Evaluation of antibody responses to pneumococcal vaccines with ELISA and opsonophagocytic assay

Antibodies to a capsular polysaccharide (PS) provide protection against Str eptococcus pneumoniae which express the homologous capsular serotype, and p neumococcal vaccines are designed to induce antibodies in the capsular PS. Levels and opsonophagocytic capacity of antibodies to the capsular PS of S. pneumoniae serotype 19F were determined by sera from adults immunized with 23-valent S. pneumoniae capsular PS vaccines. Geometric means of IgG anti- 19F antibody level and specific opsonic titer rise significantly after immu nization. The level of anticapsular PS antibodies for S. pneumoniae 19F ser otype is fairly well correlated (r(2)=0.63) with the opsonophagocytic activ ities of sera. However, 3.7% (1/27) of serum samples display strikingly les s opsonophagocytic activity than expected on the basis of their antibody le vel. Thus, antibody level may be of general use in predicting vaccine-induc ed protection among adults for 19F serotype. However, the opsonic activity data suggest that antibody levels are not always indicative of functional a ntibody.