A Study to Evaluate the Efficacy, Safety, and Immunogenicity of SARS-CoV-2 Variant (BA.4 /5) mRNA Vaccine

1. Voluntarily sign the ICF approved by the Ethics Committee before any study procedure and agree to participate in the study.
2. Healthy male or female able to provide legal identity certificate and aged 18 years and older when signing the ICF.
3. Subjects who have previously been fully vaccinated either by 2 or 3 doses of SARS-CoV-2 inactivated vaccine. The last dose of immunization should be >6 months before administration of the investigational products.
4. Be able to communicate well with the investigator, and to understand and comply with the requirements of this clinical trial.
5. Males and females with childbearing potential voluntarily take effective contraceptive methods from signing ICF to 3 months after completing the vaccination, including sexual abstinence or effective contraceptive measures (e.g., intrauterine or implanted contraceptive device, oral contraceptives, injected or implanted contraceptives, sustained-release topical contraceptives, intrauterine device [IUD], condoms [male], diaphragm, and cervical cap).

Exclusion Criteria:

1. Positive SARS-CoV-2 rapid test at screening.
2. Prior medical history of severe acute respiratory syndrome (SARS), middle east respiratory syndrome (MERS).
3. Fever (axillary temperature or equivalent ≥ 37.3℃*) on the day of vaccination with this investigational vaccine or within recent 72 hours.
4. Abnormal vital signs (pulse \< 60 bpm or > 100 bpm, systolic blood pressure ≥ 140 mmHg or diastolic blood pressure ≥ 90 mmHg when keeping awake) with clinical relevance.
5. Do not remain overall healthy (i.e., has medically deteriorated significantly since receiving the two-dose vaccination, is anticipated to have fatal outcome of uncontrolled diseases within 12 months, and is not able to provide blood as specified by the trial with anticipated, deleterious medical consequences) in the clinical judgment of the investigator based on medical history and physical examination.
6. Pregnant or lactating women, or those who plan to donate sperm or egg during the trial.
7. Prior history of allergic reaction or anaphylaxis to any vaccine or its excipients, e.g., hypersensitivity, urticaria, serious eczema, dyspnea, laryngeal edema, and angioedema etc.
8. Prior use of any other vaccine within 28 days before using the investigational products or planning to use any vaccine other than the investigational products during the study period.
9. Participation in the studies of any other interventional device or drug within 30 days before the screening, or current treatment with other investigational drug(s) or within 5 half-lives after taking the last dose of the study drug.
10. Hereditary hemorrhagic tendency or coagulation dysfunction (e.g., cytokine defects, coagulation disorders or platelet disorder), or a history of serious bleeding, or a history of massive bleeding after intramuscular injection or intravenous puncture or ecchymosis.
11. Known medical history or diagnosis confirming that subjects have diseases affecting immune system function, including cancer (except skin basal cell carcinoma), congenital or acquired immunodeficiency (e.g., infection with human immunodeficiency virus [HIV]), and uncontrolled autoimmune disease.
12. Serious or uncontrolled respiratory system disorders, cardiovascular disorders, nervous system disorders, blood and lymphatic system disorders, liver and kidney disorders, metabolism and skeletal disorders,etc. influencing study results evaluation at the investigator's discretion.
13. Asplenia or functional asplenia.
14. Long-term use (continuous use ≥14 days) of immunosuppressants or other immunomodulators (e.g., glucocorticoids: prednisone or similar drugs) within 6 months prior to administration of this investigational vaccine, except for topical medications (e.g., ointments, eye drops, inhalants or nasal sprays). And the topical medications should not exceed the recommended dose in the labels for use or induce any signs of systemic exposure.
15. Having received immunoglobulins and/or blood products within 3 months prior to administration of this investigational vaccine.
16. Suspected or known alcohol dependency or drug abuse, which may affect safety evaluation or subject's compliance at the investigator's discretion.
17. Planning to permanently move from the local area before study completion or leave the local area for a long time during the period of study visits.
18. Receiving antituberculosis treatment.
19. Staff of study site, sponsor and contract research organization (CRO) taking part in the study.
20. Other conditions that the investigators consider unsuitable for this study.