 Children eligibleoverdue for their pre school HBV vaccine booster will be identified by reviewing clinical records at the John Radcliffe Hospital. The parentslegal guardians of these children will be written to and advised that their child should receive have received a booster dose of HBV vaccine. This letter will also inform parents legal guardians that they have the option of participating in this study looking at the persistence of HBV vaccine induced antibodies through early childhood and the response to a booster dose of HBV vaccine, and that this study will be conducted by the Oxford Vaccine Group in the participants homes. Parents will be advised that if they do not wish to take part in this research study they should arrange to contact their GP to discuss whether their child requires a booster dose of hepatitis B vaccine. 