This study investigated the effectiveness of the CoronaVac vaccine against symptomatic COVID-19 infection and hospitalization in Brazilian children aged 6–11 years during the Omicron period. Using a test-negative design with data from 197,958 children, the study found that vaccine effectiveness against symptomatic infection was 39.8% (95% CI 33.7–45.4) at ≥14 days post-second dose, and 59.2% (95% CI 11.3–84.5) against hospitalization at ≥14 days post-second dose. The findings suggest that while CoronaVac offered some protection, it provided low levels of protection against symptomatic infection and modest levels against severe illness in children during the Omicron variant's dominance.

Pilar T. V. Florentino, Flávia J. O. Alves, Thiago Cerqueira-Silva, Vinicius de Araújo Oliveira, Juarey B. S. Júnior, Adelson G. Jantsch, Gerson O. Penna, Viviane Boaventura, Guilherme L. Werneck, Laura C. Rodrigues, Neil Pearce, Manoel Barral-Netto, Maurício L. Barreto, Enny S. Paixão