Safety and Immunogenicity Study of GlaxoSmithKline (GSK) Biologicals' Pneumococcal Vaccine 2830930A Administered in Toddlers

Inclusion Criteria: * Subjects who the investigator believes that their parent(s)/Legally Acceptable Representative(s) (LARs) can and will comply with the requirements of the protocol. * A male or female between, and including 12 to 23 months of age at the time of vaccination. * Written informed consent obtained from the parents/LAR(s) of the subject. * Healthy subjects as established by medical history and clinical examination before entering into the study. * Born after a gestation period of at least 36 weeks. * Previously completed three-dose vaccination course with Synflorix. Exclusion Criteria: * Child in care. * Use of any investigational or non-registered product other than the study vaccine(s) 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 since birth. * Planned administration/administration of a vaccine containing diphtheria- or tetanus- toxoid or CRM197 and not foreseen by the study protocol during any time of the study period, or of any other vaccines not foreseen by the protocol in the period starting from 30 days before the vaccine dose and ending 30 days after, with the exception of licensed influenza vaccines. * The licensed influenza vaccines are always allowed, even if concomitantly administered with the study vaccines but should be documented in the "Concomitant vaccination" of the electronic Case Report Form. * 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 * Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination * Family history of congenital or hereditary immunodeficiency. * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccine(s). * Major congenital defects or serious chronic illness, including Kawasaki's syndrome. * History of any neurological disorders or seizures, including conditions such as hypotensive-hyporesponsive episodes, encephalopathy and any convulsions (afebrile and febrile). * Acute disease and/or fever at the time of enrolment. * Administration of immunoglobulins and/or any blood products within the 3 months preceding vaccination or planned administration during study period. * Any medical condition which might interfere with the assessment of the study objectives in the opinion of the investigator. * Previous receipt of a booster dose (fourth dose) of Synflorix. * Anaphylaxis following previous administration of vaccines.