This is a single-center, randomized, blinded, placebo-controlled clinical trial to evaluate the protective efficacy, safety and immunogenicity of one dose of the SARS-CoV-2 variant (Omicron BA.5) mRNA vaccine in people aged 18 years and older who had received two or three doses of inactivated COVID-19 vaccine.

Inclusion Criteria:

1. Both male and female adults aged 18 and above who can provide identification;
2. Know the contents of the informed consent form and the situation of the vaccination, sign the informed consent form voluntarily, and have the ability to use the thermometer, scale and fill in the diary card and contact card as required;
3. Ability to communicate well with researchers, understand and comply with the requirements of the study;
4. Two or three doses of COVID-19 inactivated vaccine have been completed and ≥6 months have passed since the last dose of COVID-19 inactivated vaccine;
5. Healthy subjects or subjects with mild underlying diseases [stable condition with no worsening condition (no need for hospitalization or no major adjustment in treatment regimen, etc.) for at least 3 months prior to inclusion in the study];
6. Women who are not possibility to have children (amenorrhea for at least 1 year or surgical sterilization with medical records) or who are known not to be pregnant or lactating and who have used effective contraception for nearly 14 days before vaccination (e.g., Intrauterine or implantable contraceptive devices,oral contraceptives, injected or embedded contraceptives, slow-release topical contraceptives, intrauterine devices (IUD), condoms (male), diaphragm, cervical cap, etc.) (provide negative proof of pregnancy within 48 hours);
7. SARS-CoV-2 etiology test (RT-PCR) was negative within 48 hours.

Exclusion Criteria:

1. Abnormal vital signs that are clinically significant (e.g. abnormal controlled blood pressure);
2. Have been infected with COVID-19 or used any COVID-19 prophylactic medication other than 3 doses of COVID-19 inactivated vaccine within the last 6 months (for example, a history of any other non-COVID-19 inactivated vaccine on or off the market, or 1 or 4 doses of COVID-19 inactivated vaccine);
3. History of severe acute respiratory syndrome (SARS), Middle East Respiratory Syndrome (MERS) and other human coronavirus infection or disease history;
4. Axillary temperature ≥37.3℃ or fever within 24 hours on the day of vaccination (axillary temperature ≥37.3℃/ mouth temperature ≥37.5℃);
5. A history of severe allergic reactions or allergic reactions to vaccines or drugs, such as urticaria, severe eczema, dyspnea, laryngeal edema, angioneurotic edema, etc.;
6. Vaccination of any vaccine within 28 days prior to study vaccination;
7. Enrolled in a clinical study of another drug within 28 days before vaccination or planned to participate in a clinical study of another drug within 6 months after vaccination;
8. Have a genetic tendency to bleed or abnormal coagulation function (e.g. cytokine deficiency, coagulation disorder or thrombocytopenia), or a history of severe bleeding;
9. A known history or diagnosis of a disease affecting immune system function, such as cancer (other than basal cell carcinoma of the skin), congenital or acquired immunodeficiency (e.g., HIV infection), uncontrolled autoimmune disease, etc.;
10. Asplenia or functional asplenia;
11. Long-term use (continuous use ≥14 days) of immunosuppressants or other immunomodulatory drugs (e.g., cortisol: prednisone or similar drugs) within 6 months prior to study vaccination; Interferon, etc.), but topical use (such as ointments, eye drops, inhalants or nasal sprays) is permitted, and the dose of topical use shall not exceed that recommended in the instructions;
12. Received immunoglobulin and/or blood products within 3 months prior to study vaccination;
13. Suspected or known alcohol dependence or drug abuse;
14. Other factors considered inappropriate to be included in the study.