Primary#Objectives #Immunogenicity:To demonstrate the non-inferiority of binding antibody response in terms of geometric mean titers (GMT) of mRNA vaccine compare with mRNA COVID-19 vaccine(Pfizer) 14 days post dose.

Secondary#Immunogenicity:

1. To describe binding antibody profile at D01, D29 and D181 of each study group.
2. To describe the neutralizing antibody profile at D15, D29 and D181 of each study intervention group.

Secondary#Safety:

To assess the reactogenicity and safety of a booster dose in a heterologous vaccination regimen in subjects previously immunized with two Sinopharm doses.

Exploratory#Cell-mediated immunity:

To describe the cellular immune response profile at D01, D08, D15, in a subset of 30 participants for each study group.

Exploratory#Efficacy:

To describe theoccurrence ofvirologically-confirmedCOVID-19 like illness and serologicallyconfirmed SARS-CoV-2 infection.

Endpoint:

Immunogenicity Endpoints:

GMT of Anti-Spike IgG antibody titers will be measured with the ELISA assay on D01, D15.

Immunogenicity Endpoints:

Binding antibody titers to full length SARS-CoV-2 Spike (S) protein will be measured for each study intervention group with the ELISA method.

1. Antibody titer on D01, D29, D181.
2. Fold-rise in antibody titer at D15, D29 and D181 relative to D01Neutralizing antibody titers will be measured with the neutralization assay.
3. Antibody titer on D15, D29 and D181.
4. Fold rise in serum neutralization titer [post/pre] relative to D01 at D15, D29 and D181.

Safety Endpoints:

1. Occurrence of local and systemic AEs reported within 7 days after study vaccination (per group);
2. Occurrence of unsolicited AEs reported within 28 days after study vaccination (per group);
3. Occurrence of SAEs and AESIs within 28 days after study vaccination (per group).

Exploratory#Cell-mediated immunity Endpoints:

INF-γ will be measured in whole blood and/or cryopreserved PBMC following stimulation with pools of S-antigen peptides.

Exploratory#Efficacy Endpoints Occurrence of confirmed symptomatic cases during the study period

Inclusion Criteria:

1. Participant is willing and able to give written informed consent for participation in the study.
2. Male or Female, aged 18 years or above and in good health as determined by study clinician.
3. Female participants of childbearing potential must be willing to ensure that they or their partner use effective contraception from 1 month prior to first immunization continuously until 3months after boost immunization.
4. In the Investigator's opinion, is able and willing to comply with all trial requirements.
5. Subjects had completed the 2-dose series vaccination of COVID-19 inactivated vaccine(Sinopharm) at least 24 weeks with the completed the 2-dose Series as the primary vaccination according to the product label.

Exclusion Criteria:

-

The participant may not enter the study if ANY of the following apply:

1. Confirmed cases, suspected cases or asymptomatic cases of COVID-19;
2. Self-reported history of SARS and MERS infection;
3. Receipt of live attenuated vaccine within one month prior to vaccination and other vaccines within 14 days prior to vaccination;
4. Receipt of any SARS-COV-2 vaccine after last dose of primary vaccination
5. Participants who are pregnant at enrolment or planning to become pregnant during the first 3 months following vaccination
6. Administration of immunoglobulins and/or any blood products within the three months preceding the planned administration of the vaccines
7. History of allergic disease or reactions likely to be exacerbated by any component of study vaccines
8. Any history of anaphylaxis to any component of vaccine.
9. Bleeding disorder (e.g. factor deficiency, coagulopathy or platelet disorder), or continuous use of anticoagulants (warfarin, apixaban, rivaroxaban, dabigatran, edoxaban), or prior history of significant bleeding or bruising following IM injections or venipuncture
10. Suspected or known current alcohol or drug dependency
11. Any other significant disease, disorder or finding which may significantly increase the risk to the volunteer because of participation in the study, affect the ability of the volunteer to participate in the study or impair interpretation of the study data
12. Severe and/or uncontrolled cardiovascular disease, respiratory disease, gastrointestinal disease, liver disease, renal disease, endocrine disorder, and neurological illness (mild/moderate well controlled comorbidities are allowed)
13. Scheduled elective surgery during the trial
14. Other reasons investigators did not consider it appropriate to refer to clinical trials

This study uses an observer-blinded design, and the investigators and participants will all remain blinded before unblinding and do not know which group the participant belongs to.