Impact of polymerase chain reaction on clinical pertussis research: Finnish and Swiss experiences.

Since April 1993 in Finland and March 1994 in Switzerland, polymerase chain reaction (PCR) has been used routinely nationwide for the diagnosis of pertussis. Nasopharyngeal specimens from 3794 patients suspected of having pertussis and 1125 controls were tested. Finnish and Swiss assays found 23% and 36% of clinical specimens positive, respectively. PCR showed a higher incidence of pertussis infection among 1- to 6-year-old children in Switzerland than in Finland (P < .001). This difference may be due to the booster dose of vaccine given at 2 years of age in Finland but not in Switzerland. In Finland, PCR-confirmed asymptomatic cases were more common among children <7 years old than in older children (P < .001), whereas older children tended to have symptomatic infection. The use of PCR markedly improves the diagnosis of pertussis and opens new perspectives for epidemiologic and vaccine efficacy studies.