Defining Surrogate Serologic Tests with Respect to Predicting Protective Vaccine Efficacy: Pertussis Vaccination

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Abstract
The first efficacy trial of pertussis vaccines with defined purified antigens failed to demonstrate a serologic correlate of protection, although both tested vaccines were shown to give significant protection against typical whooping cough. The antibody response to pertussis toxoid was dose dependent. A lower anti-PT response in the two-component vaccine, containing one-half the amount of PT in the one-component vaccine, seemed to be compensated by a significant anti-FHA response, since both vaccines conferred similar protection against typical illness. However, the immunologic mechanisms whereby protection is conferred remain unclear. At present antibody responses to the antigens included in the first tested vaccines could be used as pseudoindicators of protection. The group of vaccinated infants who were protected did differ in their antibody profile as compared to unvaccinated infants. Tentatively, candidate vaccines should elicit no less antibody responses to PT and FHA as those elicited by the above one- and two-component vaccines, respectively. The response to other antigens such as pertactin and fimbriae cannot be related to protection at present. Ongoing efficacy trials of a number of pertussis vaccines of varying composition may or may not provide immunological correlates of protection against pertussis. The trials ought to be subjected to a preplanned, independent (meta)analysis with defined end points to increase the understanding of the contribution of various antigens to the protective efficacy of pertussis vaccines.