Study on Sequential Immunization of Omicron Inactivated COVID-19 Vaccine and Prototype Inactivated COVID-19 Vaccine in Population Aged 18 Years Old and Above

Inclusion Criteria: * Healthy subjects aged 18 years old and above. * By asking for medical history and physical examination, the investigator judged that the health condition is well. * Has been vaccinated 2 or 3 doses of inactivated COVID-19 vaccine manufactured by BIBP more than 3 months. * 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 12 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. * 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 clinical study protocol. Exclusion Criteria: * Confirmed or suspected cases of SARS-CoV-2 Infection. * Has a history of SARS, MERS, SARS-CoV-2 infection (self-report, on-site inquiry). * Received one dose or more than 3 doses of inactivated COVID-19 vaccine manufactured by BIBP * Received COVID-19 vaccine manufactured bu other companies (include mRNA, recombinant protein vaccines, vector vaccines, inactivated vaccines, etc.) * Axillary body temperature \> 37.3 ℃ before vaccination * 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 (self-report, on-site inquiry). * History of hospital-diagnosed thrombocytopenia or other coagulation disorder * Known immunological impairment or low level with hospital diagnosis * History of uncontrolled epilepsy, other progressive neurological disorders, or Guillain-Barre syndrome (self-report, on-site inquiry). * Known or suspected concomitant serious diseases include: respiratory disease, acute infection or active chronic disease, liver and kidney disease, severe diabetes, malignant tumor, infection or allergic skin disease, HIV infection. * Severe cardiovascular disease (cardiopulmonary failure, uncontrolled hypertension, etc.) diagnosed by the hospital, active chronic respiratory disease * Received live attenuated vaccine within 1 month before enrollment. * Received other vaccines within 14 days before enrollment. * Be participating in or plan to participate in other vaccine clinical trials during this study. * Contraindications related to vaccination as considered by other investigators.