A Phase III Clinical Study of a SARS-CoV-2 Messenger Ribonucleic Acid (mRNA) Vaccine Candidate Against COVID-19 in Population Aged 18 Years and Above

Inclusion Criteria: Subjects included in this trial must meet all of the following inclusion criteria:

1. Adults aged 18 and above (both males and females are required);
2. Individuals who are able to understand the contents listed in the informed consentform and the procedure of this clinical trial; are able to sign the informed consentform voluntarily;
3. Individuals who are able to communicate well with the investigator and has the abilityto understand and comply with the requirements of the clinical trial;
4. Individuals who are at risk of SARS-CoV-2 infection or are exposed to COVID-19 due toregional, occupational, activity and environmental factors;
5. For female participants of childbearing potential, effective contraception should beused within 2 weeks prior to participation in this study and the pregnancy testresults is required to be negative (those with amenorrhea of at least 1 year orsurgical sterilization verified by medical records could be exempted from thepregnancy test). Participants should voluntarily agree to continue using at least oneeffective methods of contraception for 12 months after complete series (effectivemethods include oral contraceptives, injectable or implantable contraceptives,sustained-release topical contraceptives, hormonal patches, intrauterine device,sterilization, abstinence, condoms (for males), diaphragms, cervical caps, etc.).
6. Healthy individuals with verified medical history: individuals who are in a stablecondition and whose current diseases will not worsen for at least 3 months prior toenrollment to this study.

Exclusion Criteria: Exclusion criteria for first dose vaccination Subjects who meet any of the following exclusion criteria shall not be enrolled:

1. Individuals with a history of SARS-CoV-2 infection or use of any preventive productsfor COVID-19 (e.g., a history of any SARS-CoV-2 vaccines that have or have not beenmarketed);
2. Individuals with SARS-CoV-2 etiological testing (RT-PCR Assay) (individuals withserological testing showing positive IgG and/or IgM antibodies may be enrolled);
3. Individuals with a previous history of severe acute respiratory syndrome (SARS),middle east respiratory syndrome (MERS) and other human coronavirus infections ordiseases;
4. Individuals who have fever within 72 hours prior to Dose 1 in this trial (oraltemperature ≥38°C);
5. Pregnant (e.g., positive pregnancy test) or lactating females;
6. Individuals who have plan of pregnancy or interruption of effective contraceptivemethods within 3 months after the second cross-vaccination in this study;
7. Personnel of the study site or the sponsor;
8. Individuals with prior history of allergic reaction or anaphylaxis to any vaccine ordrug, e.g., hypersensitivity, urticaria, serious eczema, dyspnea, laryngeal edema, andangioedema etc.;
9. Individuals who have been vaccinated with any vaccine other than the investigationalvaccine used in this clinical trial from 28 days prior to Dose 1 to 28 days after Dose 2;
10. Individuals who have participated in or plan to participate in other drug clinicaltrials form 28 days prior to Dose 1 to 12 months after Dose 4 (the second dose ofcross-vaccination) in this study;
11. Individuals who have hereditary hemorrhagic tendency or coagulation dysfunction (e.g.,cytokine defects, coagulation disorders or platelet disorder), or a history ofsignificant bleeding, or a history of injury caused by intramuscular injection orvenipuncture;
12. Individuals who are confirmed for diseases affecting immune system function, includingcancer (except skin basal cell carcinoma), congenital or acquired immunodeficiency (e.g., infection with human immunodeficiency virus (HIV)), and uncontrolled autoimmunedisease, based on known history or diagnosis;
13. Individuals who have asplenia or functional asplenia;
14. Individuals with long-term use (continuous use ≥14 days) of immunosuppressants orother immunomodulatory drugs (e.g., corticosteroids: prednisone or similar drugs)within 6 months prior to Dose 1. Drugs for topical use (e.g., ointment, eye drops,inhalants or nasal spray) are allowed in this study, and the topical medicationsshould not exceed the recommended dose in the labels for use or induce any signs ofsystemic exposure;
15. Individuals who have received immunoglobulin and/or blood products within 3 monthsprior to Dose 1;
16. Individuals who are suspected or known to have alcohol dependency problems or drugabuse that may affect safety evaluation or subject's compliance;
17. Individuals who plan to permanently relocate from the local area before the completionof the study or leave the local area for long periods during the study visits;
18. Other circumstances considered by the investigator as inappropriate to participate inthe study. Criteria for Postponement of the Subsequent Doses: If the subjects have any of the followings prior to the subsequent doses, vaccination willbe postponed. During the same immunization schedule, the second dose of vaccine will beadministered at the 28th day after the first vaccination, with a time window of +5 days:
19. Fever within 72 hours prior to the subsequent doses (oral temperature ≥38°C,probability of SARS-CoV-2 infection shall be excluded);
20. In case of acute diseases prior to the subsequent doses, the investigator shallexclude the probability of SARS-CoV-2 infection and evaluate the likelihood ofshort-term recovery for the diseases; Exclusion Criteria for the Subsequent Doses: If the subject has any of the followings prior to the subsequent doses, the vaccinationshall be terminated for the subject while other study procedures could be continued at thediscretion of the investigators:
21. Female subjects of childbearing potential who have positive pregnancy test results;
22. Subjects who have a serious allergic reaction or serious adverse event causallyrelated to vaccination after previous vaccination;
23. Other circumstances considered by the investigator as inappropriate to receive thesubsequent doses of the vaccine. Criteria for Withdrawal from the Study
24. Subjects experience disability, life-threatening adverse events or serious adverseevents and have to prematurely withdraw from this study due to treatment or otherreasons.
25. Subjects are placed at safety risks by their health conditions which prevent them fromcontinuing to participate in this study.
26. Female subjects become pregnant during the study (if at least one dose has beenadministered, the subject is not required to withdraw from this study but should notreceive the subsequent doses. Moreover, subsequent observation and follow-up visitsshould be continued);
27. Subjects actively request to withdraw from the study;
28. Subjects are considered unsuitable for continuing to participate in this study at thediscretion of the investigator.