The goal of this clinical trial is to evaluate immune response and safety of SARS-CoV-2 subunit protein recombinant vaccine (IndoVac) as a heterologous booster dose in healthy children 12-17 years of age.

Participants who had received complete primary doses of inactivated (Sinovac®) COVID-19 Vaccine will be given IndoVac as a booster dose.

This trial is open label prospective intervention study. In this study 150 subjects who had received complete primary doses of inactivated (Sinovac®) COVID-19 Vaccine and willing to participate in the booster study by signing the consent form, will be involved in this trial.

Subject will receive one booster dose of SARS-CoV-2 subunit protein recombinant vaccine (IndoVac).

Inclusion Criteria:

1. Clinically healthy children 12-17 years of age.
2. Subjects who have previously received complete primary series of inactivated (Sinovac®) COVID-19 vaccine with the last dose administered a minimum of 6 months prior to inclusion but not longer than 12 months prior to inclusion.
3. Parent/legal guardian and subject has been informed properly regarding the study, and signed the informed consent form (parent/legal guardian) and assent form (subject).
4. Parent and/or legal guardian will commit to comply with the instructions of the investigator and the schedule of the trial.

Exclusion Criteria:

1. Subject concomitantly enrolled or scheduled to be enrolled in another trial.
2. Subject who has received booster dose of COVID-19 vaccine.
3. Subject who has history of COVID-19 in the last 3 months (based on anamnesis or other examinations).
4. Evolving mild, moderate or severe illness, especially infectious disease or fever (body temperature ≥37.5℃, measured with infrared thermometer/thermal gun).
5. History of uncontrolled asthma, history of allergy to vaccines or vaccine ingredients, and severe adverse reactions to vaccines, such as urticaria, dyspnea, and angioneurotic edema.
6. History of uncontrolled coagulopathy or blood disorders contraindicating intramuscular injection.
7. Patients with serious chronic diseases (serious cardiovascular diseases, uncontrolled hypertension and diabetes, liver and kidney diseases, malignant tumors, etc) which according to the investigator might interfere with the assessment of the trial objectives.
8. Subjects who have any history of confirmed or suspected immunosuppressive or immunodeficient state, or received in the previous 4 weeks a treatment likely to alter the immune response (intravenous immunoglobulins, blood-derived products or long-term corticosteroid therapy (> 2 weeks)).
9. Subjects who have history of uncontrolled epilepsy or other progressive neurological disorders, such as Guillain-Barre Syndrome.
10. Subjects receive any vaccination (other than COVID-19 vaccine) within 1 month before and after IP immunization.
11. Female who are pregnant or planning to become pregnant during the study period (judged by self-report of subjects and urine pregnancy test results).
12. Subjects plan to move from the study area before the end of study period.