Clinical Trial of Inactivated SARS-CoV-2 Vaccine for Prevention of COVID-19 in Healthy Adults

Inclusion Criteria: * Healthy adults aged 18-59; * Have been received two doses of inactivated SARS-CoV-2 vaccine(CoronaVac)manufactured by Sinovac Research \& Development Co., Ltd at an interval of 21 to 31 days for group A.Have been vaccination two doses of inactivated SARS-CoV-2 vaccine(CoronaVac)manufactured by Sinovac Research \& Development Co., Ltd for group B; * The subjects can understand and voluntarily sign the informed consent form; * Proven legal identity. Exclusion Criteria: * History of SARS-CoV-2 infection; * Any serious adverse reactions that are causally related to vaccination during the primary immunization of the previous inactivated SARS-CoV-2 vaccine; * Severe allergic reactions after primary immunization (including urticaria/rash within 30 minutes after vaccination); * Autoimmune disease or immunodeficiency / immunosuppression; * Severe chronic diseases, severe cardiovascular diseases, hypertension and diabetes that cannot be controlled by drugs, liver or kidney diseases, malignant tumors, etc; * Severe neurological disease (epilepsy, convulsions or convulsions) or mental illness; * Immunosuppressive therapy, cytotoxic therapy, inhaled corticosteroids (excluding allergic rhinitis corticosteroid spray therapy, acute noncomplicated dermatitis superficial corticosteroid therapy) within 6 months before primary immunization and before booster immunization; * Already pregnant or are breastfeeding, planning to get pregnant (according to subjects' self-report and urine pregnancy test results) * Receipt of attenuated live vaccines within 14 days prior to booster vaccination; * Receipt of inactivated or subunit vaccines within 7 days prior to booster vaccination; * Acute diseases or acute exacerbation of chronic diseases within 7 days prior to booster vaccination; * Axillary temperature \>37.0°C prior to booster vaccination; * According to the investigator's judgment, the subject has any other factors that are not suitable for participating in the clinical trial