Abstract
BackgroundPertussis 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.MethodsData 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.ResultsFrom 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.ConclusionsToronto'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.
Highlights
Introduction of Highly Sensitivepolymerase chain reaction (PCR) (May 2005-)10.70 initial introduction of PCR in January 1999, and a marked increase in incidence coincident with the introduction of a highly sensitive pertussis assay in May 2005
We performed exploratory analyses in which we evaluated the differential effects of test volume on apparent pertussis incidence both by incorporating multiplicative interaction terms into models, and by performing restriction analyses in which testing effects were evaluated in models restricted to a single testing period
Public Health Laboratory–Toronto (PHLT) provided 82.1% of the records and 17.9% came from Hospital for Sick Children (HSC)
Summary
Introduction of Highly SensitivePCR (May 2005-)10.70 initial introduction of PCR in January 1999, and a marked increase in incidence coincident with the introduction of a highly sensitive pertussis assay in May 2005. The impact of test submission volume on apparent pertussis incidence was diminished after May 2005 as opposed to before May 2005 (IRR per 10 specimens submitted 1.30, 95% CI 1.26 to 1.35 vs IRR 7.93, 95% CI 6.80 to 9.26, P for heterogeneity < 0.001) When this time period was excluded there was no heterogeneity in the effect of test submissions before as opposed to after the introduction of PCR in January 1999 (IRR per 10 specimens submitted 9.30, 95% CI 7.74 to 11.18 vs IRR 4.59, 95% CI 3.35 to 6.27, P = 0.14). Some with infant deaths, in Nottingham and Derby (England) [12], California [13], Ireland [14] and New South Wales, Australia [15] have been a source of considerable concern It has been suggested, that increasingly sensitive PCR methods for the diagnosis of pertussis may have contributed to apparent, rather than real, increases in disease incidence [7,9,16,17,18]. The role played by young adolescents and adults in disease spread,[24] and the recognition of a gradual loss in immunity after natural infection or vaccination, have led several countries to advocate booster dosing of pertussis vaccine for young teens and adults [16,25,26,27]
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