Sequential Immunization of Two Doses of Inactivated COVID-19 Vaccine (Omicron) in Vaccinated 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/3 doses of inactivated COVID-19 vaccine or mRNA vaccine for ≥ 3 months. * Female participants who are not nursing or pregnant at the time of enrolment (negative urine pregnancy test) and have no family planning within the first 7 months after enrollment. 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 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 Severe Acute Respiratory Syndrome (SARS) and Middle East Respiratory Syndrome (MERS) (self-report, on-site inquiry); * Has vaccinated with one or four doses and above COVID-19 vaccine; * Axillary temperature ≥37.3℃ (Tympanic temperature ≥ 37.6 ℃); * Previous allergic reactions to drug or vaccination (such as acute allergic reactions, urticaria, eczema, dyspnea, angioneurotic edema or abdominal pain) or allergy to known components of COVID-19 vaccine (self-report, onsite enquiry); * Have a history of hospital-diagnosed thrombocytopenia or other coagulation disorders (self-report, onsite enquiry); * With known immunological impairment or immunocompromised diagnosed by the hospital (self-report, onsite enquiry); * Have an uncontrolled epilepsy and other progressive neurological diseases or a history of Guillain-Barre syndrome (self-report, onsite enquiry); * Received whole blood, plasma and immunoglobulin therapy within 1 month (self-report, onsite enquiry); * Known or suspected severe illness includes: respiratory illness, acute infection or active attacks of chronic illness, liver and kidney disease, severe diabetes mellitus, malignant tumor, infectious or allergic skin disease, Human Immunodeficiency Virus (HIV) infection (self-report, onsite enquiry, provide test report if available); * With hospital-diagnosed serious cardiovascular diseases (cardiopulmonary failure, drug-uncontrolled hypertension (physical examination of systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg), acute attacks of chronic respiratory illness (self-report, onsite enquiry); * Received live attenuated vaccines within 1 month before vaccination (self-report, onsite enquiry); * Received inactivated vaccines within 14 days before vaccination (self-report, onsite enquiry); * Participating or planning to participate in other interventional vaccine clinical trials during this study * Other vaccination-related contraindications considered by investigators.