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

Abstract

The outbreak of the B.1.1.529 lineage of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (Omicron) has caused an unprecedented number of Coronavirus Disease 2019 (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 6 December 2021 and 26 February 2022, during the Omicron outbreak in Chile. The estimated vaccine effectiveness was 38.2% (95% confidence interval (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 aged 3-5 years to prevent severe illness and associated complications and highlight the importance of maintaining layered protections against SARS-CoV-2 infection.

Fig. 1. Study participants and cohort eligibility, 6 December 2021 through 26 February 2022.

Participants were 3–5 years of age, affiliated to the FONASA, the public national healthcare system, and received two doses of CoronaVac, 28 days apart between 6 December 2021 and 26 February 2022 or did not receive any COVID-19 vaccination. We excluded children who had probable or confirmed COVID-19 according to RT–PCR assay for SARS-CoV-2 or antigen test before 6 December 2021.

Extended Data Fig. 1. CoronaVac vaccination rollout among children aged 3 to 5 years, by vaccination group.

unvaccinated, vaccinated with one dose, vaccinated with two doses after 28 days. The Public Health Institute of Chile extended the authorization for emergency use of CoronaVac to children starting at three years of age on November 25, 2021. The first children aged 3-5 years were vaccinated on December 6, 2021, prioritizing immunocompromised children and those with comorbidities. The median date of first and second dose for all children in the cohort were 14 and 50 days from the beginning of the follow-up respectively.

Extended Data Fig. 2. Evolution of the predominant SARS- CoV-2 lineages in Chile, according to data shared on GISAID platform, December 22, 2020, to February 24, 2022.

The Ministry of Health monitors respiratory viruses, including SARS- CoV-2, using genomic surveillance in sentinel centers. Surveillance uses non-probabilistic sampling of SARS-CoV-2 infections focusing on variants of concern (VOC) and variants of interest (VOI) through traveler (imported cases) and community surveillance (hospitalized cases and national core priority studies). The samples are sent for whole-genome sequencing (WGS) and genotyping across the country. Between December 22, 2020, and February 21, 2022, 70,186 SARS-CoV-2 samples were analyzed. Of these, 28.6% (n=20,088) were sequenced and 71.4% (n=50,098) assessed by detection of variant-associated mutations (VAM) using RT- PCR. Of these analyzed samples, 85.3% (n=59,891) correspond to VOC and 5.2% (n=3,618) to variants of interest (VOI).

Extended Data Fig. 3. Extended Fig.3 Estimated cumulative incidence of (a) symptomatic COVID- 19, (b) hospitalization, and (c) admission to incentive care unit (ICU) for unvaccinated and fully immunized individuals.

Comparison of the cumulative incidence curves between unvaccinated and fully immunized children (≥ 14 days after receiving the second dose of the CoronaVac COVID-19 vaccine) on January 1, 2022. The estimates are presented as the mean values, with 95% point-wise confidence intervals, for boys, aged 4, affiliated to FONASA insurance type A, and not having comorbidities.