This study demonstrates that a booster dose of the virosome-formulated, aluminum-free hepatitis A vaccine Epaxal (Berna Biotech) is highly immunogenic in subjects who received a single primary dose of this vaccine 18-54 months earlier. There were no significant differences in geometric mean antibody titers (GMTs) among subjects who received the booster dose 18-29 months (GMT, 2330 mIU/mL), 30-41 months (GMT, 2395 mIU/mL), or 42-54 months (GMT, 2432 mIU/mL) after primary vaccination, indicating that delays in the administration of booster vaccination do not lead to a loss of immunogenicity.