Immunogenicity and Safety of Sequential Immunization of Recombinant SARS-CoV-2 Fusion Protein Vaccine

Inclusion Criteria: 1. Healthy participants aged 18 years and older who have completed the second dose of 2-dose regimen of SARS-CoV-2 Inactivated Vaccine in the past 6 (±1) months; 2. Voluntarily participate in the study and sign the informed consent form, who can provide valid ID and follow the study protocol requirement; 3. In the past 14 days, no history of high or medium risk of the epidemic, overseas travel history or residence history; no history of contact with confirmed, asymptomatic or suspected COVID-19 cases; no history of contact with the persons from high- and medium-risk epidemic areas or contact patients with fever or respiratory symptoms; and those who are not in isolation period. 4. Males of reproductive potential and females of childbearing potential voluntarily agree to take effective and acceptable contraceptive methods from the signing of informed consent form to 3 months after vaccination. Exclusion Criteria: 1\. Confirmed COVID-19 cases, or positive for SARS-CoV-2 test by RT-PCR. 2. History of previous SARS infection. 3. History of severe allergy to any vaccine or any ingredient of the vaccine including aluminum adjuvant, e.g., anaphylactic shock, allergic laryngeal edema, allergic purpura, thrombocytopenic purpura, local allergic necrosis reaction (Arthus reaction), dyspnea, angioedema, etc. 4\. People who currently suffer from the following diseases: 1. Symptoms related to acute respiratory infections (such as: sneezing, nasal congestion, runny nose, cough, sore throat, loss of taste, chills, shortness of breath, etc.) 2. Patients with thrombocytopenia, any coagulation dysfunction, or receive anticoagulant treatment, etc. 3. Patients with congenital or acquired angioedema/neuroedema; 4. A history of congenital or acquired immunodeficiency or autoimmune disease (except for mild psoriasis, controllable autoimmune thyroid disease, vitiligo, or stable celiac disease that does not require immunosuppressive or immunomodulatory therapy); no spleen , or history of spleen surgery, history of trauma, or treatment with immunomodulators within 6 months, such as: glucocorticoid with the dose causing immunosuppressive (dose reference: equivalent to prednisone 20mg/day, more than one week); or monoclonal antibody ; or thymosin; or interferon, etc.; but local medication (such as ointment, eye drops, inhalation or nasal spray) is allowed. 5. Patients with active tuberculosis, viral hepatitis, human immunodeficiency virus or syphilis infection. 6. Patients with acute diseases, or acute attacks of chronic diseases, or uncontrolled severe chronic diseases: history of chronic respiratory diseases (including moderate to severe asthma, COPD, pulmonary fibrosis), hypertension that cannot be controlled by drugs (systolic blood pressure ≥150mmHg) And/or diastolic blood pressure ≥100mmHg), history of severe cardiovascular disease (including heart failure, coronary artery disease, cardiomyopathy), history of chronic kidney disease, history of cancer (except for basal cell carcinoma), diabetes (unsatisfied blood sugar control or diabetes related serious complications). 5\. Received inactivated vaccines or any subunit vaccines within 14 days before the vaccination and attenuated live vaccine within 28 days before the vaccination. 6\. Injection of immunoglobulin and/or other blood products within 3 months before the administration of study vaccine; or with the plan to use such product within 6 month after immunization. 7\. Pregnant (including positive urine pregnancy test for women of childbearing age) or breastfeeding women. Or women or their partners who have a pregnancy plan within 3 months after the trial vaccination. 8\. Have participated in or are participating in other COVID-19 related clinical trials, or are participating in other drug clinical trials; 9. Those considered by the investigator as inappropriate to participate in the study.