A total of 516 healthy subjects aged 18 years and above who have been vaccinated with 2 doses/3 doses of inactivated COVID-19 vaccine, will be enrolled, and randomly assigned to experimental group and control group in a 1:1 ratio, with 258 subjects in each sequential group.

Inclusion Criteria:

• Age: population aged 18 years and above;
• Judged by the investigator that the health condition is well after inquiry and physical examination;
• Vaccinated with 2 or 3 doses of inactivated COVID-19 vaccine (Vero Cell vaccine) 6 months prior to study screening and according to product insert;
• Female participants who are not pregnant or nursing or at the time of enrolment (confirmed via negative urine pregnancy test), and do not have plans to become pregnant within the first 6 months after enrollment. Effective contraceptive measures have been taken within 2 weeks before study inclusion and initiation;
• Be able and willing to provide written informed consent to participate in the study and complete all study requirements according to the study protocol;

Exclusion Criteria:

• COVID-19 infection positive patients (including suspected or asymptomatic cases);
• Have a history of SARS and MERS infection;
• Have been vaccinated by any COVID-19 Vaccines other than Vero Cell vaccine；
• Have an axillary temperature 37.3 (forehead temperature 37.8℃℃);
• Have had previous allergic reactions to vaccination (such as acute allergic reactions, urticaria, eczema, dyspnea, angioneurotic oedema or abdominal pain) or allergy to known components of COVID-19 vaccine;
• History of thrombocytopenia or other coagulation disorders;
• Patients with known immunological impairment or immunocompromised.
• Received whole blood, blood products, plasma and/or immunoglobulin therapy within 3 months before study enrollment
• Have known or suspected severe illness such as respiratory illness, acute infection or active attacks of chronic illness, liver and kidney disease, severe diabetes mellitus, malignant tumour, infectious or allergic skin disease, human immunodeficiency virus (HIV) infection (test report available);
• Diagnosed with serious cardiovascular diseases such as cardiopulmonary failure, drug-uncontrolled hypertension (Systolic blood pressure 160 mmHg and/or diastolic blood pressure 95 mmHg).
• Received live attenuated vaccines within 1 month before study enrollment;
• Received inactivated vaccines within 14 days before study enrollment;
• Received other investigational drugs within 6 months before study enrollment;
• Other vaccination-related contraindications considered by investigators.