Pertussis resurgence in Toronto, Canada: a population-based study including test-incidence feedback modeling

Abstract

Background: Pertussis continues to challenge medical professionals; recently described increases in incidence may be due to age-cohort effects, vaccine effectiveness, or changes in testing patterns. Toronto, Canada has recently experienced increases in pertussis incidence, and provides an ideal jurisdiction for evaluating pertussis epidemiology due to centralized testing. We evaluated pertussis trends in Toronto using all available specimen data, which allowed us to control for changing testing patterns and practices.

Methods: Data included all pertussis culture and PCR test records for Greater Toronto from 1993 to 2007. We estimated incidence trends using Poisson regression models; complex relationships between disease incidence and test submission were explored with vector autoregressive models.

Results: From 1993 to 2007, 26988 specimens were submitted for testing; 2545 (9.4%) were positive. Pertussis incidence was 2 per 100,000 from 1993 to 2004 and increased to 10 per 100,000 from 2005-2007, with a concomitant 6-fold surge in test specimen submissions after the introduction of a new, more sensitive PCR assay. The relative change in incidence was less marked after adjustment for testing volumes. Bidirectional feedbacks between test positivity and test submissions were identified.

Conclusions: Toronto's recent surge in pertussis reflects a true increase in local disease activity; the apparent size of the outbreak has likely been magnified by increasing use of pertussis testing by clinicians, and by improved test sensitivity since 2005. These findings may be applicable to changes in pertussis epidemiology that have been noted elsewhere in North America.

Figure 1

Comparative Best-Fit Seasonal Waveforms for Pertussis and Pertussis Testing. Pertussis incidence (thin solid curve), pertussis culture positivity (dotted curve), and pertussis PCR positivity (dashed curve) all display significant autumn seasonality; testing volumes (dark solid curve) are also distinctly seasonal but surge in winter.

Figure 2

Temporal Trends in Pertussis Testing and Estimated Crude Pertussis Incidence, Greater Toronto Area, Canada, 1993-2007. Cumulative weekly tests are presented as gray bars; annualized pertussis rates are presented as rates per million rather than per 100,000 (as in text) to maintain comparability of scales. Dashed lines represent initial introduction of PCR testing (January 1999) and introduction of more sensitive PCR assay (May 2005). It can be seen that there is a strong correlation between specimen submission and estimated pertussis incidence.

Figure 3

Proportion of Test Submissions Positive by Year. The fraction of tests positive by culture (white bars) has declined over time, but the fraction positive by PCR (gray bars) has increased since 2000, with a concomitant rebound in the overall proportion of test specimens testing positive.