Effectiveness of CoronaVac in children 3 to 5 years during the SARS-CoV-2 Omicron outbreak in Chile

The outbreak of the B.1.1.529 lineage of SARS-CoV-2 (Omicron) has caused an unprecedented number of COVID-19 cases, including pediatric hospital admissions. Policymakers urgently need evidence of vaccine effectiveness in children to balance the costs and benefits of vaccination campaigns, but to date, the evidence is sparse. Leveraging a population-based cohort in Chile of 490,694 children aged 3-5 years, we estimated the effectiveness of administering a two-dose schedule, 28 days apart, of Sinovac's inactivated SARS-CoV-2 vaccine (CoronaVac). We used inverse probability-weighted survival regression models to estimate hazard ratios of symptomatic COVID-19, hospitalization, and admission to an intensive care unit (ICU) for children with complete immunization over non-vaccination, accounting for time-varying vaccination exposure and relevant confounders. The study was conducted between December 6, 2021, and February 26, 2022, during the Omicron outbreak in Chile. The estimated vaccine effectiveness was 38.2% (95% confidence intervals, CI, 36.5-39.9) against symptomatic COVID-19, 64.6% (95%CI, 49.6-75.2) against hospitalization, and 69.0% (95%CI, 18.6-88.2) against ICU admission. The effectiveness against symptomatic COVID-19 was modest. However, protection against severe disease was high. These results support vaccination of children 3-5 years to prevent severe illness and associated complications and highlight the importance of maintaining layered protections against SARS-CoV-2 infection.