This trial is a randomized study to evaluate the immunogenicity of COVID-19 Vaccine (Vero Cell), Inactivated and in population aged 60 years old and 3-17 years old with different schedules compared to that in population aged 18-59 years old with 2-dose schedule. And subjects in the same age are randomly assigned to different immunization schedule groups.

This trial is a randomized study to evaluate the immunogenicity of COVID-19 Vaccine (Vero Cell), Inactivated and in population aged 60 years old and 3-17 years old with different schedules (D0,21,42, D0, 21,111,or D0,21,171)compared to that in population aged 18-59 years old with 2-dose schedule (D0,21). And subjects in the same age are randomly assigned to different immunization schedule groups.

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.