Second Dose of Varicella Vaccine in Healthy Children

Inclusion criteria

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

Exclusion criteria

• History of varicella-zoster virus (VZV) infection (based on parental report or medical record).
• Known exposure to VZV within 4 weeks prior to screening.
• Receipt of more than one dose of varicella vaccine.
• Interval between first dose of varicella vaccine and enrollment less than 3 months.
• Known allergy to any vaccine component or history of severe allergic reaction to any vaccine (e.g., urticaria, dyspnea, angioedema).
• 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).
• Poorly controlled chronic disease or history of severe systemic disease.
• Current or history of severe neurological disorders (e.g., epilepsy, convulsions or seizures excluding simple febrile seizures) or psychiatric disorders.
• 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.
• Receipt of blood products or immunoglobulins within 180 days prior to screening, or planned receipt during the study period.
• Receipt of other investigational drugs or vaccines within 30 days prior to screening, or planned receipt during the study period.
• 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.
• Fever (axillary temperature >37.2°C), abnormal vital signs, or clinically significant abnormal findings on physical examination on the day of vaccination.
• Skin conditions at the intended injection site (e.g., inflammation, ulceration, rash, scar, or other conditions that may interfere with administration or safety assessment).
• Acute illness or exacerbation of chronic disease within 7 days prior to vaccination, or known or suspected active infection.
• Any condition that, in the opinion of the investigator, would make the participant unsuitable for participation in the study.