The goal of this open-label clinical trial is to evaluate the safety and immunogenicity of INAVAC (Vaksin Merah Putih - UNAIR SARS-CoV-2 (Vero Cell Inactivated)) Vaccine as Homologue Booster in Adult Subjects in Indonesia. The main question it aims to answer is: "To evaluate the humoral immunogenicity profile at 28 days following vaccination with INAVAC vaccine as homolog booster administered intramuscularly in healthy adults age 18 years and above". Participants will be administered one dose of vaccination for the third dose (booster vaccination) intramuscularly.

This is an open-label trial. There will be only one group in the study. This group will receive the INAVAC vaccine as a booster. The inclusion and exclusion criteria for the subjects are listed below. All subjects will be followed for 6 months. The vaccine will be administered intramuscularly. This study will have one interim and one full analysis report. A Data Safety Monitoring Board will be commissioned for this study to evaluate safety data over the study period and to review any events that meet a specific study pausing rule or any other safety issue that may arise. They will review the 7 and 28-day safety data following the vaccine, and then the 3 and 6-month safety data. The immunogenicity data will be evaluated for 6 months.

Inclusion Criteria:

1. Healthy adults and elderly, males and females, 18 years of age and above. Healthy status will be determined by the investigator based on medical history, clinical laboratory results, vital sign measurements, and physical examination at screening.
2. Subjects already received 2 (two) doses of INAVAC vaccines (5 ug), mostly during the phase I/II/III clinical trial of this vaccine. The interval between the second primary injection and the booster is 12-18 months.
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
5. Female subjects of childbearing potential must agree to postpone pregnancy from at least 21 days before enrollment and through 6 months after the vaccination.
6. Participants agree not to donate bone marrow, blood, and blood products from the first study vaccine administration until 3 months after receiving the vaccine.
7. Participants must be willing to provide verifiable identification, have means to be contacted and to contact the investigator during the study.

Exclusion Criteria:

1. Subjects concomitantly enrolled or scheduled to be enrolled in another vaccine trial
2. Evolving mild, moderate, and severe illness, especially infectious diseases or fever (axillary temperature 37.5oC or more) concurrently or within 7 days before study vaccination. This includes respiratory or constitutional symptoms consistent with SARS-CoV-2 (cough, sore throat, difficulty in breathing, etc)
3. Known history of allergy to any component of the vaccines
4. History of uncontrolled coagulopathy or blood disorders contraindicating intramuscular injection
5. Any autoimmune or immunodeficiency disease/condition
6. Subjects who have received in the previous 4 weeks a treatment likely to alter the immune response (intravenous immunoglobulin, blood-derived products, long-term corticosteroid - more than 2 weeks, and so on), OR anticipation of the need for immunosuppressive treatment within 6 months after last vaccination. The use of topical or nasal steroids will be permitted. Inhaled glucocorticoids are prohibited.
7. Unstable chronic disease, inclusive of uncontrolled hypertension, congestive heart failure, chronic obstructive pulmonary disease, asthma, chronic urticaria, and diabetes requiring the use of medicine. The final decision regarding this condition will be made by the attending field clinicians or investigators.
8. Any abnormality or chronic disease which according to the investigator might interfere with the assessment of the trial objectives
9. Individuals who previously received any vaccines against Covid-19, other than INAVAC.
10. Subjects already immunized with any other vaccines within 4 weeks prior and expect to receive other vaccines within 60 days following the booster dose
11. Individuals who have a previously ascertained COVID-19 in 1 month (for mild, moderate, or asymptomatic people) or 3 months (for severe COVID-19) before the first recruit of this study, or in close contact in the last 14 days with a confirmed case of Covid-19.
12. Positive test for SARS-CoV-2 (Antigen or PCR) at screening before the vaccination. Testing may be repeated during the screening period if exposure to a positive confirmed case of SARS-CoV-2 is suspected, at the discretion of the investigator.
13. History of alcohol or substance abuse
14. HIV patients.
15. Malignancy patients within 3 years before study vaccination.
16. Any neurological disease or history of significant neurological disorder such as meningitis, encephalitis, Guillain-Barre Syndrome, multiple sclerosis, etc
17. Vital sign abnormalities and clinical laboratory abnormalities as decided by the investigators. Vital sign measurements and clinical laboratory testing may be repeated before the final decision.
18. Women who are pregnant or who plan to become pregnant during the study.
19. Participant has a major psychiatric problem or illness
20. Participant cannot communicate reliably with the investigator
21. Participant has contraindications to intramuscular injection and blood draws, such as bleeding disorders or phobia.
22. The participant had major surgery within 12 weeks before vaccination which will not be fully recovered, or has major surgery planned during the time the participant is expected to participate in the study or within 6 months after the last dose of study vaccine administration.
23. Any condition that in the opinion of the investigators would pose a health risk to the subject if enrolled or could interfere with the evaluation of the vaccine or interpretation of the study results
24. Study team members.
25. Subject planning to move from the study area before the end of the study period.