A Global Multi-center Clinical Study of SARS-CoV-2 mRNA Vaccine for the Prevention of COVID-19

Inclusion Criteria:

1. Adults aged 18 years and older.
2. Understand the content of the Informed Consent Form (ICF), and voluntarily sign theICF (If the participant is unable to sign the ICF on his/her own due to illiteracy, animpartial witness is needed).
3. Participants who are willing and able to comply with study requirements, including allscheduled visits, vaccinations, laboratory tests, and other study procedures.
4. Female participants of childbearing potential or partners of male participants: voluntarily agree to use effective contraception with their partners prior to thefirst vaccination and must agree to continue such precautions during the study until 3months after full vaccination (including the initial set of vaccination and crossoverset of vaccination). [Effective contraception includes oral contraceptives, injectableor implantable contraception, extended-release topical contraceptives, hormonalpatches, intrauterine devices (IUDs), sterilization, abstinence, condoms (for male),diaphragms, cervical caps, etc.].
5. For female participants: without childbearing potential (amenorrhea for at least 1year or documented surgical sterilization) or have used effective contraception with anegative pregnancy test before each vaccination in this study.
6. On the day of 1st dose of vaccination and 24 hours prior to vaccination, axillarytemperatures <37.3°C/99.1°F.
7. Healthy participants or participants with mild underlying disease [in a stable statewithout exacerbation (no admission to hospital or no major adjustment to treatmentregimen, etc.) for at least 3 months prior to enrollment in this study].

Exclusion Criteria:

1. History of Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), or other coronavirus infections.
2. Individuals using prescription medications for prophylaxis or treatment of SARS-CoV-2 (including vaccination of licensed COVID-19 vaccines).
3. History of a previous laboratory-confirmed diagnosis of SARS-CoV-2 infection orCOVID-19.
4. History of allergy to any component of the study vaccine or history of severe allergicreaction to vaccine or drug (including but not limited to anaphylaxis, allergiclaryngeal edema, anaphylactic purpura, thrombocytopenic purpura, or localized allergicnecrosis (Arthus reaction)).
5. Positive nucleic acid for SARS-CoV-2 in nasopharyngeal/oropharyngeal swab specimens.
6. Positive HIV test results.
7. A history or family history of convulsions, epilepsy, encephalopathy and psychosis.
8. Malignant tumors in the active phase, malignant tumors not receiving adequatetreatment, malignant tumors at potential risk of recurrence during the study period.
9. Asplenia or functional asplenia, complete or partial splenectomy from any cause.
10. Prolonged (defined as more than 14 days) use of immunosuppressive or otherimmunomodulatory drugs (e.g., corticosteroids, ≥20 mg/d prednisone or equivalent;however, inhaled and topical steroids are permitted) within 6 months prior to the 1stdose of investigational vaccine.
11. Any other licensed vaccines given within 28 days prior to the study vaccination, orplanned administration of vaccine(s) within 28 days after the 2nd dose in the blindedcrossover vaccination.
12. Have received immunoglobulin or other blood products within 3 months prior toenrollment or plan to receive them during the study period.
13. Blood donation or blood loss ≥ 450 mL within 1 month prior to enrollment or planned todonate blood during the study period.
14. Participants who have received any other investigational product within 1 month priorto enrollment or intent to participate in another clinical study at any time duringthe conduct of this study.
15. Women who are pregnant or breastfeeding.
16. Participants deemed unsuitable for participation in this study based on theinvestigator's assessment.