This is randomized, blinded and controlled design. Among the randomly selected subjects who have been vaccinated with two doses of Inactivated COVID-19 vaccine (Vero cell), based on a step-wise approach, the subjects will receive one dose of recombinant COVID-19 vaccine sequentially at different shedules of 1-3 months, 3-6 months and ≥ 6 months after two doses of vaccination, and the subjects vaccinated at different schedules will be randomly assigned to different sequential immunization groups. At the same time, each sequential immunization group will be matched with a control group with the inactivated COVID-19 vaccine (vero cells) as the booster dose.

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 of CNBG inactivated COVID-19 vaccine according to the product insert;
• Female subjects who are not nursing or pregnant at the time of enrolment (negative urine pregnancy test) and have no family planning within the first 6 months after enrolment. 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; Self has 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 SARS and MERS infection (self-report, on-site inquiry);
• Has vaccinated with any vaccine other than one or more doses of CNBG inactivated COVID-19 vaccine;
• Axillary temperature ≥ 37.3 ℃ (forehead temperature ≥ 37.8 ℃);
• Previous severe allergic reactions to vaccination (such as acute allergic reactions, urticaria, dyspnea, angioneurotic edema or abdominal pain) or allergy to known components of recombinant COVID-19 vaccine;
• Allergic to any component of the study vaccine (e.g. aluminum, histidine, etc.)
• Known or suspected diseases include：Severe respiratory diseases, severe liver and kidney diseases, hypertension (systolic blood pressure ≥ 150 mmHg, diastolic blood pressure ≥ 90 mmHg), diabetic complications, malignant tumors, various acute diseases or acute attacks of chronic diseases;
• Has been diagnosed with congenital or acquired immunodeficiency, HIV infection, lymphoma, leukemia or other autoimmune diseases;
• Has a history of convulsions, epilepsy, encephalopathy，long term history of alcohol and drug abuse, history of thyroidectomy, infectious diseasesor mental illness or family history（lineal）;
• With Congenital malformation or developmental disorder, genetic defect, severe malnutrition, etc.;
• Has a history of coagulation dysfunction (e.g. coagulation factors deficiency and coagulation diseases);
• Absence of spleen or splenectomy, functional absence of spleen caused by any condition
• Anti -TB (TB) treatment is under way.
• Patients receiving immunoenhancement or inhibitor therapy within 3 months (continuous oral or IV administration for more than 14 days);
• Received other vaccines within 14 days before vaccination；
• Received blood products before within 3 months before vaccination;
• Received other investigational drugs within 6 months before vaccination;
• Plan to move before the end of the study or leave the local area for a long time during the scheduled study visit
• Other circumstances judged by investigators that are not suitable for this clinical trial