S. pneumoniae is a gram-positive, alpha haemolytic bacterium that is surrounded by an external polysaccharide capsule. There is a difference in composition of this capsule between the 92 different serotypes of S. pneumoniae. Each serotype differs in virulence and prevalence. The external polysaccharide capsule is the main trigger for the specific antibody response and is the basis for pneumococcal vaccines. As it is impossible to develop a vaccine that protects children, immune compromised adults and older adults to all serotypes, the vaccines are based on the most common and virulent serotypes.