Incidence and reproduction numbers of pertussis: estimates from serological and social contact data in five European countries

Abstract

Background: Despite large-scale vaccination programmes, pertussis has remained endemic in all European countries and has been on the rise in many countries in the last decade. One of the reasons that have been discussed for the failure of vaccination to eliminate the disease is continued circulation of the pathogen Bordetella pertussis by mostly asymptomatic and mild infections in adolescents and adults. To understand the impact of asymptomatic and undiagnosed infection on the transmission dynamics of pertussis we analysed serological data from five European countries in combination with information about social contact patterns from five of those countries to estimate incidence and reproduction numbers.

Methods and findings: We compared two different methods for estimating incidence from individual data on IgG pertussis toxin (PT) titres. One method combines the cross-sectional surveys of titres with longitudinal information about the distribution of amplitude and decay rate of titres in a back-calculation approach. The second method uses age-dependent contact matrices and cross-sectional surveys of IgG PT titres to estimate a next generation matrix for pertussis transmission among age groups. The next generation approach allows for computation of basic reproduction numbers for five European countries. Our main findings are that the seroincidence of infections as estimated with the first method in all countries lies between 1% and 6% per annum with a peak in the adolescent age groups and a second lower peak in young adults. The incidence of infections as estimated by the second method lies slightly lower with ranges between 1% and 4% per annum. There is a remarkably good agreement of the results obtained with the two methods. The basic reproduction numbers are similar across countries at around 5.5.

Conclusions: Vaccination with currently used vaccines cannot prevent continued circulation and reinfection with pertussis, but has shifted the bulk of infections to adolescents and adults. If a vaccine conferring lifelong protection against clinical and subclinical infection were available pertussis could be eliminated. Currently, continuing circulation of the pathogen at a subclinical level provides a refuge for the pathogen in which it can evolve and adjust to infect vaccinated populations. Please see later in the article for the Editors' Summary.

Figure 1. Seroincidence of pertussis in five European countries.

Maximum likelihood estimates and (likelihood-based) 95% confidence intervals for the infection incidence as a function of age (5-y age classes).

Figure 2. Seropositive fraction per age class from serological data using a standard diagnostic test (dots) and fitted model (solid line).

The error bars represent 95% bootstrapping confidence intervals.

Figure 3. Estimates for the age-dependent force of infection (risk per year of becoming infected) for five European countries.

DE, Germany; FI, Finland; IT, Italy; NL, The Netherlands.

Figure 4. Comparison of incidence estimates from both estimation methods.

Dots, seroincidence with error estimates; solid lines, incidence of infections that test positively with diagnostic tests with 95% credible intervals (shaded).