Bordetella pertussis Isolates, Finland

To the Editor: Pertussis, or whooping cough, is a highly contagious respiratory disease in humans caused by Bordetella pertussis. Reemergence of pertussis has been observed in many countries with high vaccination coverage. In the United States, reported cases of pertussis in adolescents and adults have increased since the 1980s, despite increasingly high rates of vaccination in infants and children (1). At the same time, clinical B. pertussis isolates have become antigenically divergent from vaccine strains (2,3). This observation has raised the question of whether vaccination has caused selection for the variant strains, and whether the reemergence of pertussis in vaccinated populations is due to vaccination not protecting against these antigenic variants as effectively as it protects against vaccine type strains. On the other hand, vaccine-induced immunity wanes over time, and pertussis is not only a childhood disease but also a frequent cause of prolonged illness in adults and adolescents today (4). In Finland, children are vaccinated with diphtheria-tetanus whole-cell pertussis vaccine at 3, 4, and 5 months, and from 20 to 24 months of age. The whole-cell vaccine contains 2 strains and has remained unchanged since 1976. The vaccine strain 18530 contains fimbriae 3 (Fim3), pertussis toxin S1 subunit D (PtxS1D), and per-tactin 1(Prn1); the other vaccine strain, 1772, contains Fim2,3, PtxS1B, and Prn1. Ninety-six percent of Finland's population has been vaccinated with 4 doses of pertussis. The incidence of pertussis is highest in infants <1 year of age and in school-children from 6 to 14 years old, although about 30% of the cases occur in adults older than 20 years. In Finland, as in many other countries with large-scale vaccination programs, several outbreaks of pertussis occurred in the 1990s. We studied prospectively 3 pertussis outbreaks in 2 elementary schools and 1 municipality in southwestern Finland (5,6). The aim of the study was to characterize the strains circulating and causing outbreaks and to track the transmission of B. pertus-sis during these outbreaks. Sample were collected and primary cultures were done as described earlier (5,6). Rusko with 9,900 inhabitants. The isolates were obtained from schools and local health centers. In addition, 1 isolate was obtained from a household contact (See online Table 3 at Most of the cases occurred in school-children >8 years of age and in adults. Various DNA fingerprinting techniques , such as IS1002-based restriction fragment length polymorphism (IS1002-RFLP) and pulsed-field gel electrophoresis (PFGE) have been used to study …