This is a phase Ⅳ clinical trial of live attenuated varicella vaccine manufactured by Sinovac (Dalian) Vaccine Technology Co., Ltd.The primary objective of this study is to evaluate the immunogenicity of live attenuated varicella vaccine co-administered with MMR vaccine or DTaP vaccine. The secondary objective is to evaluate the safety of the vaccines when administered simultaneously.

A total of 720 children aged 18~24 months who have not received varicella vaccine, the second dose of MMR and the fourth dose of DTaP (or vaccines containing related ingredients) will be recruited and randomly assigned to one of three study groups (1:1:1 ratio): Group A, Group B and Group C. Participants in Group A will receive varicella vaccine and DTaP simultaneously on Day 0 and receive MMR on Day 30. Participants in Group B will varicella vaccine and MMR simultaneously on Day 0 and receive DTaP on Day 30. Participants in Group C will receive varicella vaccine only.

Inclusion Criteria:

• Infants aged 18-24 months;
• have completed 3 doses of DTaP for primary immunization in their first year of life without the fourth dose of Dtap-containing vaccine;
• have completed the first dose of MMR in their first year of life without a second dose of MMR;
• Guardians of participants who are able to understand and voluntarily sign informed consent;
• Provision of legal proof of identity.

Exclusion Criteria:

• Having a history of previous varicella vaccination;
• Having a history of chickenpox, pertussis, diphtheria, tetanus, measles, mumps, and rubella;
• Having a history of uncontrolled chronic or serious diseases, including but not limited to cardiovascular diseases, hematological diseases, liver and kidney diseases, digestive diseases, respiratory diseases, malignant tumors, major functional organ transplantation, etc.;
• Presence of autoimmune diseases, immunodeficiency diseases (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid disease, asplenia, functional asplenia, HIV infection);
• Presence of abnormal coagulation function (e.g. coagulation factor deficiency, platelet abnormality);
• Having/Previous having a severe neurological disorder (epilepsy, seizures, or convulsions) or psychosis, or have a family history of psychosis;
• Acute onset of various acute or chronic illnesses within the last 7 days, or known or suspected active infection;
• Receipt of > 14 days of immunosuppressive or other immunomodulatory therapy (prednisone ≥2 mg/kg/ day or its equivalent, except topical or inhaled corticosteroids), cytotoxic therapy within the past 6 months, or planned to receive such therapy during the trial;
• Having received immune globulin or other blood products within the past 3 months or plan to receive such treatment during the trial;
• Receipt of another study drug or vaccine within the past 30 days or plans to receive such drug or vaccine during the trial;
• Administration of live attenuated vaccine within the past 28 days or subunit, inactivated, or other process vaccine within the past 7 days;
• Known allergies to the vaccine or vaccine components, such as urticaria after vaccination, dyspnea, angioedema;
• Having fever on the day of scheduled vaccination (axillary temperature > 37.0 ° C);
• Failure of medical examination on the planned vaccination day;
• Participants have any other factors that, in the judgment of the investigator, make them ineligible to participate in a clinical trial.