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.