Relationship of immunogenicity to protective potency in acellular pertussis vaccines

Comparison of the immunogenicity response and resistance to challenge in the modified intracerebral challenge assay induced by various acellular pertussis vaccines showed that these were not closely linked. The immunogenicity assay was effective for confirming the presence of specific antigenic components and was invaluable for detecting minor components present in co-purified vaccines. However, the magnitude of antibody responses was not consistently related to antigen concentration nor did it correlate with protection in the modified intracerebral challenge assay. The immunogenicity assay detected degradation of pertussis toxin and pertactin components but not of filamentous haemagglutinin or fimbriae 2 and 3 in denatured acellular pertussis vaccines. The modified intracerebral challenge assay was effective in detecting antigen degradation in all types of acellular pertussis vaccines including those of European/North American origin but was dominated by the response to pertussis toxin. Aerosol challenge was more sensitive in detecting denaturation of filamentous haemagglutinin or fimbriae. The modified intracerebral challenge assay was the only assay that provided a quantitative indication of protective activity. Both immunogenicity and challenge assays provided useful data on acellular pertussis vaccine properties but were complementary and not alternatives.