Aim/Background: This study aims to investigate the necessity and efficacy of a hepatitis B virus (HBV) vaccine booster in children after liver transplantation. A universal mass vaccination program of HBV was launched for 20 years in Taiwan. The coverage rate is high and the effect is great. The carrier rate of the population under vaccine coverage decreased from 10-15% to < 1%. In Taiwan, most children who receive organ transplantation were vaccinated with HBV vaccine in infancy and well before the transplantation procedure. This vaccination background information on Taiwanese children is quite unique and not similar to the other countries in the world. The antibody generated by the vaccine usually wanes after a certain period even in normal subjects, let alone in subjects who receive organ transplantation and immunosuppressive agents after transplantation. At present, Taiwan is still an HBV hyperendemic area and the risk of exposure to HBV cannot be overlooked. Should children be given a booster dose of HBV vaccine after transplantation? And how about the immunogenicity of this booster dose in these immunocompromised hosts? If these children cannot obtain an adequate antibody titer, will the risk of HBV infection increase? This study is designed to answer these questions. As a pediatric hepatologist, the author's routine work is to take care of children who underwent liver transplantation. To take advantage of this, the investigators decided to study the efficacy and necessity of HBV booster vaccine in these patients. However, the results of this study should be able to be applied to any kind of solid organ transplanted patients.