Objective: To optimize the serum bactericidal assay (SBA), detect and analyze the bactericidal antibody level against Neisseria meningitidis serogroup C strains after divalent polysaccharide (A plus C) vaccine immunization.
Methods: Two Neisseria meningitidis serogroup C strains, vaccine candidate strain (C11) and epidemic strain (053442), were selected as targets. The national Neisseria meningitidis standardized serum was used as reference serum. Pel-Freez infant rabbit complements was available. The optimized SBA method was used to detect bactericidal antibody against strain C11 and 053442 for 122 pairs of sera before and after immunization with a divalent polysaccharide (A and C) vaccine.
Results: The strain C11 and 053442 both could be used as targeted strain for SBA. The optimized concentration of targeted strain was achieved when a whole-cell suspension of 0.35 A at 600 nm was diluted 4 x 10(4) times. Before immunization, SBA geometric mean titers (GMT) of 122 sera against strain C11 and 053442 were 1:1.75 and 1:2.63 respectively, and the protective rates were 9.8% and 17.2% respectively. After immunization, the GMTs and the protective rates of 122 sera both rose significantly (P<0.01), the GMTs against strain C11 and 053442 were 1:483.73 and 1:412.57 respectively. The protective rates against strain C11 and 053442 were 100% and 95.9% respectively.
Conclusion: Immunization with a divalent polysaccharide (A and C) vaccine could elevate remarkably the population SBA titer against Neisseria meningitidis serogroup C strains of different subtypes, but the surveillance of vaccine effect against different targeted strains remains necessary.