Sinovac Biotech Ltd. announced The Lancet Infectious Diseases published results from the Phase I/II clinical trials of an inactivated COVID-19 vaccine developed by SINOVAC, also known as CoronaVac®, in healthy children and adolescents aged 3-17 in China. The double-blind, randomized, placebo-controlled study evaluated the safety, tolerability and immunogenicity of CoronaVac in pediatric populations. The findings indicate that, after receiving 2 doses of the vaccine in a 28-day interval, CoronaVac was safe, well tolerated and induced a strong antibody response in children and adolescents aged 3-17 years. This is the world's first published study on the use of a COVID-19 vaccine for children as young as 3 years old. Prior to this, China had approved the emergency use of CoronaVac in the 3-17 age group on June 3, 2021. The study enrolled 552 participants, comprised of healthy children and adolescents aged 3-17. From October 31 to December 2, 2020, 72 were enrolled in the phase I clinical study. Between December 12 and December 30, 2020, 480 were enrolled in the phase II clinical study. The participants received intramuscular injections of the vaccine candidate in a two-dose interval on days 0 and 28 (with low-dose of 1.5µg/dose or medium-dose of 3µG /dose) or the control vaccine (aluminum hydroxide diluent). The combined safety results from Phase I/II showed that, among 550 subjects who received at least one dose of the vaccine candidate or placebo, the incidence of adverse reactions within 28 days after the injection was 26% (56 /219) in the low-dose group, 29% (63 /217) in the medium-dose group and 24% (27 /114) in the control group, with no significant difference among the groups. Most adverse reactions were mild to moderate in severity, recorded within 7 days after each dose. The most common reaction was injection site pain (13%). The results of immunogenic evaluation found, in Phase I, 28 days after receiving the second dose of low- or medium-dose vaccine, a positive seroconversion rate of neutralizing antibody was observed in 100% of participants and increased geometric mean titer (GMT) of neutralizing antibody of 55.0 and 117.4 in children and adolescents, respectively. The immune response in the medium-dose group was stronger than that in the low-dose group. In Phase II, 28 days after full vaccination, seroconversion was observed in 96.8% of participants in the low-dose group and 100% in the medium-dose group with GMT of 86.4 and 142.2, respectively. The immune response of the medium-dose group was still higher than that of the low-dose group, and there were no detectable antibody responses in the control group. In addition, the study found that children and adolescents had stronger immune responses compared to adults aged 18-59 and those aged 60 and older, following two doses of the medium-dose vaccine. The study results indicate a decrease in vaccine response with increase in age, which has been observed in other vaccine candidates as well.