Immunogenicity of an Inactivated COVID-19 Vaccine

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.