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Streptococcus pneumoniae is the most common cause of invasive bacterial disease and otitis media in infants and young children. Licensed pneumococcal polysaccharide vaccines are not reliably immunogenic in children younger than 2 years of age; therefore pneumococcal glycoprotein conjugate vaccines are currently being evaluated for safety, immunogenicity and efficacy in various age groups.During a 12-month period we determined the kinetics of pneumococcal IgG antibody in 60 children who received primary immunization with one dose of bivalent (serotypes 6A and 23F) pneumococcal polysaccharide-CRM197 vaccines at 18 to 30 months of age. To assess immunologic priming a subgroup of 20 subjects received secondary immunization with pneumococcal polysaccharide vaccine, including serotypes 6B and 23F, at 11 to 20 months after primary immunization. Pneumococcal-specific IgG subclass distributions were also evaluated in the subgroup.In the 12 months after primary immunization with glycoprotein conjugate vaccine, geometric mean pneumococcal IgG antibody concentrations to 6B and 23F serotypes remained stable. Pneumococcal polysaccharide vaccine induced a greater anamnestic response in children primed with the glycoprotein conjugate vaccines (13- to 40-fold increases to geometric mean concentrations of 6 to 30 μg/ml for type 23F), than in those primed with polysaccharide (2- to 4-fold increases). A greater IgG response to pneumococcal serotype 23F than to 6B was observed with both primary and secondary immunization. The serotype-specific pneumococcal IgG antibody response was virtually restricted to the IgG1 subclass after primary immunization, but secondary immunization elicited antibodies of IgG1 and IgG2 subclasses.These glycoprotein conjugate vaccines appear to prime for anamnestic IgG antibody responses to subsequent immunization with polysaccharide vaccine, suggesting that the polysaccharide-CRM197 vaccine effectively induces a predominantly T cell-dependent immune response. The greater IgG response to 23F than to 6B indicates that pneumococcal serotype is a major determinant of immunogenicity of pneumococcal glycoprotein conjugate vaccines.