An open experimental design was used in this study, and 240 subjects were planned to be enrolled. To evaluate the immunogenicity and safety of recombinant Novel Coronavirus vaccine (CHO cells) after sequential intensification in populations vaccinated with two doses of marketed Novel Coronavirus inactivated vaccine.

One dose of recombinant novel coronavirus vaccine (CHO cell) for test was injected into the upper arm deltoid muscle muscle of the subject

Inclusion Criteria:

1. Healthy subjects aged 3 to 17 years who can provide vaccination information proving that they have received two doses of commercially available Novel Coronavirus inactivated vaccine in the last 6-9 months;
2. The subject voluntarily agrees to participate in the study (and/or the subject's legal guardian voluntarily agrees to allow the child to participate in the study), and the guardian and the subject (aged 8-17) sign the informed consent, and can provide valid identification; Understand and comply with test protocol requirements;
3. Armpit temperature < 37.3℃ (> 14 years old), armpit temperature < 37.5℃ (≤14 years old);
4. Female subjects of reproductive age agree to use effective contraceptive measures from the beginning to the end of the study.

Exclusion Criteria:

1. History of confirmed or asymptomatic coronavirus infection or positive nucleic acid test of novel Coronavirus infection;

2. SARS virus disease history or SARS-COV-2 history;

3. Have taken antipyretics or painkillers within 24 hours before vaccination;

4. Have a history of severe allergy to any component of the test vaccine, including aluminum preparation, such as anaphylactic shock, allergic laryngeal edema, allergic purpura, thrombocytopenic purpura, local allergic necrosis reaction (Arthus reaction), dyspnea, angiopantic edema, etc.;

5. Persons suffering from the following diseases:

   1. In the past 7 days, have suffered from digestive diseases (such as diarrhea, abdominal pain, vomiting, etc.);
   2. congenital malformations or developmental disorders, genetic defects, severe malnutrition, etc.;
   3. have thrombocytopenia, any coagulation dysfunction or receive anticoagulant treatment, etc.;
   4. Neurological disorders or neurodysplasia (e.g., migraines, epilepsy, stroke, seizures in the last three years, encephalopathy, focal neurological deficits, Guillain-Barre syndrome, encephalomyelitis or transverse myelitis); History of mental illness or family history;
   5. History of congenital or acquired immune deficiency or autoimmune disease or treatment with immunomodulators within 6 months, such as immunosuppressive doses of glucocorticoids (dose reference: equivalent to prednisone 20mg/ day, over a week); Or monoclonal antibodies; Or thymosin; Or interferon; However, topical use (such as ointments, eye drops, inhalants or nasal sprays) is allowed;
   6. Known to have been diagnosed with an infectious disease, such as active tuberculosis, viral hepatitis or treponema pallidum;

6. Subunit vaccine and inactivated vaccine should be administered within 14 days before experimental vaccine, and live attenuated vaccine should be administered within 30 days before experimental vaccine;

7. Have received blood or blood-related products, including immunoglobulin, within 3 months prior to the trial vaccine; Or planned for use during the study period;

8. People aged 3-17 years who have participated in or are participating in other COVID-19 related clinical trials or have received other COVID-19 vaccines, except those who have completed two doses of marketed Novel Coronavirus inactivated vaccine within 6-9 months;

9. Pregnant (including positive urine pregnancy), or breast-feeding; The Investigator believes that the subject has any disease or condition that may place the subject at unacceptable risk; Subjects cannot meet the requirements of the program; Conditions that interfere with the assessment of vaccine response.