Epidemiology and Infection



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


M.  VAN BOVEN  a1 c1, H. E.  DE MELKER  a1, J. F. P.  SCHELLEKENS  a2 and M.  KRETZSCHMAR  a1
a1 Department of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, The Netherlands
a2 Diagnostic Laboratory for Infectious Diseases and Perinatal Screening, National Institute of Public Health and the Environment, PO Box 1, 3720 BA Bilthoven, The Netherlands

Abstract

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.

(Accepted April 5 2001)


Correspondence:
c1 Author for correspondence, present address: Institute for Animal Science and Health, PO Box 65, 8200 AB Lelystad, the Netherlands.


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