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.

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.

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.

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.