Objective  To evaluate the safety of an inactivated coronavirus disease 2019 (COVID-19) vaccine in healthy children and adolescents aged 3 – 17 years.

  Methods  Totally 500 healthy children and adolescents aged 3 – 17 years were recruited in Zanhuang county of Hebei province and randomly assigned into a vaccination group (n = 375) and a placebo group (n = 125) during May 2021. Two doses of inactivated COVID-19 vaccine (Sinovac Life Sciences Co., Ltd, Beijing) or placebo (day 0 and day 28) were administered to the participants. The participants were observed for adverse reactions within 30 minutes after each vaccination. The participants′ information on local and systemic solicited adverse events from day 0 to day 7, and unsolicited adverse events from day 0 to day 28 were collected after the vaccination. Serious adverse events were recorded from the beginning of vaccination to 6 months after the second dose vaccination to evaluate the safety of the vaccine.

  Results  The incidence of adverse reactions was 19.2% (72/375) in the vaccination group and 15.2% (19/125) in the placebo group, without significant difference between the incidence of the two groups (P > 0.05). The most common adverse reactions were injection site pain and fever. There was no significant statistical difference in the incidence of other adverse reactions between the two groups except for pain at the injection site. Most observed adverse reactions were grade 1 and 2 in severity, only 3 participants reported grade 3 adverse reactions. The vaccination group′s incidences of adverse reactions of grade 1 (14.93% vs. 12.8%), grade 2 (7.73% vs. 4%), and grade 3 (0.53% vs. 0.8%) were not significantly different from those of the placebo group. No serious vaccination-related adverse event was reported in either group. The incidence of adverse reactions was higher (28%) in the 3 – 5 years old participants than those in the participants aged 6 – 11 and 12 – 17 years (16% and 15.5%) and the age-group-specific adverse reaction incidences were not significantly different between vaccination group and placebo group. The incidence of adverse reactions for the first dose vaccination was significantly higher than that for the second dose (14.6% vs. 5.69%, P < 0.05) and there was no significant difference in the dose order-specific adverse reaction incidence between the vaccination group and the placebo group.

  Conclusion  The inactivated COVID-19 vaccine is of good safety when administered in 3 to 17 years old healthy children.