Factors influencing the immunogenicity of polysaccharide (PS)-protein conjugate vaccines are not entirely known but may include variables such as the quantity and ratio of serotype-specific PS and carrier protein, and concurrent use of vaccines with the same carrier protein. We evaluated these factors in a study of two recently produced lots of pneumococcal conjugate(Pn-conj) vaccine. Both lots consisted of seven Pn serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F) each conjugated to the outer membrane protein complex(OMPC) of N. meningitidis. Lot A had slightly more total PS (17.6μg vs 16.1 μg) and less OMPC (123 μg vs 140 μg) per dose than lot B, but the PS/OMPC ratios varied for each serotype. These vaccines were given to healthy infants at 2, 4, 6, and 12 mo of age. Lot A was given concurrently with PRP-CRM (Grp A1) or PRP-OMPC (Grp A2) (randomized), and lot B was given with PRP-CRM (Grp B). Pneumococcal antibody concentrations (GMTs) were measured by ELISA at UCLA. For each serotype, pre-vaccination GMTs were similar between groups. Table

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p<0.05: * grp A1 vs A2 and B; ° grp A1 vs B; # grp A2 vs B. In the interval b/w 7 and 12 mo of age, the GMTs in grps A1 and A2 ↓'ed 2-5-fold(except 6B, unchanged), then ↑'ed 6-9-fold after the booster dose at 12 mo. Grp B infants were boosted with Pn-conj or Pn PS (data pend). In this study, 1) concurrent use of an OMPC-containing Hib-conj vaccine did not affect anti-Pn responses except to 6B, and 2) compared to lot A, Pn-conj lot B induced poorer antibody responses to all four serotypes measured. To develop an effective Pn-conj vaccine for use in infants, it is critical that we discern the etiology of lot B's reduced immunogenicity. Supported in part by a grant from Merck & Co., Inc.