- Background. Pneumococcal nasopharyngeal colonization is important for transmission of the organisms. We assessed the ability of two tetravalent conjugate vaccines administered in early infancy to prevent carriage of vaccine-related pneumococci. Methods. A vaccine containing pneumococcal type 6B, 14, 19F and 23F polysaccharide conjugated to tetanus toxoid (Pnc-T) and a vaccine containing the same four polysaccharides conjugated to diphtheria toxoid (Pnc-D) were compared with placebo, in a double blinded study (25 infants per group). Vaccines (or placebo) were injected at 2, 4 and 6 months of age. At 12 months of age a native (nonconjugate) polysaccharide vaccine was administered as a booster. Serum type-specific anticapsular antibody concentrations were measured and nasopharyngeal cultures were obtained at 2, 4, 6, 7, 12 and 13 months of age. Results. In general carriage of all pneumococci (vaccine- and non-vaccine-related) was low at age 2 months and increased with age. However, for the vaccine-related serotypes (6A, 6B, 14, 19F and 23F) carriage was not increased with age in Pnc-D or Pnc-T recipients. Of all cultures obtained after the full primary series, 7 of 72 (10%), 3 of 62 (5%) and 19 or 70 (27%) were positive for the vaccine-related pneumococcal serotypes among the Pnc-D, Pnc-T and placebo recipients, respectively (P = 0.001 for Pnc-D vs. placebo; P = 0.014 for Pnc-T vs. placebo). Most of the antibiotic-resistant isolates belonged to the vaccine-related serotypes. Conclusions. A significant reduction in the carriage of vaccine-related strains after administration of conjugate vaccines was observed. These preliminary results suggest that transmission of specific pneumococcal serotypes most often associated with disease and antibiotic resistance may at least partially be controlled by immunization.