To evaluate the immunogenicity and safety of the third dose SARS-CoV-2 Vaccine, Inactivated (Vero Cell) in adults aged 18 years and above, who inoculated the third dose after 3, 4, 5, or 6 months since finished two doses schedule of CoronaVac or BBIBP-CorV.

Inclusion Criteria:

1. Axillary temperature ≤37.0 ℃.
2. aged 18 years and above, after 3, 4, 5, or 6 months since finished two doses schedule of CoronaVac or BBIBP-CorV, or one does CoronaVac and one does BBIBP-CorV.
3. Proven legal identity, could come each visit.
4. Participants should understand the contents of the informed consent form, the vaccine in this trial, voluntarily sign the informed consent form.

Exclusion Criteria:

1. Persons with a clear history of SARS-CoV-2 infection.
2. Using blood products after basic immunization or receiving immunosuppressive therapy.
3. Participants who have a positive pregnancy test, or are breastfeeding, or plan to become pregnant, or plan to donate sperm or eggs from the screening to 6 months after vaccination.
4. Allergic to the active substance in the vaccine, any inactive substance or substance used in the preparation process (aluminum hydroxide, glycine);
5. History of abnormal clinical manifestations and serious diseases to be excluded, including but not limited to nervous system, cardiovascular system, blood and lymphatic system, immune system, kidney, liver, gastrointestinal tract, respiratory system, metabolism, bones and other system diseases, and a history of malignant tumors.
6. Diseases beyond drug control, such as high blood pressure, diabetes, asthma.
7. Those who developed acute disease within 2 weeks, or had symptoms of fever or upper respiratory tract infection within 7 days.
8. Immunization with any vaccine within 14 days.
9. Any other situations judged by investigators as not suitable for participating in this study.