Primary vaccination with pneumococcal protein D conjugate vaccine in the first year of life induced clear ELISA and OPA responses, which varied considerably for the different serotypes. Antibody levels declined following primary vaccination but were restored (except for serotype 3) to above post-primary levels by booster vaccination in the second year of life. Antibody levels declined when measured in the fourth year of life, although remaining higher than in the non-immunized children. For some serotypes, antibody levels did not decline indicating exposure to pneumococci or cross-reacting bacteria. Development of natural immunity to several serotypes was evident from the increase in opsonophagocytic activity in the control group between booster and plain polysaccharide vaccination. Vigorous and rapid OPA and ELISA responses were elicited to all vaccine serotypes including serotype 3 following administration of plain polysaccharide vaccine in both the conjugate and control groups, being higher in the conjugate group (Study ID: 104083/NCT00169507).