Immunogenicity and Safety of GlaxoSmithKline (GSK) Biologicals' Herpes Zoster Subunit (HZ/su) Vaccine GSK1437173A in Adults With a Prior Episode of Herpes Zoster

Inclusion Criteria: * Subjects with a physician-documented history of HZ. * Subjects who, in the opinion of the investigator, can and will comply with the requirements of the protocol. * A male or female aged 50 years or older at the time of the first vaccination. * Written informed consent obtained from the subject. * Female subjects of non-childbearing potential may be enrolled in the study. * Non-childbearing potential is defined as pre-menarche, current tubal ligation, hysterectomy, ovariectomy or post-menopause. * 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 during the entire treatment period and for 2 months after completion of the vaccination series. Exclusion Criteria: * Active Herpes Zoster infection (a case is considered no more active when all lesions have at least turned to crusts). * Use of any investigational or non-registered product other than the study vaccine/product within 30 days preceding the first dose of study vaccine/product, or planned use during the study period. * Chronic administration (defined as \> 14 consecutive days) of immunosuppressants or other immune-modifying drugs within six months prior to the first vaccine dose. For corticosteroids, a prednisone dose of \<20 mg/day, or equivalent, is allowed. Inhaled, topical and intra-articular corticosteroids are allowed. * Administration of long-acting immune-modifying drugs within six months prior to the first vaccine dose or expected administration at any time during the study period. * Administration or planned administration of a live vaccine in the period starting 30 days before the first dose of study vaccine and ending 30 days after the last dose of study vaccine, or, administration or planned administration of a non-replicating vaccine within 8 days prior to or within 14 days after either dose of study vaccine. * Concurrently participating in another clinical study, at any time during the study period, in which the subject has been or will be exposed to an investigational or a non-investigational vaccine/product. * Previous vaccination against VZV or HZ and/or planned administration during the study of an HZ vaccine other than the study vaccine. * Any confirmed or suspected immunosuppressive or immunodeficient condition resulting from disease or immunosuppressive/cytotoxic therapy. * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccine/product. * Acute disease and/or fever at the time of enrolment. * Fever is defined as temperature ≥ 37.5°C /99.5°F for oral, axillary or tympanic route, or ≥ 38.0°C /100.4°F on rectal route. The preferred route for recording temperature in this study will be oral. * Subjects with a minor illness (such as mild diarrhoea, mild upper respiratory infection) without fever may, be enrolled at the discretion of the investigator. * Administration of immunoglobulins and/or any blood products within the 3 months preceding the first dose of study vaccine or planned administration during the study period. * Any condition which, in the judgment of the investigator, would make intramuscular injection unsafe. * Pregnant or lactating female. * Female planning to become pregnant or planning to discontinue contraceptive precautions before Month 4 (i.e. 2 months after the last dose of study vaccine).