This is a follow-on, multi-centre, open-label, clinical trial. The purpose of this trial is to investigate the concentrations of serotype-specific antibodies to IgG included in PCV13 in children who have received either the PCV7 or PCV13 primary immunisation at 2, 4 and 12 months of age. We intend to recruit all interested participants who completed the Wyeth-sponsored PCV13 infant trial study (6096A1-007) at selected study sites (i.e. those that recruited the majority of the children in the original study). The study will start in March 2010, at which time the eldest participants in the 6096A1-007 study will be approximately 42 months of age. There will be two visits per participant, 1 month apart from each other. At visit one, all participants will have a blood test and receive a dose of PCV13. At visit 2, all participants will have a blood test and will be offered the remaining pre-school booster vaccinations unless they have already received them.