Revisiting the epidemiology of pertussis in Canada, 1924-2015: a literature review, evidence synthesis, and modeling study

Abstract

Background: Disease surveillance is central to the public health understanding of pertussis epidemiology. In Canada, public reporting practices have significantly changed over time, creating challenges in accurately characterizing pertussis epidemiology. Debate has emerged over whether pertussis resurged after the introduction of adsorbed pertussis vaccines (1981-1985), and if the incidence fell to its pre-1985 after the introduction of acellular pertussis vaccines (1997-1998). Here, we aim to assemble a unified picture of pertussis disease incidence in Canada.

Methods: Using publicly available pertussis surveillance reports, we collected, analyzed and presented Canadian pertussis data for the period (1924-2015), encompassing the pre-vaccine era, introduction of vaccine, changes to vaccine technology, and the introduction of booster doses. Information on age began to be reported since 1952, but age reporting practices (full, partial or no ages) have evolved over time, and varied across provinces/territories. For those cases reported without age each year, we impute an age distribution by assuming it follows that of the age-reported cases.

Results: Below the age of 20 years, the adjusted age-specific incidence from 1969 to 1988 is substantially higher than existing estimates. In children < 1 year, the incidence in some years was comparable to that during the 1988-1999 resurgence.

Conclusions: The results presented here suggest that the surge in the average yearly incidence of pertussis that began in 1988 was weaker than previously inferred, and in contrary to the past findings, below age 5, the average yearly incidence of pertussis from 1999 to 2015 (when the incidence dropped again) has been lower than it was from 1969 to 1988.

Keywords: Canada; Epidemiology; Modeling; National surveillance; Pertussis; Pertussis vaccines.

Fig. 1

Summary of the current state of knowledge of this history of pertussis epidemiology in Canada: Yearly incidence rates for total reported cases of pertussis since the start of national reporting in 1924 (blue), and yearly incidence of cases without age information (red). Prior to 1952, no age information was reported at all. See Fig. S1 for a more detailed view of the period 1952–2015. In most years between 1969 and 1988, the majority of cases had no age information

Fig. 2

Yearly age-stratified incidence rates for age-supplied reported cases ($\widehat{{\mathit{R}}_{\mathit{j}}}$) and “Unknown” group $\widehat{{\mathit{R}}^{\mathit{U}}}$. Note that some of the age groupings changed during the years of reporting; these changes are denoted by gaps in the graph. Contrary to Fig. 1, the incidence rate for age-unknown cases is comparatively small throughout, since its denominator is the entire population rather than just one age group

Fig. 3

Yearly adjusted age-stratified incidence rates for combined age-supplied reported cases and proportionally-distributed “Unknown” group $\widetilde{{\mathit{R}}_{\mathit{j}}}$, also calculated via the bootstrapping method. Note that some of the groupings changed during the years of reporting; these changes are denoted by gaps in the graph