Study to Evaluate Immunogenicity and Safety Study of GSK Biologicals' Herpes Zoster (HZ) Vaccine GSK1437173A When Co-administered With Pneumovax 23™ 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 other than the study vaccines 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 vaccine not foreseen by the study protocol within the period starting 30 days before the first dose of study vaccine(s) and e…