This study will assess the safety, reactogenicity and immunogenicity of different formulations of GSK Biologicals' pneumococcal vaccine 2189242A when administered alone or in combination with the 10-valent pneumococcal conjugate vaccine (GSK1024850A vaccine) as a 2-dose primary vaccination course followed by a booster dose in healthy children aged 12-23 months at the time of first vaccination. Considering that febrile reactions are frequently observed following pneumococcal vaccination, usually co-administered with other routine paediatric vaccines, the primary study objective will focus on evaluating the increase in grade 3 fever (i.e. rectal temperature >40.0°C).