Elsevier

Vaccine

Volume 26, Issue 36, 26 August 2008, Pages 4697-4703
Vaccine

Co-morbidities associated with influenza-attributed mortality, 1994–2000, Canada

https://doi.org/10.1016/j.vaccine.2008.06.087Get rights and content

Abstract

The elderly and persons with specific chronic conditions are known to face elevated morbidity and mortality risks resulting from an influenza infection, and hence are routinely recommended for annual influenza vaccination. However, risk-specific mortality rates have not been established. We estimated age-specific influenza-attributable mortality rates stratified by the presence of chronic conditions and type of residence based on deaths of persons who were admitted to hospital with a respiratory complication captured in our national database. The majority of patients had chronic heart or respiratory conditions (80%) and were admitted from the community (80%). Influenza-attributable mortality rates clearly increase with age for all risk groups. Our influenza-specific estimates identified higher risk ratios for chronic lung or heart disease than have been suggested by other methods. These estimates identify groups most in need of improved vaccines and for whom the use of additional strategies, such as immunization of household contacts or caregivers should be considered.

Introduction

While immunization programs remain the cornerstone for the control of influenza [1], [2], each year influenza is associated with considerable respiratory and all-cause mortality [3], [4], [5], [6]. Although most public health programs target persons thought to be at higher risk of complications of influenza for vaccination [1], [2]; they generally do not differentiate the degree of risk associated with various co-morbid conditions. Quantification of the relative mortality risks associated with influenza infection by age and chronic conditions would help identify target groups for further vaccine development, inform public communications planning for annual vaccination campaigns, assist with prioritization in the case of vaccine shortages and guide physicians in their recommendation of additional measures to prevent or treat influenza illness during seasonal outbreaks.

In previous work we established statistical estimates of influenza-attributable mortality [6] and hospitalization rates [7]. However, the relative risks of the high-risk conditions for influenza complications have not been evaluated in a national dataset. We sought to identify cohorts based on age, health status and place of residence that accounted for the largest influenza mortality burden. By combining estimates from several comprehensive national databases, we calculated age-specific influenza-attributed mortality rates from the estimated number of influenza-attributed deaths for those cohorts for which age-specific measures of health status for the Canadian population were available.

Section snippets

Methods

Statistical methods, which involve matching the weekly pattern of various proxy variables for influenza activity to the weekly pattern of deaths using Poisson regression, have been established for the estimation the number of deaths or hospitalizations attributable to influenza from national databases. These methods estimate, for example, the number influenza-attributable deaths from records of all cause mortality and proxy variables for influenza activity. We used these same methods to

Results

Over the study period, approximately 4000 influenza-attributable deaths occurred per year for a rate of 14 per 100,000 population. The number of deaths per year varied with rate estimates ranging from 4 to 21/100,000. Mortality rates increased with age from 23/100,000 persons aged 65–69 years to 831/100,000 persons aged 90 and over (Table 1), resulting in a median age at death of 80–84 years. For those 65 years of age and over, the average influenza-attributable mortality rate was estimated as

Discussion

This study of age-specific mortality rates according to health status (known co-morbidity) identifies that age as well as co-morbidity contribute independently to elevated risk of influenza-associated mortality. Our influenza-specific estimates identified higher risk ratios for chronic lung or heart disease than have been suggested by other methods [20], [22]. Increasing risk with age was apparent for all health status groups. Regardless of health status, over 90% of the mortality burden was in

Acknowledgements

The authors acknowledge the support of Statistics Canada and the provincial and territorial vital statistics registries in providing access to the Canadian vital statistics databases, the Canadian Institute for Health Information, the National FluWatch Network and the Data Coordination and Access Program of the Public Health Agency of Canada, and all those involved in the collection and compiling of this data.

Contributors: Dena Schanzer, Joanne Langley and Theresa Tam conceived and designed the

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