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