The binding of complement factor H (fH) to meningococci was recently found to be specific for human fH. Therefore, passive protective antibody activity measured in animal models of meningococcal bacteremia may overestimate protection in humans, since in the absence of bound fH, complement activation is not downregulated. We developed an ex vivo model of meningococcal bacteremia using nonimmune human blood to measure the passive protective activity of stored sera from 36 adults who had been immunized with an investigational meningococcal multicomponent recombinant protein vaccine. Before immunization, human complement-mediated serum bactericidal activity (SBA) titers of \textgreater or = 1:4 against group B strains H44/76, NZ98/254, and S3032 were present in 19, 11, and 8\% of subjects, respectively; these proportions increased to 97, 22, and 36\%, respectively, 1 month after dose 3 (P \textless 0.01 for H44/76 and S3032). Against the two SBA-resistant strains, NZ98/254 and S3032, passive protective titers of \textgreater or = 1:4 were present in 11 and 42\% of sera before immunization, respectively, and these proportions increased to 61 and 94\% after immunization (P \textless 0.001 for each strain). Most of the sera with SBA titers of \textless1:4 and passive protective activity showed a level of killing in the whole-blood assay (\textgreater1 to 2 log(10) decreases in CFU/ml during a 90-min incubation) similar to that of sera with SBA titers of \textgreater or = 1:4. In conclusion, passive protective activity was 2.6- to 2.8-fold more frequent than SBA after immunization. The ability of SBA-negative sera to kill Neisseria meningitidis in human blood where fH is bound to the bacteria provides further evidence that SBA titers of \textgreater or = 1:4 measured with human complement may underestimate meningococcal immunity.