The Safety and Immunogenicity Following a Heterologous Booster Dose of Recombinant SARS-CoV-2 Vaccine LYB002

Inclusion Criteria: 1. Healthy subjects aged 18-59 years, including both males and females; 2. Subjects who agree to participate in this clinical trial voluntarily and sign the informed consent form, are capable of providing valid identification, understanding and complying with the requirements of the clinical protocol. 3. Subjects who have completed three-dose inactivated COVID-19 vaccine at 6 months earlier; 4. S-protein binding antibody IgG concentration was less than 300BAU/mL before booster vaccination in cohort 1, while S-protein binding antibody IgG concentration was not screened in cohort 2; Subjects in cohort 1 and cohort 2 were negative in nucleic acid test or antigen test before booster vaccination. 5. For female participants of childbearing potential, effective contraception measures should be used within 2 weeks prior to participation in this study and the results of pregnancy test is required to be negative. Participants should voluntarily agree to use effective contraceptive measures from the time of signing the informed consent form to the end of the study (effective contraceptive measures including oral contraceptives (excluding emergency contraceptives), injectable or implantable contraceptives, sustained-release topical contraceptives, hormonal patches, intrauterine device, sterilization, abstinence, condoms (for males), diaphragms, cervical caps, etc.). Exclusion Criteria: 1. Receipt of any COVID-19 prophylactic medication, or previous vaccination history other than other than three doses of inactivated vaccination; 2. Abnormal vital signs with clinical significance prior to enrolment, systolic blood pressure ≥140mmHg and/or diastolic blood pressure ≥90mmHg (systolic blood pressure ≥150mmHg and/or diastolic blood pressure ≥100mmHg for subjects aged ≥60 years), or axillary body temperature ≥37.3℃; 3. The results of laboratory tests before enrollment were abnormal and clinically significant as judged by clinicians; 4. Known allergy, or history of anaphylaxis or other serious adverse reactions to vaccines or their excipients; 5. History of severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS); 6. Administration of antipyretics, painkillers or anti-allergy drugs within 24 hours prior to enrolment; 7. Receipt of any live attenuated vaccine within 28 days prior to vaccination and other vaccines, such as subunit and inactivated vaccine within 14 days prior to vaccination; 8. Receipt of blood or blood-related products, including immunoglobulins, within 3 months prior to vaccination; or any planned use during the study period. 9. Subjects with the following diseases: 1. Any acute diseases or acute attacks of chronic diseases within 7 days prior to enrolment； 2. Congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.； 3. Congenital or acquired immunodeficiency or autoimmune disease, or long-term receipt (\>14 consecutive days) of glucocorticoid (reference value for dose: ≥20 mg/day prednisone or equivalent) or other immunosuppressive agents within the past 6 months, with exception of inhaled or topical steroids, or short-term use (≤14 consecutive days) of oral corticosteroids； 4. Currently suffering from or diagnosed with infectious diseases, positive screening results for hepatitis B surface antigen, hepatitis C antibody, treponema pallidum antibody, human immunodeficiency virus antibody； 5. History or family history of neurological disorders (convulsions, epilepsy, encephalopathy, etc.) or psychiatric disorders； 6. Asplenia, or functional asplenia； 7. Presence of severe, uncontrollable or hospitalized cardiovascular diseases, diabetes, blood and lymphatic diseases, immune diseases, liver and kidney diseases, respiratory diseases, metabolic and skeletal diseases, or malignant tumors; 8. Contraindications to IM injections and blood draws, such as coagulation disorders, thrombotic or bleeding disorders, or conditions that needs continuous anticoagulant usage. 10. Drug or alcohol abuse (alcohol intake ≥ 14 units per week) which in the investigator's opinion would compromise the participant's safety or compliance with the study procedures; 11. Pregnant or lactating females; 12. Having participated or participating in COVID-19 related clinical trials, and those participating or planning to participate in other clinical trials during the study period; 13. Presence of any underlying disease or condition which, in the opinion of the investigator, may place the subject at unacceptable risk, is unable to meet the requirements of the protocol, or interfere with the assessment of vaccine response.