This trial is a phase II clinical trial of SARS-CoV-2 Bivalent mRNA Vaccine (LVRNA021). The trial was randomized, blinded, placebo controlled. To evaluate the safety and immunogenicity of the study vaccine in participants aged 18 years and older who have received SARS-CoV-2 Vaccine.

Inclusion Criteria:

1. Healthy adults aged 18 years and older, both male and female, who can provide legal identity certificate of participants;
2. The subject understands the contents of the Informed Consent Form and the vaccination situation of this vaccination, voluntarily signs the Informed Consent Form, and has the ability to use the thermometer, scale and fill in the Diary Card and Contact Card as required (if the subject is unable to sign the Informed Consent Form by himself/herself due to limited reading and writing ability, the informed consent form and signature of the Informed Consent Form can be completed under the witness' s witness);
3. Able to communicate well with investigators and understand and comply with the requirements of this trial;
4. Completion of basic immunization with SARS-CoV-2 vaccine ≥ 6 months;
5. Negative nucleic acid test for SARS-CoV-2 within 3 days prior to vaccination;
6. Women who have used effective contraception within 2 weeks prior to inclusion in this trial, have a negative pregnancy test (pregnancy test can be exempted for those who have been amenorrheic for at least 1 year or have a documented history of surgical sterilization), and voluntarily agree to continue using at least 1 effective contraception within 6 months after vaccination [effective contraception includes: oral contraceptives, injected or implanted contraception, sustained-release local contraceptives, hormonal patches, intrauterine device (IUD), sterilization, abstinence, condoms (men), diaphragms, cervical caps, etc.];
7. Healthy participants or participants with mild underlying disease [stable condition without worsening (requiring no hospitalization or major modification of treatment regimen, etc.) for at least 3 months prior to inclusion in this trial].

Exclusion Criteria:

1. Participants with abnormal electrocardiogram during screening and who were judged unsuitable for vaccination by investigators; Blood pressure indicators during the screening period: systolic blood pressure ≥160 or diastolic blood pressure ≥100mmHgHg, or abnormal blood pressure, and the health status determined by the investigator is uncertain and requires further diagnosis, or the investigator determines that the vaccination is not suitable for participants with medical history and clinical manifestations ;
2. Body mass index (BMI) <18 kg/m 2 or >30 kg/m 2;
3. Individuals with any infectious disease, acute infection, acute phase of chronic infection (such as active untreated pulmonary tuberculosis, etc.) or any advanced immune disease (inquiry);
4. Infected within last 6 months or likely infected with SARS-CoV-2;
5. Positive HIV test result at screening;
6. Fever on the day of investigational vaccination (axillary temperature ≥ 37.3℃) or within 3 days, or use of antipyretic and analgesic drugs within 3 days;
7. Women with a positive pregnancy test (surgical sterilizers who are menstruating or amenorrheic for at least 1 year or have medical records may be exempted from pregnancy testing), or breastfeeding women, or women planning to become pregnant from screening through 6 months after booster vaccination, men whose partners plan to become pregnant, or plan to donate sperm and eggs;
8. Previous history of allergic or allergic reactions to vaccines or drugs, such as urticaria, severe skin eczema, dyspnea, laryngeal edema, angioneurotic edema, etc.;
9. Administration of any vaccine within 28 days prior to booster vaccination with investigational vaccine;
10. Have participated within 28 days prior to booster vaccination with investigational vaccine or plan to participate in other drug clinical trials within 12 months after booster vaccination;
11. Patients with a history of thrombocytopenia or other coagulation disorders, which may cause contraindications to subcutaneous blood sampling or injection, and patients with a history of thrombosis;
12. Known history or diagnosis of diseases affecting immune system function, such as cancer (except basal cell carcinoma of the skin), congenital or acquired immunodeficiency, uncontrolled autoimmune diseases, etc. (such as systemic lupus erythematosus, autoimmune thyroid disease, multiple sclerosis);
13. Absence of spleen or functional absence of spleen;
14. Chronic use (≥14 consecutive days) of immunosuppressants or other immunomodulatory drugs (eg, corticosteroids: prednisone or drugs of the same class) within 6 months prior to booster vaccination with the investigational vaccine, but topical medications (eg, ointments, eye drops, inhalers, or nasal sprays) are allowed and should not exceed the dose recommended in the package insert;
15. Immunoglobulins and/or blood products received within 3 months prior to the investigational booster vaccination;
16. Suspected or known alcohol dependence or drug abuse that could compromise the safety evaluation or subject compliance;
17. Other conditions that the investigator considers inappropriate for participation in this trial.