• For each serotype assayed (23 for ELISA and 11 for functional assays) the proportion of children responding to the micro-PPS dose at 18 months and the GMC of the response will be compared between children who receive PPS at 12 months and those who do not [ Time Frame: 19 months of age ] [ Designated as safety issue: No ]
  

• Proportion of children showing hyporesponse at 18 months to more than half of all 23 serotypes [ Time Frame: 18 weeks; 12.5 months of age ] [ Designated as safety issue: No ]
  
• Immunogenicity and carriage: Immune responses following the primary series of conjugate vaccination, measured by ELISA and OPA will be compared between the group receiving two doses of PCV and those receiving 3 doses of vaccine [ Time Frame: 18 months of age ] [ Designated as safety issue: No ]
  
• Rate of decline: assessment of antibody levels [ Time Frame: 12 months ] [ Designated as safety issue: No ]
  

This research project began as a study of alternative strategies for Pnc (Pneumococcus) immunization for children in developing countries. In the original study, infants presenting to a health center in urban Fiji were randomized to receive 1, 2, or 3 doses of PCV (Prevnar) followed by a dose of 23-valent PPS (Pneumococcal polysaccharide vaccine) at 6 or 9 months of age. The regimens were compared with each other and with 2 control groups with respect to immunogenicity, impact on carriage of vaccine type Pnc, and response to a small dose of PPS at 15 months of age. After the trial was underway and 228 infants had been recruited, concerns were raised about the safety of PPS in infancy. Specifically concerns were raised that it might result in later immunological hyporesponsiveness in some of the recipients, to some of the serotypes. After a thorough review it was decided to proceed with the study, but to modify it so that it addressed directly the issue of potential hyporesponsiveness, while not giving PPS to any children under 12 months of age. This protocol represents the completion of the study with the new design for the children already enrolled, and a new design for a further cohort of children who will be enrolled. The newly designed trial will be a single blind, open-label, randomized, controlled trial of 550 healthy infants. Infants will be randomized to 1 of 8 equal groups to receive 0, 1, 2, or 3 doses of PCV (Pneumococcal conjugate vaccine), with or without a booster of PPS at 12 months of age. Two control groups will be recruited, 1 will receive no PCV in infancy and the other will receive a dose of PPS at 12 months of age. At 18 months of age, all infants will receive a 20 percent dose of PPS to stimulate and allow the assessment of immunological memory. At 2 years of age, any child who has received no or one dose of PCV (Groups E, F, G and H) will receive a single dose of PCV. Blood samples will be taken at 18 weeks age, 12 months of age, before the 18 months dose and 4 weeks later. In addition half of the children will have a blood sample taken at 9 months and the other half will have a sample taken 2 weeks after their 12 month dose of PPS. The 9- and 12-month blood sample will assess the long-term persistence of circulating antibody and avidity maturation following a 1, 2, or 3 dose primary series of PCV. The primary objective is to demonstrate noninferiority at 19 months of age, of those groups receiving PPS at 12 months and those who do not with respect to the proportion of children in each group, with a satisfactory immune response at 19 months of age, measured by OPA to each of the 11 serotypes included in the PPS for which an OPA has been developed. The secondary objective is to assess the immunogenicity and impact on carriage of various Pnc vaccination regimens that combine 1 to 3 doses of PCV with a dose of PPS, concerns regarding the potential for the development of hyporesponsiveness. Endpoints to be evaluated will include serotype specific immunoglobulin G antibody measured by enzyme-linked immunosorbent assay to the 23 serotypes in the PPS, avidity assays to the same serotypes, opsonophagocytic assays to the 11 serotypes for which these assays are currently available, and Pnc carriage by serotype. With hyporesponsiveness as the primary endpoint of interest a scheme of analysis has been proposed which will require a minimum sample size of 500.