It has been repeatedly observed that mixing

Functional anti-N. meningitidis serogroup A (MenA) activity in human serum is detected by serum bactericidal assay (SBA), using either rabbit (rSBA) or human (hSBA) complement, with F8238 as the recommended MenA SBA target strain. However, the F8238 strain may not be optimal for this purpose because, as we show here, it expresses the L11 immunotype, whereas most MenA invasive strains express the L(3,7)9 or L10 immunotype. Moreover, SBA results may be strain dependent, because immunotypes differ in their sensitivity to complement, emphasizing the need to choose the most appropriate strain. Sera from random subsets of infants, toddlers, children, and adolescents in clinical trials of MenA conjugate vaccines were tested by rSBA using strains 3125 (L10) and F8238 (L11). In unvaccinated subjects from all age groups, the percentages of seropositive samples (rSBA-MenA titer, >1:8) was lower using strain 3125 than using strain F8238. However, in toddlers and adolescents immunized with a conjugate MenA vaccine, the percentages of seropositive samples generally were similar using either strain in the rSBA. In two studies, sera also were tested with hSBA. Using hSBA, the differences in the percentages of seroprotective samples (hSBA-MenA titer, >1:4) between strains 3125 and F8238 was less apparent, and in contrast with rSBA, the percentage of seroprotective samples from unvaccinated subjects was slightly higher using strain 3125 than using strain F8238. In adults vaccinated with plain MenA polysaccharide, the percentage of seroprotective samples was higher using strain 3125 than with strain F8238, and the vaccine response rates using strain 3125 were better aligned with the demonstrated efficacy of MenA vaccination. In conclusion, SBA results obtained using the MenA L10 3125 strain better reflected vaccine-induced immunity.The development of effective polysaccharide-protein conjugate vaccines targeting Neisseria meningitidis relies upon the measurement of bactericidal antibodies (4, 32). To ensure the quality, safety, and efficacy of N. meningitidis serogroup A (MenA) conjugate vaccines, the WHO recommends the use of any MenA strain in the serum bactericidal assay (SBA), provided that the strain is not killed by the complement in the absence of MenA-specific antibodies (33). However, strain A1, originally used by Goldschneider et al. in their classic studies (8), was shown to be too sensitive to complement killing (20). The SBA against MenA that has been proposed by Maslanka et al. (20) uses rabbit complement and MenA strain F8238.Clinical trials with MenA polysaccharide-protein conjugate vaccines showed a high percentage of control subjects, not vaccinated with a MenA vaccine, who became SBA positive (with rabbit complement 2nd strain F8238) between 18 weeks of age and the second year of life (5). However, these subjects did not display a parallel increase in anti-capsular polysaccharide A IgG, suggesting that part of the positive SBA results, referred to as natural immunity, was related to either noncapsular surface an...

The lowest dose of DTPw-HBV/Hib-MenAC polysaccharide conjugate vaccine was well tolerated, immunogenic, had good persistence of antibodies, and demonstrated immune memory, and consequently was selected for further development.