Prospective survey on carriage of Neisseria meningitidis and protective immunity to meningococci in schoolchildren in Niamey (Niger): focus on serogroup W135

We investigated the carriage of serogroup W135 meningococci and its relationship with protective immunity in Niamey. Between February and May 2003, three oropharyngeal swabs and two serum samples were each taken from 287 school children. Serogroup W135 isolates were obtained from 8.9% of children. Specific IgG ≥ 2 μg/ml using ELISA or serum bactericidal assay (SBA) titre ≥ 8 were supposed to represent the protective immunity to a serogroup. The proportion of children with serogroup W135-specific IgG ≥ 2 μg/ml increased significantly during follow-up (13.9% to 19.1%), but not the proportion of those with SBA titre ≥ 8 (10.1% to 11.6%). At the end of the follow-up, we observed a significant association between carriage of serogroup W135 strains and presumed protective immunity to this serogroup, using either ELISA or SBA. Among 240 children having an initial SBA titre <8, 20 carried serogroup W135 strains at least once. In May, 25% of carriers had an SBA titre ≥ 8, vs. 2.3% of non-carriers. For ELISA, 230 children had specific IgG < 2 μg/ml in February, with 22 having at least one swab positive for serogroup W135 meningococci later. In May, 45.5% of them had specific IgG ≥ 2 μg/ml vs. 5.3% among non-carriers.