This primary purpose of this study is the evaluation of the immunological persistence following completion of the 3-dose primary vaccination course with either a clinical or a commercial lot of pneumococcal conjugate vaccine GSK1024850A in study NCT00808444. In addition, the study will also assess the safety, reactogenicity and immunogenicity of a fourth dose of pneumococcal conjugate vaccine GSK1024850A (commercial lot) when co-administered with Infanrix-IPV/Hib at 18-21 months of age in children primed in study NCT00808444.