A model based evaluation of the 1996-7 pertussis epidemic in the Netherlands

In the Netherlands a strong epidemic outbreak of pertussis took place in 19 96-7. Here we investigate the possible causes of the epidemic, using an age -structured epidemic model. Motivated by the observation that during the ep idemic the number of cases in vaccinated children had increased considerabl y compared to the preceding period, we focus on two vaccination related cha nges. First, we consider the possibility that the potency of the vaccine de creased 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 a ll individuals. This may be the case if the pathogen population changed suc h that the current vaccine confers less protection. A comparison of the obs erved and simulated age-distribution of infections indicates that the secon d scenario is more in line with the observed pattern of the 1996-7 epidemic . We discuss the implications of this conclusion for B. pertussis circulati on, and for the design of vaccination programmes in the face of a polymorph ic B. pertussis population that may adapt itself to vaccination.