Antibody Persistence & Immune Memory in Healthy Adults Previously Vaccinated With Twinrix Adult

Blood sampling at Year 16, 17, 18, 19 and 20 and at the time of challenge dose administration and 14 days and one month after challenge dose administration (if challenge dose needed).

Engerix-B will be administered to subjects who are not seroprotected against hepatitis B

Havrix will be administered to subjects who are seronegative for anti-HAV antibodies