Polysaccharide-
protein conjugate vaccines elicit higher concentrations of serum anticapsular antibody in infants and children than do unconjugated
polysaccharide vaccines. The conjugate-induced
antibodies also have higher avidity and
complement-mediated bactericidal activity. Similar
vaccine-related differences in the magnitude or functional activity of antibody are observed infrequently in immunized adults. We compared the antibody responses of adults immunized with an investigational group A and C meningococcal
conjugate vaccine to those elicited by an unconjugated
meningococcal polysaccharide vaccine. Although there were no significant differences between the respective geometric mean bactericidal titers of the two
vaccine groups, it took, on average, three- to fourfold higher concentrations of
polysaccharide-induced serum anticapsular antibody to achieve 50%
complement-mediated bacteriolysis than conjugate-induced antibody (P < 0.001 for groups A and C). At limiting doses, the
polysaccharide-induced anticapsular
antibodies also were less effective in conferring passive protection against meningococcal
bacteremia in infant rats challenged with a group C strain (P < 0.04). The avidity index of the group C
antibodies was higher in the
conjugate vaccine group than in the
polysaccharide vaccine group (P < 0.005). The disparities in the functional activity of the anticapsular
antibodies elicited in adults by the two
vaccines imply fundamental differences in the respective B-cell populations stimulated.