The purpose of this study is to investigate the humoral and cell-mediated immune response to the trivalent subviron influenza vaccine in pediatric liver transplant recipients. Additionally, researchers will attempt to correlate the humoral and cell-mediated immune response with parameters that may predict a subject's ability to respond to the vaccine, such as time from transplant, immunosuppressive agents, previous episodes of rejection or cytomegalovirus infection, T-cell numbers and T-cell responses to mitogens or antigens. Understanding the immune response to influenza will provide insight not only into the response to the trivalent subviron influenza vaccine but to other vaccines being developed against agents of bioterrorism, in particular those that may use influenza or related viruses as a backbone. This is a single center, prospective comparative study of the humoral and cell-mediated immune responses to the trivalent subviron influenza vaccine for the 2004-2005 and 2005-2006 influenza seasons in outpatient pediatric liver transplant recipients and their healthy siblings. There are two study arms, the liver transplant recipients (n=44) and the healthy controls (n=22). All study subjects will receive the vaccine at enrollment. Enrollment into this study will be extended into the 2005-2006 influenza season. The trivalent subviron influenza vaccine for the 2005-2006 influenza season will differ from that for the 2004-2005 influenza season with respect to the H3N2 viral strains. The cell-mediated and humoral immune responses of patients to the two H3N2 viral strains will be analyzed together and separately to assess potential differences in immunogenicity between the two viral strains. Humoral and cell-mediated immune responses will be obtained at baseline and following immunization. Healthy siblings are used as controls to ensure equal burden of influenza disease among study participants. Participants will be involved in study related procedures for up to 8 months.