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Relative avidity of IgG to Streptococcus pneumoniae type 6B and 23F polysaccharides (PSs) was measured in sera of children immunized with pneumococcal vaccines, using an EIA and the chaotropic agent thiocyanate. Infants were immunized at 2, 4, and 6 months of age with pneumococcal PS-diphtheria toxoid conjugate vaccine, and boosted at 14 months with the homologous conjugate or a pneumococcal PS vaccine. The concentrations of antibodies to 6B and 23F PSs increased significantly after the booster with both vaccines. A significant increase in the avidity of anti-6B and anti-23F antibodies was seen after the booster with conjugate but not with PS vaccine. Avidity also increased in another group of infants primed and boosted with pneumococcal PS-meningococcal protein conjugate but not in a group boosted with PS vaccine after priming with pneumococcal PS-tetanus toxoid conjugate. In the latter group, however, the avidity of anti-6B was high before boosting.

SUMMARYThe contribution of serotype-speci®c IgG concentration, subclasses, and avidity to opsonophagocytic activity (OPA) against Streptococcus pneumoniae (Pnc) was evaluated in sera of adults and infants immunized with different pneumococcal vaccines. Antibody concentrations and avidities were measured by enzymeimmunoassay (EIA) and OPAs by killing assay of Pnc. The most important factor contributing positively to OPA was the speci®c IgG level. In infants, a tendency to negative correlation was found between the concentration needed for killing of bacteria and avidity, suggesting that less antibodies of high rather than low avidity were required for killing. No such correlation was seen in adults. However, in adults the avidity was high already before vaccination and the variation was narrow. Thus, avidity was probably not a limiting factor in¯uencing OPA. The effect of IgG2/IgG1 ratio on OPA was mostly negative but insigni®cant.

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