Participants use contact cards and phone to investigators to report their AEs. study based on a double-blind, randomised, placebo-controlled phase 2 clinical trial (NCT04551547) to assess the safety and immunogenicity of a third dose of CoronaVac. In this study, 384 participants in the vaccine group were assigned to two cohorts. One received the third dose at a 10-months interval (cohort 1) and the other one at a 12-months interval (cohort 2). The primary endpoint is safety and immunogenicity following a third dose of CoronaVac. The secondary endpoint is antibody persistence following the primary two-dose schedule. Severities of local and systemic adverse reactions reported within 28 days after PF 750 dose 3 were mild and moderate in both cohorts. A third dose of CoronaVac increased GMTs to 681.0 (95%CI: 545.2C850.7) in cohort 1 and 745.2 (95%CI: 577.0C962.3) in cohort 2. Seropositivity rates against the prototype were 100% on day 28 after dose 3. Seropositivity rates against the Omicron variant were 90.6% (cohort 1) and 91.5% (cohort 2). A homologous booster dose of CoronaVac is safe and induces a significant neutralising antibody levels increase in children and adolescents. Subject terms: Randomized controlled trials, Epidemiology, SARS-CoV-2, Inactivated vaccines Few countries have approved SARS-CoV-2 booster doses in children and adolescents due to insufficient evidence about the safety and interval vaccination. Here, the authors assess the safety and immunogenicity of a homologous booster dose of CoronaVac in a cohort of 3C17 year olds. PF 750 Introduction On November 24, 2021, the novel SARS-CoV-2 Omicron (B.1.1.529) variant was first reported to WHO from South Africa1, and it has rapidly replaced the highly transmissible Delta (B.1.617.2) variant as the predominant variant worldwide2. The significant immune escape of the Omicron variant in convalescent patients infected with the prototype SARS-CoV-2 and other variants of concern (VOC) was reported3C5. The levels of neutralising antibodies against the Omicron are low and only short-lived after two-dose primary vaccination, while are enhanced with a third dose booster dose in adults6C10. Studies also found that three doses of inactivated COVID-19 vaccine could induce the cross-neutralising potency against the Omicron variant in adults11,12. In our previous study, two doses of inactivated COVID-19 vaccine (CoronaVac, Sinovac Life Sciences Co., Ltd) induced higher neutralising antibody concentrations in children and adolescents aged 3C17 years compared with the adults13C15. At present, individuals 12 years of age and older are eligible for a single booster dose of the Pfizer-BioNTech COVID-19 Vaccine, Bivalent if it has been at least two months since they have completed primary vaccination16. However, Data on immunity and safety elicited by a third booster dose of PF 750 inactivated COVID-19 vaccine in children and IGF2 adolescents are scarce. Few countries approve of booster doses in this PF 750 population. In our previous study, antibody levels in children were higher than those in adults and the elderly at 6C8 months after a primary schedule17. Due to the waning antibody levels over time, whether giving a booster with 10C12 months interval is reasonable in children and adolescents should be evaluated. It is unclear whether the immunity and safety elicited by a booster dose of inactivated COVID-19 vaccine are maintained for the Omicron variant in this population. To fill this knowledge gap, we assessed the immune persistence after primary immunisation with CoronaVac, and the safety, immunogenicity, especially the cross-neutralising activity against the Omicron variant after a third dose of CoronaVac in children and adolescents aged 3C17 years. Results Between December 12 and December 20, 2020, 515 individuals were screened, 480 participants were enroled in phase 2 trial, of whom 96 were randomly allocated to the placebo group, and 384 were randomly allocated to the 1.5?g or 3.0?g vaccine group. All 96 participants in placebo groups withdrew from the study and received COVID-19 vaccines since the inactivated COVID-19 vaccine was approved for emergency use among children and adolescents aged 3C17 years18. Of these 384 participants, 192 (50%) participants were allocated to cohort 1, and 192 (50%) participants were allocated to cohort 2. In cohort 1, 180 (94%) participants completed blood sampling at 3 months, and 171 (89%) participants completed blood sampling at 10 months after the second dose. In cohort 2, 185 (96%) participants completed blood sampling at 6 months, and 175 (91%) participants completed blood sampling at 12 months after the second dose. 171 participants in cohort 1 (89% of the 192 participants eligible.