Immunocompetent adults do not require routine booster vaccinations. Although the anti-HBs titer decreases gradually, the duration of the protective effect is long. 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. Booster vaccination is required only for the following groups of people with antibody levels up to <10 mIU/mL: hemodialysis patients, because vaccine protection persists only with antibody levels ≥10 mIU/mL. Immunocompromised individuals at continued risk of exposure, e.g., HIV-infected individuals. Routine single-antigen booster vaccination is not recommended for immunocompetent children and adults who have responded to a complete series of vaccinations. Long-term follow-up studies support that immune memory exists more often in people who have lost protective antibody levels.