This is a multi-center, randomized, blinded, placebo-controlled, phase 3 clinical study to evaluate the efficacy, safety and immunogenicity of SARS-CoV-2 bivalent mRNA vaccine (LVRNA021) as booster in participants aged 18 years and older who completed primary/1 booster dose(s) of SARS-CoV-2 vaccination.

Inclusion Criteria:

1. Adults aged 18 years and older;
2. Understand the content of the ICF, and voluntarily sign the ICF (If the participant is unable to sign the ICF on his/her own due to illiteracy, an impartial witness is needed);
3. Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, 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 the first vaccination and must agree to continue such precautions during the study until 3 months after booster vaccination [Effective contraception includes oral contraceptives, injectable or implantable contraception, extended-release topical contraceptives, hormonal patches, intrauterine devices (IUDs), sterilization, abstinence, condoms (for male), diaphragms, cervical caps, etc.);
5. For female participants: without childbearing potential (amenorrhea for at least 1 year or documented surgical sterilization) or have used effective contraception with a negative pregnancy test before booster vaccination in this study；
6. On the day of vaccination and 24 hours prior to vaccination, axillary temperatures<37.3°C/99.1°F;
7. Healthy participants or participants with mild underlying disease [in a stable state without exacerbation (no admission to hospital or no major adjustment to treatment regimen, etc.) for at least 3 months prior to enrollment in this study];
8. Participants who have received primary/1 booster dose(s) of SARS-CoV-2 vaccination (including primary series of inactivated vaccine, mRNA vaccine, adenovirus vaccine or 1 homologous/heterologous booster), with the last dose received at least 6 months before enrolment. Documented confirmation of prior SARS-CoV-2 vaccination receipt must be obtained prior to randomization;

Exclusion Criteria:

1. History of Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), or other coronavirus infections at any time;
2. History of hepatitis A, hepatitis B, hepatitis C, syphilis infection based on medical inquiry.;
3. History of severe adverse reaction associated with a vaccine or drug and/or severe allergic reaction (e.g., anaphylaxis) to any component of the study intervention(s);
4. Receipt of medications intended to treat COVID-19 within 6 months;
5. Virologically confirmed SARS-CoV-2 diagnosis within 6 months before screening visit;
6. Positive nasopharyngeal/oropharyngeal swab SARS-CoV-2 RT-PCR test result at screening;
7. Positive HIV test result at screening;
8. A history or family history of convulsions, epilepsy, encephalopathy and psychosis;
9. Malignant tumors in the active phase, malignant tumors not receiving adequate treatment, malignant tumors at potential risk of recurrence during the study period;
10. Asplenia or functional asplenia, complete or partial splenectomy from any cause;
11. Individuals who receive treatment with radiotherapy or immunosuppressive therapy, including cytotoxic agents or systemic corticosteroids (if systemic corticosteroids are administered for ≥14 days at a dose of ≥20 mg/day of prednisone or equivalent), e.g., for cancer or an autoimmune disease, or planned receipt throughout the study. Inhaled/nebulized, intra-articular, epidural, or topical (skin or eyes) corticosteroids are permitted;
12. Any other licensed vaccines given within 28 days prior to vaccination, planned administration of any other vaccines within 28 days after vaccination, or planned administration of other COVID-19 vaccines during the entire study duration;
13. Receipt of blood/plasma products, immunoglobulin, or monoclonal antibodies, from 60 days before vaccine administration, or receipt of any passive antibody therapy specific to COVID-19, from 90 days before vaccine administration, or planned receipt throughout the study;
14. Blood donation or blood loss ≥ 450 mL within 1 month prior to enrollment or planned to donate blood during the study period;
15. Participation in other studies involving study intervention within 28 days prior to study entry, and/or during the study;
16. Women who are pregnant or breastfeeding;
17. Participants deemed unsuitable for participation in this study based on the investigator's assessment.](streamdown:incomplete-link)