Variation in the Bordetella pertussis virulence factors pertussis toxin and pertactin in vaccine strains and clinical isolates in Finland

There is evidence that pertussis is reemerging in vaccinated populations. We have proposed, and provided evidence for, one explanation for this phenomenon in The Netherlands: antigenic divergence between vaccine strains and circulating strains. Finland has a pertussis vaccination history very similar to that of The Netherlands, and yet there is no evidence for an increase in the incidence of pertussis to the extent that it was observed in The Netherlands. A comparison of the Bordetella pertussis strains circulating in the two countries may shed light on the differences in pertussis epidemiology. Here we investigated whether temporal changes had occurred in pertussis toxin and pertactin types produced by the Finnish B. pertussis population. We show that strains isolated before 1964 produced the same pertussis toxin and pertactin variants as the vaccine strains. However, these vaccine types were replaced in later years, and in the 1990s most strains were distinct from the vaccine strains with respect to the two proteins. These trends are similar to those found in the Dutch B. pertussis population. An interesting difference between the contemporary Finnish and Dutch B. pertussis populations was found in the frequencies of pertactin variants, possibly explaining the distinct epidemiology of pertussis in the two countries.