Pertussis resurgence in Canada largely caused by a cohort effect

Background. Beginning in 1990 Canada experienced a resurgence of pertussis. Changes in incidence and hospitalization according to age in the province of Quebec between 1983 and 1998 were examined to assess the presence of a cohort effect resulting from a poorly protective vaccine. Methods. The source of data on incident cases was pertussis notifications to the Quebec Ministry of Health and Social Services. Hospitalization data were extracted from the administrative database that collects information on each hospitalization. Results. The mean annual incidence before 1990 was 3.8 cases per 100 000 population which increased to 37.2 thereafter. Infants had the smallest increase (2.7-fold) when compared with children between 1 and 19 years who experienced a 9- to 15-fold increase and with adults (22.5-fold). The mean annual hospitalization rates increased from 2.7 per 100 000 before 1990 to 5.2 afterward. Ninety percent of hospitalizations occurred in children <5 years of age. The proportion of cases in 0- to 4-year-old children decreased, whereas it increased steadily in all other age groups during the entire study period. Between 1990 and 1998 the median age of cases shifted from 4.4 to 7.8 years. Pertussis affected predominantly children who were immunized with a vaccine introduced in the mid-1980s. The evolution of the age distribution of cases paralleled the aging of this cohort with a slow but steady drift of disease from early childhood to adolescence. Conclusion. The sudden increase in pertussis incidence in Canada can be largely attributed to a cohort effect resulting from a poorly protective pertussis vaccine used between 1985 and 1998.