Co-morbidities associated with influenza-attributed mortality, 1994–2000, Canada
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.
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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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2021, International Journal of Infectious DiseasesCitation Excerpt :Interestingly, a slightly larger proportion of patients with Influenza have underlying comorbidities such as hypertension, diabetes, or COPD. In previous studies, these factors have been associated with a complicated course of Influenza infection (Lina et al., 2020; Schanzer et al., 2008; Segaloff et al., 2018). In accordance, the reported comorbidities are similarly frequent in patients with COVID-19 with severe disease progression, increase with advanced age, and might be associated with a poor outcome (Aggarwal et al., 2020; Brück, 2020; Richardson et al., 2020; Zhou et al., 2020).