Seroprevalence of pertussis in The Netherlands: evidence for increased circulation of Bordetella pertussis

Abstract

Background: In many countries, the reported pertussis has increased despite high vaccination coverage. However, accurate determination of the burden of disease is hampered by reporting artifacts. The infection frequency is more reliably estimated on the basis of the prevalence of high IgG concentrations against pertussis toxin (IgG-Ptx). We determined whether the increase in reported pertussis in the last decade is associated with an increase in the number of infections.

Methodology/principal findings: In a cross-sectional population-based serosurveillance study conducted in 2006-07, from a randomly selected age-stratified sample of 7,903 persons, serum IgG-Ptx concentrations were analyzed using a fluorescent bead-based multiplex immuno assay. In 2006-07, 9.3% (95%CI 8.5-10.1) of the population above 9 years of age had an IgG-Ptx concentration above 62.5 EU/ml (suggestive for pertussis infection in the past year), which was more than double compared to 1995-96 (4.0%; 95%CI 3.3-4.7). The reported incidence showed a similar increase as the seroprevalence between both periods.

Conclusions: Although changes in the vaccination program have reduced pertussis morbidity in childhood, they have not affected the increased infection rate in adolescent and adult pertussis. Indeed, the high circulation of B. pertussis in the latter age-categories may limit the effectiveness of pediatric vaccination.

Figure 1. Incidence per 100,000 population of reported pertussis cases in the Netherlands in 1993–2007, for 0-2-year-olds and for all ages.

Figure 2. Age-specific seroprevalence of IgG-Ptx concentrations in children 0–9 years of age in 1995-96 (upper figure) and in 2006-07 (lower figure).

Note: on the x-axis the age-group, number tested and in brackets the percentage targeted by the acellular vaccine are indicated. In 2006-07 children below 4 years of age could have been primed with either whole-cell or acellular vaccine in infancy (nationwide coverage circa 96%), and children 4–9 years of age could have been primed with whole-cell vaccine and may have received a preschool booster with acellular vaccine (nationwide coverage circa 90%).

Figure 3. Age-specific seroprevalence of IgG-Ptx concentrations in individuals >9 years in 1995-96 (upper figure) and in 2006-07 (lower figure).

Figure 4. Risk ratios and 95% confidence intervals for the comparison of seroprevalence and reported incidence of pertussis in individuals >9 years in 2006-07 vs. 1995-96.