Reemergence of Pertussis in the Highly Vaccinated Population of The Netherlands: Observations on Surveillance Data

Hester E. de Melker, J.F.P. Schellekens, S.E. Neppelenbroek, F.R. Mooi, H.C. Rümke, M.A.E. Conyn-van Spaendonck, National Institute of Public Health and the Environment, Bilthoven, the Netherlands

Disclosures

Emerging Infectious Diseases. 2000;6(4) 

In This Article

Abstract and Introduction

We analyzed pertussis reporting, death, hospitalization, and serodiagnostic data from 1976 to 1998 to help explain the cause of the 1996 pertussis outbreak in the Netherlands. The unexpected outbreak was detected by an increase in pertussis reporting and by other surveillance methods. In 1996, according to reporting and serologic data, the increase in pertussis incidence among (mostly unvaccinated) children less than 1 year of age was similar to the increase in hospital admissions. Among older (mostly vaccinated) persons, the increase in hospital admissions was relatively small. The increase in pertussis incidence was higher among vaccinated than among unvaccinated persons of all ages. This resulted in lower estimates of vaccine effectiveness. The proportion of pertussis infections resulting in recognizable symptoms may have increased among vaccinated persons because of a mismatch of the vaccine strain and circulating Bordetella pertussis strains. The small immunogenicity profile of the Dutch vaccine may have resulted in greater vulnerability to antigenic changes in B. pertussis.

The incidence of pertussis has been greatly reduced by mass vaccination; however, even in countries with high vaccination coverage, the disease is reemerging[1,2,3,4]. A sudden increase in cases reflecting a pertussis outbreak in the Netherlands in 1996[5] could not be explained by a decrease in vaccination coverage, which remained stable at 96% for at least three vaccinations in the first year of life. Until January 1999, children were vaccinated at 3, 4, 5, and 11 months of age with a diphtheria, tetanus, pertussis, and inactivated polio vaccine. In 1999, the schedule changed, and vaccine was administered at 2, 3, 4, and 11 months of age. The vaccine used meets international standards; no sign of an abrupt or gradual deterioration of vaccine quality, as determined at product release by the mouse protection test, was found. The introduction of vaccination against Haemophilus influenzae type b in 1993 did not interfere with the immunoresponse to pertussis[6], and no cohort effect in children vaccinated for H. influenzae type b was observed[5]. A mismatch between the vaccine and circulating strains of Bordetella pertussis [5,7,8,9] may have contributed to pertussis reemergence.

Determining the epidemiology of pertussis by case-reporting data is hampered by changes in case definitions, availability and interpretation of laboratory diagnostic tests, case-reporting rates, and diagnostic practice. To ascertain the current epidemiology of pertussis in the Netherlands and to try to determine the cause of the 1996 epidemic, we compared case-reporting data from January 1976 to September 1998 with other surveillance data (deaths, hospitalizations, and positive serodiagnoses).

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