Antibody response after immunization of Brazilian children with serogroup C meningococcal polysaccharide noncovalently complexed with outer membrane proteins.

We have studied the antibody response of Brazilian vaccinees to C meningococcal polysaccharide (C-PS) after one or two doses of a vaccine composed of C-PS, outer membrane proteins of B meningococci and aluminum hydroxide. Total IgG, IgG1 and IgG2 as well as bactericidal activity mediated by complement were measured in serum samples from children 3 to 83 months of age (post-vaccination IgG, IgG1 and IgG2 levels of 2.4 to 13.4 micrograms/ml; less than 18 to 67.8 U/ml and less than 18 to 106.8 U/ml, respectively) and from individuals 10 to 14 years of age (post-vaccination IgG, IgG1 and IgG2 levels of 14.6 micrograms/ml, 23.7 U/ml and 112.0 U/ml, respectively). The antibody response, measured as IgG levels, was age-dependent. Although high antibody levels were demonstrable by enzyme-linked immunosorbent assay (ELISA), bactericidal activity was not demonstrable (less than 1:4) in serum from children aged less than 24 months. A significant bactericidal activity was detected in serum of children older than 49 months of age and in individuals 10 to 14 years of age. A predominance of IgG2 was observed in post-vaccination serum samples from children belonging to those two age groups. The antibody concentration sufficient to confer protection as well as the possible causes of the poor correlation observed between ELISA and bactericidal activity results are discussed.