Context: The prevalence of Bordetella pertussis and Bordetella parapertussis infections among outpatients in an immunized population is not known.
Objective: To study the prevalence of these infections in outpatients with paroxysmal cough in Finland, where the pertussis vaccine coverage of 4 doses is 98%.
Design: Prospective cohort study.
Setting: Thirty-two health centers in southwestern Finland.
Patients: A total of 584 patients with paroxysmal cough seen at local health centers from October 1994 through March 1997 from whom nasopharyngeal swabs were collected.
Main outcome measures: Prevalence of positive cultures for B pertussis or B parapertussis and/or positive polymerase chain reaction (PCR) results and frequency of symptoms in those with pertussis and parapertussis.
Results: A total of 153 subjects (26.2%) had Bordetella infection by culture or PCR: 93 (60.8%) had B pertussis infection, 49 (32.0%) had B parapertussis infection, and 1 1 (7.2%) had both. Of these cases, 39 (25.5%) had positive cultures and 95 (62.1%) had positive PCR results for B pertussis, and 19 (12.4%) had positive cultures and 55 (35.9%) had positive PCR results for B parapertussis. At the time of diagnosis, no difference was found in the frequency of symptoms between patients with B parapertussis infection and those with B pertussis infection. Bordetella parapertussis infection was as common as B pertussis infection in children before school entry, whereas in schoolchildren and adults, B pertussis infection was more common than B parapertussis infection (P<.001).
Conclusion: Bordetella infections are common in an immunized population, and B parapertussis infections apparently are more prevalent than previously documented.