Pertussis in rural populations of Saskatchewan (1995 to 2003): incidence, seasonality, and differences among cases.

backgroundFew Canadian studies have studied re-emergent pertussis in rural areas. This study described the epidemiology of pertussis in the rural areas of the Saskatoon Regional Health Authority in Saskatchewan, and comparisons were made to the City of Saskatoon.

methodsAnalyses were based on passive surveillance data collected between 1995 and 2003. Estimates of the cumulative incidence (per 10,000 population) measured the occurrence of pertussis. Kaplan-Meier curves were plotted to compare a case's time until disease from their last vaccination by different vaccine types (whole-cell (WCV), or acellular (ACV)) and vaccine histories (complete or partial). Epidemic curves identified peaks in the incidence and checked for seasonal variation in case reporting.

resultsOver the 9-year period, 1,135 cases were reported. Rural areas had higher incidence rates and lower proportions of cases vaccinated than urban areas. Overall, the highest age-specific incidence was observed in people aged 10-19 years. Cases aged 0-9 years vaccinated with the ACV, from both rural and urban areas, presented a shorter time to disease (14 months (95% CI: 13-16) and 17 months (95% CI: 11-21), respectively) when compared to cases vaccinated with the WCV (47 months (95% CI: 40-51) and 36 months (95% CI: 31-41), respectively), or with a combination of the two vaccines (40 months (95% CI: 27-47) and 44 months (95% CI: 36-51), respectively, p<0.01). Epidemic curves revealed that reported cases are occurring earlier in successive years (1997, 1999, and 2003) in rural areas.

conclusionEpidemiologic differences among cases from rural areas exist when compared to urban areas. This study further emphasizes the need to better understand age-, vaccine-, and seasonally-related aspects of pertussis epidemiology in rural areas.