Background: From October 2005 to March 2006, a laboratory-confirmed outbreak of pertussis occurred in preschool-aged children (1–4 years) in Toronto, Canada. A case-control study in children was done to identify the risk factors for being positive for Bordetella pertussis by polymerase chain reaction (PCR).
Methods: A case was defined as an individual 0 to 18 years of age who tested positive for B. pertussis by PCR in Toronto from October 2005 to March 2006. Each case was matched to 2 controls (negative for B. pertussis by PCR) by: (1) age, (2) geographic region of testing, and (3) date of testing.
Results: One hundred eighty-nine cases and 296 controls were enrolled. Only 42% of cases (vs. 25% controls, P < 0.0001) met a clinical definition of pertussis, 3 cases (4 controls) were hospitalized, and the secondary household attack rate was 8%. One-third of cases were positive for another respiratory pathogen and >90% of cases and controls were up-to-date with pertussis immunization. Children attending school (matched odds ratio [ORm] = 5.2; 95% confidence intervals [CI]: 1.4–19.0), day care (ORm = 2.2; 95% CI: 1.2–4.0), visiting a doctor's office (ORm = 1.6; 95% CI: 1.0–2.5), or exposed to a household member (median age: 4–5 years) with a pertussis-like cough (ORm = 1.9; 95% CI: 1.0–3.6) were significantly more likely to have a positive PCR test for B. pertussis.
Conclusions: The main risk factors for PCR positivity for B. pertussis were school or day care attendance. Atypical symptoms were likely moderated by high immunization rates or may have been caused by other respiratory pathogens. In some cases, a positive PCR result might simply have reflected transient nasopharyngeal carriage of B. pertussis.
From the *Division of Infectious Diseases, Department of Pediatrics, Hospital for Sick Children, University of Toronto, Toronto, Canada; †Clinical and Environmental Bacteriology, Public Health Laboratories, Ontario Agency for Health Protection and Promotion, Toronto, Canada; ‡Division of Microbiology, Department of Pediatric Laboratory Medicine, Hospital for Sick Children, University of Toronto, Toronto, Canada; §Communicable Disease Control, Toronto Public Health, Toronto, Canada; ∥Department of Pediatrics, Children's Hospital, London Health Sciences Centre, University of Western Ontario, London, Canada; ¶Canadian Center for Vaccinology, IWK Health Centre, Dalhousie University, Halifax, Canada; and Departments of #Pediatrics and **Microbiology and Immunology, IWK Health Centre, Dalhousie University, Halifax, Canada.
Accepted for publication December 8, 2008.
Supported by an unrestricted grant from the Ontario Ministry of Health and Long-Term Care and Sanofi Pasteur.
The funders did not participate in the design and conduct of the study, in the collection, analysis, and interpretation of the data, or in the preparation, review, or approval of the manuscript.
Address for correspondence: Valerie Waters, MD, Division of Infectious Diseases, Hospital for Sick Children, 555 University Ave, Toronto, Ontario M5G 1X8, Canada. E-mail: email@example.com.