Pneumococcal polysaccharide vaccines (PPV) have been licensed in the U.S. for over 40 years. In contrast, pneumococcal conjugate vaccines are immunogenic and efficacious in normal infants and children, and offer hope of reducing pneumococcal infections in immunocompromised children. However, conjugate pneumococcal vaccine can only protect against a limited number of the 90 pneumococcal serotypes.