In most countries, routine single-antigen booster vaccination is not recommended for immunocompetent children and adults who have responded to a complete series of vaccinations. Booster vaccines should be given to the following groups if their antibody levels fall to <10mIU/mL: 1. hemodialysis patients, because vaccine protection persists only if antibody levels are ≥10 mIU/mL. 2. immunocompromised individuals at continued risk of exposure. Although anti-HBs titers decrease gradually, the duration of protection is longer. The protective effect of the vaccine is estimated to last up to 30 years in individuals who respond to the first vaccine series. Even when anti-HBs levels are reduced or undetectable, the vaccine still protects against clinical disease due to the priming of memory cells (capable of triggering a recall response upon viral attack) and long-term cellular immunity.