Varicella is a vaccine-preventable disease, which can be severe in immunosuppressed children. Currently, the (live) vaccine is not recommended in pediatric orthotopic liver transplant recipients. Furthermore, protection due to naturally acquired immunity to VZV or post-immunization isn't well described in this population.The questions asked are:

• What is the influence of the immunosuppression required after orthotopic liver transplantation (OLT) on the maintenance of VZV-specific immunity elicited by wild-type varicella infection before OLT transplantation?
• What is the influence of the immunosuppression required after OLT on VZV-specific immunity elicited by varicella immunization before OLT transplantation?
• What is the influence of the residual immunosuppression at ≥ 12 months after OLT transplantation on the induction of VZV-specific B and T cell responses elicited by VZV vaccination after OLT transplantation?
• What is the influence of the residual immunosuppression at ≥ 12 months after OLT transplantation on the persistence / waning of B and T cell responses elicited by VZV vaccination?

VZV vaccine given to pediatric liver transplant recipients at least one year after transplantation if tehy fulfill inclusion criteria and give informed consent.

Inclusion Criteria:

• Children either awaiting or recipients of a liver transplantation followed at the Children's Hospital of Geneva, Switzerland
• If vaccination offered: > 12 months of age

Exclusion Criteria:

• Known wild type varicella exposure within four weeks of the initial vaccine
• Immunoglobulins administered within the 5 months preceding the receipt of varicella vaccine.
• Antiviral agents administered during the preceding 4 weeks
• Febrile illness (>38.5°) in the 72 hours before vaccine administration
• Chronic aspirin therapy
• Any other live vaccinations within four weeks of receipt of varicella vaccine
• Female patients in childbearing age will have a pregnancy test at enrollment, and at the time of the second vaccine.