Jara, AlejandroUndurraga Fourcade, Eduardo AndresZubizarreta, Jose R.Gonzalez, CeciliaAcevedo, JohannaPizarro, AlejandraVergara, VeronicaSoto-Marchant, MarioGilabert, RosarioFlores, Juan CarlosSuarez, PamelaLeighton, PaulinaEguiguren, PabloRios, Juan CarlosFernandez, JorgeGarcia-Escorza, HeribertoAraos Bralic, Rafael2023-02-102022-05-23Jara, Alejandro, Eduardo A Undurraga, José R Zubizarreta, Cecilia González, Johanna Acevedo, Alejandra Pizarro, Verónica Vergara, et al. 2022. “Effectiveness of CoronaVac in Children 3-5 Years of Age During the SARS-CoV-2 Omicron Outbreak in Chile.” Nature Medicine 28 (7): 1377–80.https://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37374329Abstract 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 the evidence is sparse or non-existing. Leveraging a population-based cohort of 490,694 children aged 3–5 years, we estimated the effectiveness of administering a two-dose schedule, 28 days apart, of CoronaVac using inverse probability-weighted survival regression models to estimate hazard ratios of 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%CI, 36.5–39.9) against Covid-19, 64.6% (95%CI, 49.6–75.2) against hospitalization, and 69.0% (95%CI, 18.6–88.2) to prevent intensive care unit admission. The effectiveness was modest; however, protection against severe disease remained high.en-USJournal Article2023-02-1010.21203/rs.3.rs-1440357/v1