Immunogenicity and Safety Study of GlaxoSmithKline (GSK) Biologicals' Pneumococcal Vaccine (Synflorix™) When Administered to Children Who Are at an Increased Risk of Pneumococcal Infection

Inclusion Criteria: * Subjects who the investigator believes that parent(s)/Legally Acceptable Representative(s) \[LAR(s)\] can and will comply with the requirements of the protocol. * Written informed consent obtained from the parent(s)/LAR(s) of the subject and informed assent obtained from the subject, if appropriate, prior to enrollment. * Female subjects of non-child bearing potential may be enrolled in the study. (Non-child bearing potential is defined as pre-menarche, current tubal ligation, hysterectomy or ovariectomy). * Female subjects of child bearing potential may be enrolled in the study, if the subject: * has practiced adequate contraception for 30 days prior to the first 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. Priming status: * Children who have not been previously vaccinated with any pneumococcal vaccine, i.e. either plain polysaccharide pneumococcal vaccine, Synflorix (10Pn-PD-DiT), Prevenar or Prevenar13 will be considered for inclusion in the unprimed groups. Children who have been previously vaccinated with: * at least one dose of a pneumococcal conjugate vaccine, i.e. either Synflorix (10Pn-PD-DiT), Prevenar or Prevenar13 * with plain polysaccharide pneumococcal vaccine more than 2 years and less than 5 years before enrollment. * will be considered for inclusion in the primed groups. Additional inclusion criteria for the At-risk groups: * A male or female aged between, and including, 2 and 17 years at the time of first vaccination. * For the purpose of this study, at-risk subject is a subject with: * Congenital or acquired asplenia such as anatomic, surgical or functional asplenia or * Splenic dysfunction, chronic gastrointestinal disorders, liver disease, infiltrative disorders, vascular disorder etc or Note: All individuals who are diagnosed by the investigator as with splenic dysfunction are eligible for enrollment in the At-risk group. When available, investigator will collect medical documentation for reduced splenic function diagnosed with an appropriate technique in the At-risk subject's medical records. No further assessment will be necessary. A maximum of 35 individuals with sickle-cell disease can be enrolled in the At-risk group. These subjects do not require assessment of the splenic function as sickle-cell disease is invariably associated with severe splenic dysfunction. \- Complement deficiencies. For all subjects defined as At-risk the Investigator will make all efforts to collect information from the subject/subject's parent(s)/LAR(s) during the interview and/or from previously available medical documentation on the date and conditions which have made a child at-risk of pneumococcal infection and/or the results of tests determining spleen dysfunction or complement deficiency. This should be documented in the medical records of the At-risk subject. No originals/copies of medical documentation are needed. Additional inclusion criteria for the Healthy group: * A male or female matched (for age and country) to a subject aged 24-59 months from the At-risk group. * Healthy subjects as established by medical history and clinical examination before entering into the study. 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. * Planned administration/administration of a vaccine not foreseen by the study protocol during the period starting 30 days before each dose of vaccine(s) and ending 30 days after\*. \* In case an emergency mass vaccination for an unforeseen public health threat is organised by the public health authorities, outside the routine immunization program, vaccines can be administered at any time during the study period provided it is licensed and used according to its Summary of Product Characteristics or Prescribing Information and according to the local governmental recommendations and that a written approval of the Sponsor is provided. Vaccines that are recommended for subjects with an increased risk of bacterial infection, can be administered at any time to the subjects enrolled in the At-risk group. * 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 (pharmaceutical product or device). * History of any neurological disorders or seizures. * Acute disease and/or fever at the time of enrollment. * History of chronic alcohol consumption and/or drug abuse. * Any confirmed or suspected Human Immunodeficiency virus (HIV) infection, based on medical history and physical examination. * History of any reaction or hypersensitivity likely to be exacerbated by any component of the vaccine. * 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. * Major congenital defects except medical conditions that define an At-risk subject. * Previous vaccination against pneumococcal infection with pneumococcal conjugate vaccine within the last 8 weeks. * Previous vaccination against pneumococcal infection with plain polysaccharide vaccine within the last 2 years. Additional exclusion criteria for the Healthy group: * Chronic administration of immunosuppressants or other immune-modifying drugs within six months prior to the first vaccine dose. Inhaled and topical steroids are allowed. * Serious chronic illness. * Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination. * Family history of congenital or hereditary immunodeficiency.