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)

ABSTRACT 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 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 represented by a directed acyclic graph (DAG), were implemented using random-effects Poisson regression models. We also analyzed alternative vaccination schedules and obtained age-specific estimates of effectiveness. Results The cohort comprised 37.6 million person-months of follow-up and 1,384 COVID-19-attributed SARI hospitalizations, including 27 associated deaths. The 3-dose series of BNT162b2 vaccine had an effectiveness of 97% (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. No deaths occurred among children who completed the 3-dose series, whereas four deaths were observed among those with fewer doses. The effectiveness of CoronaVac was small and not statistically significant (IRR 0.96, 95%CI 0.57-1.62). However, no deaths were recorded among children who received any number of CoronaVac doses, and no COVID-19-attributed SARI hospitalizations were observed among those who received a third dose of this vaccine. Conclusions The 3-dose series of the mRNA vaccine (BNT162b2) had high and consistent effectiveness in protecting against severe COVID-19 in children aged 6 months to 4 years. These findings support the maintenance of routine COVID-19 vaccination in this age group.