Consistency, Immunogenicity and Safety Study of GlaxoSmithKline (GSK) Biologicals' Herpes Zoster (HZ) Vaccine GSK1437173A in Adults ≥ 50 Years of Age

Inclusion Criteria: * 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: * Use of any investigational or non-registered product other than the study vaccine within 30 days preceding the first dose of study vaccine, or planned use during the study period. * Chronic administration of immunosuppressants or other immune-modifying drugs within six months prior to the first vaccine dose. A prednisone dose of \< 20 mg/day is allowed. Inhaled, topical and intra-articular corticosteroids are allowed. * 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 (E.g. inactivated and subunit vaccines) within 8 days prior to or within 14 days after either dose of study vaccine. * 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. * 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 product. * Previous vaccination against HZ or varicella. * Planned administration during the study of an HZ or varicella vaccine other than the study vaccine. * History of HZ. * 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. * Acute disease and/or fever at the time of enrollment. * Fever is defined as temperature ≥ 37.5°C (99.5°F) on 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 without fever may be enrolled at the discretion of the investigator. * Administration of immunoglobulins and/or any blood products within 3 months preceding the first dose of study vaccine or planned administration during the study period. * Pregnant or lactating females. * Females planning to become pregnant or planning to discontinue contraceptive precautions (if of childbearing potential) before Month 4. * Any condition which, in the opinion of the investigator, prevents the subject from participating in the study. * Any other condition that, in the opinion of the investigator, might interfere with the evaluations required by the study. * Any condition which, in the judgment of the investigator would make intramuscular injection unsafe.