Clinical Trial on Sequential Immunization of Recombinant COVID-19 Vaccine (CHO Cells) and Inactivated COVID-19 Vaccine (Vero Cells) in Population Aged 18 Years and Above

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