The incidence of pertussis in the Netherlands has remained high since an outbreak occurred in 1996

SE Neppelenbroek1, HE de Melker1, JFP Schellekens2, MAE Conyn-van Spaendonck1
1 Department of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment, Bilthoven, the Netherlands
2 Diagnostic Laboratory of Infectious Diseases and Perinatal Screening, National Institute of Public Health and the Environment, Bilthoven, the Netherlands

The incidence of pertussis throughout the Netherlands, determined by notifications, rose suddenly in 1996 to reach 27.2 cases per 100 000 population, compared with 2.3/100 000 on average from 1989 to 1995. Earlier increases in 1989-1990 and 1994 reflected the usual patterns seen in other countries with epidemic cycles every three to five years, so the outbreak in 1996 was unexpected. It could not be explained by a change in vaccine quality, interference with the introduction of vaccination against Haemophilus influenzae type b in 1993, or a fall in vaccination coverage (96% at the age of 12 months) (1,2,3). By analysing data on cases confirmed by serology and culture and on hospital admissions we concluded that the increase of notifications represented a true increase in morbidity (1,2). The largest increase was observed among vaccinated children aged 1 to 9 years; the incidence peaked in 4 year olds. This shift towards older, vaccinated cases was reflected in a decrease in vaccine efficacy estimated retrospectively using notification data. The re-emergence of pertussis was found to be associated with an upsurge in strains that were antigenically distinct from those used in the whole cell vaccine used in the Netherlands (4,5). The increase of the proportion of vaccinated individuals of a broad age-range -which already started in 1994, i.e. two years before the epidemic- suggested a causative role for mismatch between vaccine strains and circulating strains. Studies to explain the unexpected course of pertussis in the Netherlands are still in progress.Annual notifications of pertussis decreased in 1997 (17.2/100 000) and 1998 (16.0/100 000), but have remained – also in the first six months of 1999 – higher than from 1989 to 1995. Recent notification data should be interpreted with caution, however, due to changes over time. By linking notification data and serological data we concluded that the notification rate has increased, probably because physicians have been more alert since 1996. In addition, however, the case definition for notification, which was not changed from 1989 to 1996 (clinical symptoms with laboratory confirmation (1)) changed in April 1997 to accept a single high antibody titre as well as a rise in antibody titre between two specimens, positive culture, or polymerase chain reaction. Since then, a larger proportion of notifications has been confirmed on the basis of a single high titre. These two factors do not fully explain why the numbers of notifications since the 1996 epidemic have remained higher than in 1989 to 1995, however, because rates of hospital admission and Bordetella pertussis isolates have doubled as well.

In response to the epidemic a whole cell vaccine with a slightly higher pertussis toxin content was introduced in November 1997. In addition, since January 1999 infants have received pertussis vaccine at 2, 3, 4, and 11 months rather than at 3, 4, 5, and 11 months in order to protect younger infants, who are at the greatest risk of severe pertussis. The effect of these measures is not yet known. The National Health Council is considering whether a booster vaccination is needed. Results of a study on booster response after acellular and whole cell vaccine for 4 year olds and the current epidemiological situation will provide evidence with which to guide decision making (6). Surveillance, using several sources of data, should continue in order to monitor the effects of such interventions.



References1. Melker HE de, Conyn-van Spaendonck MAE, Rümke HC, Wijngaarden JK van, Mooi FR, Schellekens JFP. Pertussis in the Netherlands: an outbreak despite high levels of immunization with whole cell vaccine. Emerg Infect Dis 1997; 3:175-8.

2. Melker HE de, Conyn-van Spaendonck MAE, Schellekens JFP. The pertussis epidemic in 1996: description and evaluation based on surveillance data from 1976 to 1996. Bilthoven: National Institute of Public Health and the Environment (RIVM), 1997. (RIVM report no. 128507005).

3. Mooi FR, Oirschot H van, Heuvelman K, Heide HGJ van der, Gaastra W, Willems RJL. Polymorphism in the Bordetella pertussis virulence factors P.69/pertactin and pertussis toxin in the Netherlands: temporal trends and evidence of vaccine-driven evolution. Infect Immun 1998; 66: 670-5.

4. Labadie J, Sundermann LC, Rümke HC and the DTP-IPV-Hib Vaccine Study Group. Multicenter study on the simultaneous administration of DTP-IPV and Hib PRP-T vaccines. Part 1. Immunogenicity.Bilthoven: National Institute of Public Health and the Environment, 1996. (RIVM report no. 124001003).

5. Mastrantonio P, Spigaglia P, Oirschot H van, Heide HGJ van der, Heuvelman K, Stefanelli P, et al. Antigenic variants in Bordetella pertussis strains isolated from vaccinated and unvaccinated children. Microbiology 1999; 145: 2069-75.

6. Berbers GAM, Lafeber AB, Labadie J, Vermeer-de Bondt PE, Bolscher DJA, Plantinga AD. A randomised controlled study with whole-cell or acellular pertussis vaccines in combination with regular DT-IPV vaccine and a new poliomyelitis (IPV-Vero) component in children 4 years of age in the Netherlands. Bilthoven: National Institute of Public Health and the Environment (RIVM), 1999. (RIVM report no. 105000001).

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