Homologous Booster Study of COVID-19 Protein Subunit Recombinant Vaccine

Inclusion Criteria: 1. Clinically healthy subjects aged 18 years and above. 2. Subjects had previously received complete primary doses of IndoVac® with the last dose administered minimum 12 months but no longer than 18 months prior to inclusion. 3. Subjects have been informed properly regarding the study and signed the informed consent form. 4. Subjects will commit to comply with the instructions of the investigator and the schedule of the trial. Exclusion Criteria: 1. Subjects concomitantly enrolled or scheduled to be enrolled in another trial. 2. Subjects had received booster dose of COVID-19 vaccine. 3. History of COVID-19 within 3 months prior to enrollment (based on anamnesis or other examinations). 4. Evolving mild, moderate, or severe illness, especially infectious disease, or fever (body temperature ≥37.5°C, measured with infrared thermometer/thermal gun). 5. Women who are pregnant or planning to become pregnant during the study period (judged by self- report of subjects and urine pregnancy test results). 6. History of uncontrolled asthma, allergy to vaccines or vaccine ingredients, and severe adverse reactions to vaccines, such as urticaria, dyspnea, and angioneurotic edema. 7. History of uncontrolled coagulopathy or blood disorders contraindicating intramuscular injection. 8. Patients with serious chronic diseases (serious cardiovascular diseases, uncontrolled hypertension or diabetes, liver or kidney diseases, malignant tumors, etc.) which according to the investigator might interfere with the assessment of the trial objectives. 9. History of confirmed or suspected immunosuppressive or immunodeficient state or in the previous 4 weeks had received a treatment likely to alter the immune response (intravenous immunoglobulins, blood-derived products, or long- term corticosteroid therapy (\> 2 weeks)). 10. History of uncontrolled epilepsy or other progressive neurological disorders, such as Guillain- Barre Syndrome. 11. Subjects had received any vaccination (other than COVID-19 vaccine) within 1 month before IP immunization. 12. Subjects plan to move from the study area before the end of study period.