Study to Assess the Immunogenicity and Safety of GlaxoSmithKline (GSK) Biologicals' Herpes Zoster Subunit (HZ/su) Vaccine (GSK1437173A) When Co-administered With GSK Biologicals' Diphtheria, Tetanus and Pertussis Vaccine (Boostrix®) in Adults Aged 50 Years and Older

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 with the study vaccine(s). * 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 (drug or vaccine) other than the study vaccines within 30 days preceding the first dose of study vaccine, or planned use during the study period. * Chronic administration (defined as more than 14 consecutive days) of immunosuppressants or other immune-modifying drugs within six months prior to the first vaccine dose (for corticosteroids, this will mean prednisone ≥ 20 mg/day, or equivalent). A prednisone dose of \< 20 mg/day is allowed. Inhaled, topical and intra-articular corticosteroids are allowed. * Administration or planned administration of a vaccine not foreseen by the study protocol within the period starting 30 days before the first dose of study vaccine(s) and ending 30 days after the last dose of study vaccine. This includes any type of vaccine such as (but not limited to) live, inactivated and subunit vaccines (e.g., inactivated and subunit influenza vaccines). * Administration of long-acting immune-modifying drugs (e.g. infliximab) 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 vaccine/product (pharmaceutical product or device). * Previous vaccination against VZV or HZ and/or planned administration during the study of an HZ or VZV vaccine (including an investigational or non-registered vaccine) other than the study vaccine. * History of HZ. * Vaccination against diphtheria, or tetanus in the last five years or planned vaccination against diphtheria or tetanus during the study period, other than the study vaccine(s). * Administration of a combined tetanus, diphtheria and pertussis (Tdap) vaccine at any time prior to study entry. * Any confirmed or suspected immunosuppressive or immunodeficient condition resulting from disease (e.g., malignancy, human immunodeficiency virus \[HIV\] infection) or immunosuppressive/cytotoxic therapy (e.g., medications used during cancer chemotherapy, organ transplantation or to treat autoimmune disorders). * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccines including prior severe allergic reaction following tetanus-toxoid, diphtheria-toxoid or pertussis-containing vaccine. * Hypersensitivity to latex. Note: The investigational HZ/su vaccine does not contain latex. * Acute disease and/or fever at the time of enrolment. * Fever is defined as temperature ≥ 37.5°C /99.5°F by oral route. The preferred route for recording temperature in this study will be oral. * Subjects with a minor illness (such as mild diarrhea, 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. * Pregnant or lactating female. * Female planning to become pregnant or planning to discontinue contraceptive precautions before 2 months after the last dose of study vaccine. * Any condition which, in the opinion of the investigator, prevents the subject from participating in the study. * Any condition which, in the judgment of the investigator, would make intramuscular (IM) injection unsafe. * Encephalopathy (e.g. coma, decreased consciousness, prolonged seizures) not attributable to an identifiable cause within 7 days of administration of a previous pertussis antigen-containing vaccine. * Progressive or unstable neurologic disorder. * History of Arthus-type hypersensitivity reaction following a prior dose of a tetanus-toxoid containing vaccine within the last 10 years. * History of Guillain-Barré syndrome within 6 weeks of receipt of a prior vaccine containing tetanus toxoid.