This study is an open phase 4 clinical trial of an inactivated COVID-19 vaccine (CoronaVac) manufactured by Sinovac Research and Development Co., Ltd. The purpose of this study is to evaluate the immunogenicity of the CoronaVac in healthy children aged 3-11 years old.

This study is an open phase 4 clinical trial to evaluate the immunogenicity of an inactivated COVID-19 vaccine(CoronaVac)in healthy children aged 3-11 years old.A total of 400 healthy subjects who have received two doses of inactivated COVID-19 vaccine(CoronaVac) manufactured by Sinovac Research & Development Co., Ltd and is currently 28-42 days after the second dose will be enrolled, including 200 subjects aged 3-5 years and 200 subjects aged 6-11 years old.All of subjects will be collected 3ml venous blood to evaluate the immunogenicity of the CoronaVac.

Inclusion Criteria:

• Have received two doses of inactivated COVID-19 vaccine(CoronaVac) manufactured by Sinovac Research & Development Co., Ltd and is currently 28-42 days after the second dose;
• The subjects and/or guardians can understand and voluntarily sign the informed consent form (for subjects aged 8 years, both subjects and guardians need to sign the informed consent form);
• The subjects and their legal guardians voluntarily participate the study and comply with the study procedure to collect 3ml venous blood;
• Proven legal identity.

Exclusion Criteria:

• Haven't received two doses of inactivated COVID-19 vaccine(CoronaVac);
• Have received COVID-19 vaccines from other manufacturers;
• The interval between blood collection and the second dose is less than 28 days or more than 42 days;
• The subjets and their legal guardians can't cooperate to complete 3ml venous blood collection.
• According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial.