It is reasonable to anticipate that the introduction of PCV13 may help reduce the chances of severely immunocompromised children getting pneumococcal infections. Many of these children have been previously immunized with a full series of a 7-valent pneumococcal conjugate vaccine. These children will benefit from an additional dose of the new 13-valent vaccine. The degree to which SOT-recipient children are protected by prior immunizations and are responsive to new immunizations is still largely undefined. This study aims to expand the knowledge regarding the safety and immunogenicity of PCV13 immunization in this growing and vulnerable population.