Data on safety and immunity elicited by a third booster dose of inactivated COVID-19 vaccine in children and adolescents are scarce. Here we conducted a 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 dose 3 were mild and moderate in both cohorts. A third dose of CoronaVac increased GMTs to 681.0 (95%CI: 545.2-850.7) in cohort 1 and 745.2 (95%CI: 577.0-962.3) 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.