Inclusion Criteria:

- Participants are eligible to be included in the study only if the following
conditions are met:

1. Male or female aged 12-17 years old when signing ICF;

2. Participants who were fully vaccinated with 2 doses of mRNA COVID-19 vaccine
(mRNA-1273 from Moderna or Comirnaty from Pfizer, participants previously
vaccinated with mRNA1273 are more preferred) and the interval between the last
dose and this study vaccination is 3 to 12 months;

3. The participant and/or his legal guardian can sign written ICF, and can fully
understand the trial procedure, the risk of participating in the trial, and
other interventions that can be selected if they do not participate in the
trial;

4. The participant and/or his legal guardian has the ability to read, understand,
and fill in record cards;

5. Healthy participants or participants with pre-existing medical conditions who
are in stable condition. The "pre-existing medical conditions" include but not
limited to hypertension, diabetes, chronic cholecystitis and cholelithiasis,
chronic gastritis that meet the described criteria. A stable medical condition
is defined as disease not requiring significant change in therapy or no need
for hospitalization as a consequence of worsening disease state for at least 3
months prior to enrollment;

6. Fertile men and women of childbearing potential voluntarily agree to take
effective contraceptive measures from signing ICF to 6 months after the study
vaccination; the pregnancy test results of women of childbearing potential are
negative on screening.

Exclusion Criteria:

- A participant who conforms to any of the following criteria should be excluded from
the study:

1. Presence of fever within 3 days before the study vaccination;

2. A positive result of nucleic acid test for SARS-CoV-2 during the screening
period or previously diagnosed with COVID-19;

3. A history of infection or disease related to severe acute respiratory syndrome
(SARS), Middle East respiratory syndrome (MERS), or other disease with
corresponding use of immunosuppressants;

4. A history of allergic reactions to any vaccine or drug, such as allergy,
urticaria, severe skin eczema, dyspnea, laryngeal edema, and angioneurotic
edema;

5. A medical or family history of seizure, epilepsy, encephalopathy and psychosis;

6. Immunocompromised patients suffering from immunodeficiency diseases, important
organ diseases, immune diseases (including Guillain-Barre Syndrome [GBS],
systemic lupus erythematosus, rheumatoid arthritis, asplenia or splenectomy
caused by any circumstances, and other immune diseases that may have an impact
on immune response in the investigator's opinion), etc.;

7. Long-term use of immunosuppressant therapy or immunomodulatory drugs for ≥14
days within the first six months prior to enrollment. Whereas short-term (≤14
days) use of oral, inhaled and topical steroids are allowed;

8. Patients on antituberculosis therapy;

9. Presence of severe or uncontrollable cardiovascular diseases, or severe or
uncontrollable disorders related to endocrine system, blood and lymphatic
system, liver and kidney, respiratory system, metabolic and skeletal systems,
or malignancies (skin basal cell carcinoma and carcinoma in-situ of cervix are
exceptions and will not be excluded), such as severe heart failure, severe
pulmonary heart disease, unstable angina, liver failure, or uremia;

10. Contraindications for intramuscular injection or intravenous blood sampling,
including thrombocytopenia and other blood coagulation disorders;

11. Participants who received any immunoglobulin or blood products in the previous
3 months before enrollment, or plan to receive similar products during the
study;

12. Participants who received other investigational drugs within 1 month before the
study vaccination;

13. Participants who is at the acute stage of illness, such as acute onset of
chronic heart failure, acute sore throat, hypertensive encephalopathy, acute
pneumonia, acute renal insufficiency, acute cholecystitis;

14. Participants received other drugs or vaccines used to prevent COVID-19, but
participants previously received Vaxzevria will not be excluded;

15. Participants vaccinated with influenza vaccine within 14 days or with other
vaccines within 28 days before the study vaccination;

16. Those who donated blood or had blood loss (≥450 mL) within 3 months before the
vaccination or plan to donate blood during the study period;

17. Those who are pregnant or breast-feeding or plan to be pregnant during the
study period;

18. Those who plan to donate ovum or sperms during the study period;

19. Those who cannot follow the trial procedures, or cannot cooperate to complete
the study due to planned relocation or long-term outing;

20. Those unsuitable for participating in the clinical trial as determined by the
investigator because of other abnormalities that are likely to confuse the
study results, or non-conformance with the maximal benefits of the
participants;

21. Those who are tested positive for HIV.