The purpose of this study is to evaluate how well children responds to the recommended trivalent subviron influenza vaccine (flu shot). Children who have had a liver transplant and receive their post-transplant care at Mount Sinai and their siblings are being invited to participate in this study. Study procedures will include a review of medical records, physical examination, and up to 2 flu shots. All participants will receive the flu vaccine. Four weeks after the first flu shot, participants will return to the clinic for a physical exam, blood sample collection, and a second flu shot as recommended. Following each vaccination, parents will be asked to record their children's temperatures and any experienced side effects in a diary card for two weeks. Participants will be involved in study related procedures for up to 8 months.