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

Inclusion Criteria: All subjects must satisfy the following criteria at entry into each of the long-term follow-up visits: * Subjects who the investigator believes that they can and will comply with the requirements of the protocol should be enrolled in the study. * A male or female who received the complete primary vaccination course in the primary study Hepatitis A and B vaccine (HAB), HAB-028 (208127/021). * Written informed consent obtained from the subject. All subjects must satisfy the following criteria at entry into the challenge dose phase: * Subjects who the investigator believes that they can and will comply with the requirements of the protocol. * A male or female who received the complete primary vaccination course in the primary study HAB-028 (208127/021). * Written informed consent obtained from the subject. * Subjects who participated in the long-term follow-up (LTFU) phase of the HAB-028 (208127/021) study and for whom the antibody concentrations were below the cut-off at the last available follow-up time-point. * Female subjects of non-childbearing potential may be enrolled in the study. * Female subjects of childbearing potential may be enrolled in the study, if the subject: * has practiced adequate contraception for 30 days prior to vaccination, and * has a negative pregnancy test on the day of vaccination, and * has agreed to continue adequate contraception for two months after the administration of the challenge dose. Exclusion Criteria: The following criteria should be checked before entry into each of the long-term follow-up visits. If any exclusion criterion applies, the subject must not be included in the study: * Use of any investigational or non-registered product within 30 days prior to blood sampling. * Administration of a hepatitis A, hepatitis B or combined hepatitis A and B vaccine outside the study procedures, since the primary study HAB-028 (208127/021). * History of hepatitis A or hepatitis B infection since the primary study HAB-028 (208127/021). * Administration of hepatitis A or hepatitis B immunoglobulins and/or any blood products within three months prior to blood sampling. The following criteria should be checked before the challenge dose is administered. If any apply, the subject must not be included in the challenge dose phase: * Use of any investigational or non-registered product within 30 days prior to study start or planned use during the study. * Administration of a hepatitis A, hepatitis B or combined hepatitis A and B vaccine between the last LTFU visit and the challenge dose visit. * History of hepatitis A or hepatitis B infection between the last LTFU visit and the challenge dose visit. * History of anaphylactic reactions following the administration of vaccines. * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccines. * Acute disease and/or fever at the time of enrolment. * Pregnant or lactating female. * Female planning to become pregnant or planning to discontinue contraceptive precautions.