A Immuno-bridging and Immunization Schedules Study of COVID-19 Vaccine (Vero Cell), Inactivated

Inclusion Criteria: * Aged 3 years and above (after enrolled, subjects will be allocated according to age). * By asking for medical history and physical examination, the health condition judged by the investigators is well. * Female subjects of childbearing age are not nursing or pregnant at the time of enrolment (negative urine pregnancy test) and have no family planning within the first 3 months after enrolment. Effective contraceptive measures have been taken within 2 weeks before inclusion. * No vaccination history of COVID-19 vaccine before enrollment. * 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 be able to comply with the requirements of the protocol. Exclusion Criteria: * Confirmed cases, suspected cases or asymptomatic infections of SAR-CoV-2 infection (check "China Disease Prevention and Control Information System"). * Has a history of SARS, MERS infection (self-report, on-site inquiry). * \>14-year-old subjects axillary temperature ≥37.3℃, ≤14-year-old subjects axillary temperature ≥37.5℃. * Previous severe allergic reactions to vaccination (such as acute allergic reactions, urticaria, dyspnea, angioneurotic edema or abdominal pain) or allergy to known ingredients of inactivated SARS-CoV-2 vaccine have occurred. * With severe liver diseases, severe kidney diseases, uncontrollable hypertension (systolic blood pressure ≥160 mmHg, diastolic blood pressure ≥100 mmHg), diabetic complications, malignant tumors, various acute diseases, or acute attack period of chronic diseases. * Has been diagnosed with congenital or acquired immune deficiency, HIV infection, lymphoma, leukemia or other autoimmune diseases. * Has a history of convulsion, epilepsy, encephalopathy or mental illness or family history. * With congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc. * Has a history of coagulation dysfunction (e.g. Coagulation factor deficiency, coagulation disease). * Received immunotherapy or inhibitor therapy within 3 months (continuous oral or infusion for more than 14 days). * Received live attenuated vaccinewithin 1 month before this vaccination, other vaccines are inoculated within 14 days before this vaccination. * Received blood products within 3 months before enrolment. * Received other research drugs within 6 months before enrolment. * Other circumstances judged by investigators are not suitable for this clinical trial.