inclusion criteria: * subjects aged 60 years and above; * provide legal identification; * have completed two-dose of covid-19 vaccine (vero cell), inactivated manufactured by sinovac research\& development co., ltd. for 3\~6 months（the interval between two doses was 21\~35 days）； * be able to understand and voluntarily sign the informed consent form and be willing to complete the study according to the research plan.

inclusion criteria: * subjects aged 60 years and above; * provide legal identification; * have completed two-dose of covid-19 vaccine (vero cell), inactivated manufactured by sinovac research\& development co., ltd. for 3\~6 months（the interval between two doses was 21\~35 days）； * be able to understand and voluntarily sign the informed consent form and be willing to complete the study according to the research plan.

inclusion criteria: subjects aged 60 years and above; provide legal identification; have completed two-dose of covid-19 vaccine (vero cell), inactivated manufactured by sinovac research& development co., ltd. for 3~6 months（the interval between two doses was 21~35 days）； be able to understand and voluntarily sign the informed consent form and be willing to complete the study according to the research plan.

inclusion criteria: subjects aged 60 years and above; provide legal identification; have completed two-dose of covid-19 vaccine (vero cell), inactivated manufactured by sinovac research& development co., ltd. for 3~6 months（the interval between two doses was 21~35 days）； be able to understand and voluntarily sign the informed consent form and be willing to complete the study according to the research plan.