Additional Study Information: In this study, we will collect information from the medical record of pediatric liver transplant recipients who received live vaccines as part of their routine post-transplant care. This data will be combined with data from other centers across the United States and Canada to help give us a better understanding of the safety and efficacy of live vaccines in the patient population. Vaccine-preventable illness continues to have resurgence in the United States and globally. Specifically, mumps and measles outbreaks have occurred with increasing frequency and distribution in the past several years. Pediatric solid organ transplant recipients are at increased risk for infection. Liver transplant recipients may be at increased risk as up to 25% are transplanted prior to 1 year of age and did not receive a primary vaccination series. Live-viral vaccination was previously not recommended after pediatric solid organ transplantation due to continued immunosuppression and limited data on the risk of side effects. However, there is new data to suggest that live vaccines can be safe and efficacious in certain transplant recipients. If you have any questions regarding this study, please contact the Transplant Clinical Research Center (TCRC).