Overview

This study aim to assess the immunogenicity and safety of interchangeable administration of the second dose of varicella vaccine. A total of 300 healthy participants aged 15 months - 12 years will be enrolled. Written informed consent form will be obtained from participants' parents or legally acceptable representatives (and assents will be obtained from participants aged above 9 years old and written consent forms will be prepared and consent will be obtained in writing if possible from the participants aged 3-8 years old.) before enrollment. Participants will be assigned to 5 groups according to the brand of first dose varicella vaccine they received at the age of 12 months. All participants will receive the second dose of varicella vaccine manufactured by Sinovac.

The duration of individual participation from enrollment to the last onsite visit will be a maximum of 42 days. The end of study is considered the completion of the last visit for the last participant in this study.

Blood samples (2.5 - 3.0ml per time) for immunogenicity assessment will be collected. The blood samples will be collected before vaccination and 42 days after vaccination.

Eligibility

Inclusion Criteria:

1. Healthy children aged 15 months to 12 years.
2. Have received one dose of varicella vaccine at 12 months of age in Turkey (verified by vaccination card or official electronic immunization record).
3. Written informed consent obtained from parent(s) or legally authorized representative(s); assent obtained from children as applicable according to local regulations.
4. Able to comply with study procedures, as assessed by the investigator.
5. Able to provide verifiable identification and be contacted for study follow-up during the study period.

Exclusion Criteria:

1. History of varicella-zoster virus (VZV) infection (based on parental report or medical record).
2. Known exposure to VZV within 4 weeks prior to screening.
3. Receipt of more than one dose of varicella vaccine.
4. Interval between first dose of varicella vaccine and enrollment less than 3 months.
5. Known allergy to any vaccine component or history of severe allergic reaction to any vaccine (e.g., urticaria, dyspnea, angioedema).
6. Known or suspected immunodeficiency or autoimmune disease (including but not limited to systemic lupus erythematosus, ankylosing spondylitis, autoimmune thyroid disease, asplenia, functional asplenia, or HIV infection).
7. Poorly controlled chronic disease or history of severe systemic disease.
8. Current or history of severe neurological disorders (e.g., epilepsy, convulsions or seizures excluding simple febrile seizures) or psychiatric disorders.
9. Receipt of ≥14 days of immunosuppressive or immunomodulatory therapy (e.g., prednisone ≥20 mg/day or ≥2 mg/kg/day or equivalent), or cytotoxic therapy within 180 days prior to screening, or planned use during the study period.
10. Receipt of blood products or immunoglobulins within 180 days prior to screening, or planned receipt during the study period.
11. Receipt of other investigational drugs or vaccines within 30 days prior to screening, or planned receipt during the study period.
12. Receipt of live attenuated or nucleic acid vaccines within 28 days prior to screening, or subunit or inactivated vaccines within 7 days prior to screening.
13. Fever (axillary temperature \>37.2°C), abnormal vital signs, or clinically significant abnormal findings on physical examination on the day of vaccination.
14. Skin conditions at the intended injection site (e.g., inflammation, ulceration, rash, scar, or other conditions that may interfere with administration or safety assessment).
15. Acute illness or exacerbation of chronic disease within 7 days prior to vaccination, or known or suspected active infection.
16. Any condition that, in the opinion of the investigator, would make the participant unsuitable for participation in the study.