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.