This trial adopts a randomized, double-blind and positive control design, it is planned to recruit 1800 healthy participants who have been vaccinated with 2/3 doses of COVID-19 inactivated vaccine or mRNA vaccine for 3 months. The participants will be divided into two strata according to the types of vaccines administered, including 900 participants of COVID-19 inactivated vaccine and 900 participants of mRNA vaccine. According to the ratio of 1:1:1, each stratum was randomly assigned to three groups: sequential BIBP- COVID-19 inactivated vaccine (Omicron), WIBP-COVID-19 inactivated vaccine (Omicron) or COVID-19 inactivated vaccine (prototype strain). And according to D0, D28 immunization schedule, two doses of corresponding group vaccines are sequentially administered.

Inclusion Criteria:

• Age range: populations aged 18 years and above.
• Judged by the investigator that the health condition is well after inquiry and physical examination.
• Vaccinated with 2 doses/3 doses of inactivated COVID-19 vaccine or mRNA vaccine for ≥ 3 months.
• Female participants who are not nursing or pregnant at the time of enrolment (negative urine pregnancy test) and have no family planning within the first 7 months after enrollment. Effective contraceptive measures have been taken within 2 weeks before inclusion.
• During the whole follow-up period of the study, be able and willing to complete the whole prescribed study plan.
• With self ability to understand the study procedures, the informed consent & voluntarily sign an informed consent form and is able to comply with the requirements of the clinical study protocol.

Exclusion Criteria:

• Confirmed cases, suspected cases or asymptomatic cases of COVID-19;
• With a history of Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) (self-report, on-site inquiry);
• Has vaccinated with one or four doses and above COVID-19 vaccine;
• Axillary temperature ≥37.3℃ (Tympanic temperature ≥ 37.6 ℃);
• Previous allergic reactions to drug or vaccination (such as acute allergic reactions, urticaria, eczema, dyspnea, angioneurotic edema or abdominal pain) or allergy to known components of COVID-19 vaccine (self-report, onsite enquiry);
• Have a history of hospital-diagnosed thrombocytopenia or other coagulation disorders (self-report, onsite enquiry);
• With known immunological impairment or immunocompromised diagnosed by the hospital (self-report, onsite enquiry);
• Have an uncontrolled epilepsy and other progressive neurological diseases or a history of Guillain-Barre syndrome (self-report, onsite enquiry);
• Received whole blood, plasma and immunoglobulin therapy within 1 month (self-report, onsite enquiry);
• Known or suspected severe illness includes: respiratory illness, acute infection or active attacks of chronic illness, liver and kidney disease, severe diabetes mellitus, malignant tumor, infectious or allergic skin disease, Human Immunodeficiency Virus (HIV) infection (self-report, onsite enquiry, provide test report if available);
• With hospital-diagnosed serious cardiovascular diseases (cardiopulmonary failure, drug-uncontrolled hypertension (physical examination of systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg), acute attacks of chronic respiratory illness (self-report, onsite enquiry);
• Received live attenuated vaccines within 1 month before vaccination (self-report, onsite enquiry);
• Received inactivated vaccines within 14 days before vaccination (self-report, onsite enquiry);
• Participating or planning to participate in other interventional vaccine clinical trials during this study
• Other vaccination-related contraindications considered by investigators.