COVID-19 lethality. The analysis of the period from 2019 to 2024 reveals the transition from a pandemic scenario centered on COVID-19 to a new viral profile, with the reemergence of RSV and Influenza as the main agents of SARI. RSV stands out due to its high prevalence and severity in the pediatric population, while Influenza maintains an important role in mortality among older adults. These findings reinforce the need for integrated surveillance, expansion of vaccination coverage, and incorporation of new prevention technologies, such as the RSV vaccine, in addition to rapid testing strategies, genomic surveillance, and seasonal campaigns.