Eligible participants were randomly assigned (1:1:1), using block randomization stratified, to either 3 groups. Group 1 will be immunized inactivated SARS-CoV-2 vaccine and varicella vaccine in day 0 followed by measles-mumps-rubella vaccine(MMR) in day 28.

Group 2 will be immunized varicella vaccine in day 0 followed by SARS-CoV-2 vaccine and MMR in day 28.

Group 3 will be immunized inactivated SARS-CoV-2 vaccine in day 0 followed by MMR and varicella vaccine in day 28.

Vaccines were administered 28 days apart, with blood samples taken on day 0 and day 28 before vaccination, and on day 56. Local and systemic symptoms and serious adverse events following immunzation were collected.

Inclusion Criteria:

• Aged 4 years old
• With legal guardian signing the informed consent
• Able to participate in all planned visits and comply with all research procedures (such as completing diary cards of adverse events and participating visits)
• With clear vaccination records including no SARS-CoV-2 vaccine history, with 1 varicella vaccine and 2 measles-containing vaccines history
• the last attenuated vaccine was administered ≥28 days ago, and other vaccines administered ≥14 days ago
• With an axillary temperature ≤37.5℃ at the time of vaccination

Exclusion Criteria:

First dose exclusion criteria:

• With SARS-CoV-2 infection in the last 3 months (any of the following conditions shall be met: positive nucleic acid test; positive antigen test; suspected SARS-CoV-2 infection symptoms of subject or close contacts)
• allergic to any substance of the vaccine or with a severe allergic reactions history of vaccines (such as acute allergic reactions, angioneurotic edema, breathing difficulties, etc.)
• Suffering uncontrolled epilepsy or progressive nerve system diseases, or with a history of Gribali syndrome
• With acute illness, severe or acute attack of chronic illness or fever
• With immunodeficiency or weakened immune system or being treated with immunosuppressant drugs (oral steroids)
• Received non specific immunoglobulin within 3 months
• Any situation that researchers thought that might influence the consequence of the clinical trial Second dose exclusion criteria：
• Occurrence of any serious adverse event that may be related to the previous dose of study vaccine
• Occurrence of systemic adverse reactions or allergic reactions with a severity level ≥ 3 recognized by researchers after vaccination
• Any new situations that fit the first dose exclusion criteria
• Any situation that researchers thought that might influence the consequence of the clinical trial