Few studies have examined the duration of protection following vaccination against hepatitis A virus (HAV) with currently licensed HAV vaccines. This study explored the long-term immunogenicity in individuals vaccinated with the virosomal hepatitis A virus, Epaxal. Adult volunteers (N = 130) previously enrolled into four different studies between 1992 and 1994 and who had completed a 0/12-month immunization regimen (primary and booster dose) were asked to participate in this follow-up study. Yearly anti-HAV titers up to 6 years following booster vaccination, and then once 9-11 years after booster were measured using two assays, Enzygnost and AxSYM HAVAB 2.0. Based on the Enzygnost assay, the seroprotection rate 9-11 years after booster was 100%, with a geometric mean concentration (GMC) of anti-HAV antibodies of 526 mIU/ml. Females had markedly higher GMCs than males (741 mIU/ml vs. 332 mIU/ml). Using an anti-HAV cut-off titer of >or=10 mIU/ml, a linear mixed mathematical model predicted a median duration of protection of 52.1 years. A duration of protection >or= 35.7 years was predicted for 95% of subjects. A more stringent cut-off of >or=20 mIU/ml shortened the median predicted duration of protection to 45.0 years. In conclusion, a two-dose Epaxal vaccination regimen confers in healthy adults a real-time protection of at least 9-11 years; this protection is predicted to last at least 30 years in over 95% of individuals. Further studies are necessary to assess the real duration of seroprotection and whether an additional booster is necessary later.