Genetic evolution under vaccine pressure: the Bordetella pertussis model.

A possible genetic selective pressure related to the long-term use of vacci nes has been the object of recent theoretical thought and publications. For more than thirty years, an effective vaccine has been in use against whoop ing cough on a wide scale basis in several countries. Thus, the Bordetella pertussis model may contribute to the analysis of an evolutionary risk link ed to the vaccine. To maintain and improve the control of whooping cough, b etter vaccination coverage must be achieved in countries where prevalence i s low. In countries where high vaccination coverage has been achieved over a long period, a trend toward the resurgence of the disease has been observ ed. Efforts are therefore now being directed toward primary vaccination and boosters. These two targets require new vaccines with fewer side effects. Outbreaks in highly vaccinated populations have been reported, raising the issues of vaccine efficacy, of the long-term effect of vaccines on the tran smission of the disease, and of genetic selective pressure. Time trend modi fications of circulating strains related to vaccination practices and vacci ne types have been observed and are compatible with a selective pressure of the vaccine on related pathogens. However evidence for a causal relation i s lacking. In order to monitor and understand the various effects the vacci ne may be having on the effectiveness of immunisation against whooping coug h, further surveillance is needed, integrating a standardised characterisat ion of circulating strains and vaccines by way of a space-time sampling mod el.