Effectiveness of COVID-19 vaccination schedules against severe COVID-19 in children aged 6 months to 4 years in Brazil: A population-based cohort study (2023-2024).
BACKGROUND Although the impact of COVID-19 vaccination is widely documented in the general population, the evidence on its effectiveness in children under 5 years of age is still limited. In this context, the continuation of vaccination programs in this age group has been debated globally. Consequently, we estimated the effectiveness of the complete 3-dose series of BNT162b2 (Pfizer-BioNTech) in children aged 6 months to 4 years and the complete 2-dose series of CoronaVac (Sinovac) in children aged 3 to 4 in reducing the risk of hospitalizations due to COVID-19-attributed severe acute respiratory infection (SARI) in Brazil. METHODS We conducted a retrospective cohort study in 24 Brazilian municipalities, using surveillance data. We evaluated vaccine effectiveness in reducing the incidence rate of COVID-19-attributed SARI hospitalizations from July 2023 to December 2024. Covariate adjustments, defined a priori based on a conceptual model, were implemented using random-effects Poisson regression models. RESULTS The cohort comprised 37.7 million person-months of follow-up and 1384 COVID-19-attributed SARI hospitalizations, including 27 associated deaths. The 3-dose series of BNT162b2 vaccine had an effectiveness of 97% (Incidence rate ratio [IRR] 0.03, 95% CI 0.01-0.10) in the group aged 6 months to 4 years, with no significant differences among age-specific estimates. The effectiveness of CoronaVac was small and not statistically significant (IRR 0.96, 95% CI 0.57-1.62). CONCLUSIONS In this population-based cohort, completion of the 3-dose BNT162b2 primary series was associated with substantially lower rates of COVID-19-attributed SARI among children aged 6 months to 4 years.