In the Netherlands a strong epidemic outbreak of pertussis took place in 1996–7. Here we investigate the possible causes of the epidemic, using an age-structured epidemic model. Motivated by the observation that during the epidemic the number of cases in vaccinated children had increased considerably compared to the preceding period, we focus on two vaccination related changes. First, we consider the possibility that the potency of the vaccine decreased so that it confers protection for a shorter period of time in newly vaccinated children. Second, we consider the possibility that at a certain point in time the duration of protection after vaccination decreased for all individuals. This may be the case if the pathogen population changed such that the current vaccine confers less protection. A comparison of the observed and simulated age-distribution of infections indicates that the second scenario is more in line with the observed pattern of the 1996–7 epidemic. We discuss the implications of this conclusion for B. pertussis circulation, and for the design of vaccination programmes in the face of a polymorphic B. pertussis population that may adapt itself to vaccination.
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