Streptococcus pneumoniae capsular polysaccharide-diphtheria toxoid conjugate vaccine is immunogenic in early infancy and able to induce immunologic memory

BACKGROUND.Pneumococcal polysaccharide vaccines are not protective against the most common pneumococcal infections in infancy. The importance of pneumococcal diseases and emerging antimicrobial resistance emphasize the need for prophylaxis. METHODS.Pneumococcal conjugate vaccine, containing capsular polysaccharides from serotypes 6B, 14, 19F and 23F conjugated to diphtheria toxoid (PncD), was given to 75 infants at 2, 4 and 6 months of age. Three dosages (1, 3 or 10 μg of each) were used. A placebo group of 49 infants received physiologic saline. Children were given a booster dose of either polysaccharide or conjugate vaccine at 14 months of age; the placebo group received conjugate vaccine. Antibody concentrations were determined with an enzyme immunoassay. RESULTS.The highest dose induced the strongest response after primary immunization, but booster response was greatest in the group primed with the lowest dose. Polysaccharide and conjugate vaccines induced booster responses of the same magnitude. At 24 and 36 months of age the antibody concentrations were similar in children who had received the PncD in infancy and in children immunized at 14 months of age only. CONCLUSIONS.The PncD conjugate vaccine is immunogenic and able to induce immunologic memory.