Effectiveness of a whole cell pertussis vaccine in child-care centers and schools

Background. Pertussis has substantially increased in Quebec, Canada, since 1990, We estimated pertussis vaccine effectiveness and vaccine coverage in child-care centers and elementary schools. Methods. Two retrospective cohort studies were simultaneously conducted, One included 4482 children attending 88 public child-care centers and the other included 3429 pupils in 14 elementary schools. Cough and pertussis symptoms were assessed through a questionnaire and medical records; immunization status was ascertained by examination of written records. Results. In child-care centers 95% of children had received at least three vaccine doses at the beginning of the follow-up; in schools more than 98% of pupils had received at least 4 doses. With greater than or equal to 4 doses of vaccine and a standard case definition used for surveillance (cough greater than or equal to 2 weeks, greater than or equal to 1 pertussis symptom and no other apparent cause for cough), vaccine effectiveness was estimated at 61% (95% confidence interval, 44 to 72%) in child-care centers and at 60% (95% confidence interval, 10 to 82%) in schools. With the same number of doses but a case definition requiring a cough greater than or equal to 5 weeks, vaccine effectiveness increased to 71% (95% confidence interval, 49 to 83) in child-care centers and to 86% (95% confidence interval, 66 to 94%) in schools. Conclusions. The increase in pertussis in Quebec is not caused by a low vaccine coverage. A low vaccine effectiveness may contribute to the resurgence of pertussis in the past decade.