Influenza and Acute Myocardial Infarction

Excess winter mortality is widely recognized in countries with temperate climates, although the cause has been debated for .80 years [1]. Some studies have suggested that environmental factors such as temperature and humidity could be responsible for triggering thrombotic and cardiovascular events [2–5]. The association of winter excess cardiovascular and respiratory mortality with the circulation of seasonal influenza has also been postulated [1, 2, 5–9]. Winter surges in mortality correlate with hospital respiratory and cardiovascular admissions, particularly in the elderly [7, 10, 11]. Estimation of influenza-related excess mortality often includes not only influenza but excess cardiovascular and other respiratory-related deaths, as deaths coded as influenza tend to be few and greatly underestimate the impact of influenza, especially in the elderly [2, 6]. Including excess causes of death other than influenza to measure influenzaassociated disease burden is justified because influenza testing is not performed in most cases or when performed, rapid antigen tests with submoptimal sensitivity are often used [12]. In addition, persons with influenza can die from pneumonia, exacerbation of chronic respiratory disease, or secondary complications related to other underlying medical conditions [9, 13]. The direct and indirect effects of influenza on cardiovascular and respiratory mortality are difficult to estimate [13]. Although an association between influenza and cardiovascular events has been described as plausible and demonstrated in many studies [1, 2, 5–9, 14], the specificity of this association has been questioned because the relationship of influenza to specific causes of cardiovascular mortality has not been well described. In this issue of the Journal, WarrenGash et al [15] shed considerable light on the complex relationship of climate, influenza, and a major source of cardiovascular mortality, acute myocardial infarction (AMI). Warren-Gash et al assessed the relationship between the circulation of influenza viruses in the community and hospital admissions and death due to AMI in the temperate climate of England and Wales, where influenza has a marked winter peak, and the subtropical climate of Hong Kong, where influenza viruses circulate yearround with much less pronounced peaks during winter and summer months. This natural experiment allowed the evaluation of the relationship of influenza to AMI, independent of cold weather effects. ICD9– and ICD-10–coded data on AMIrelated hospitalizations and deaths were obtained from both jurisdictions from January 1999 through December 2008. General practitioner consultation rates for influenza-like illness (ILI) in the United Kingdom and the percentage of respiratory specimens positive for influenza in Hong Kong were used as measures of influenza activity and compared with AMI hospitalizations and death rates by week. Data were modeled using Poisson regression, where level of influenza activity was the primary exposure, controlling for temperature and relative humidity. Sensitivity analyses were performed for the independent variables. Finally, the percentages of AMI deaths and hospitalizations attributable to influenza were calculated. The authors also modeled coded data on colon cancer and fractured neck of the femur with the same independent variables as controls. There was a strong association between population levels of influenza and AMI hospitalizations and deaths in both temperate and subtropical climates after adjusting for seasonality and environmental factors. In both countries, the strongest association between influenza and AMI was among the oldest age groups. The sensitivity analyses had little effect on the magnitude or direction of the estimates. Conversely, colon cancer and fractured neck of the femur were not associated with influenza activity. In this study, 3.9%–5.6% of AMI deaths in Hong Kong and 3.1%–3.4% AMI of deaths in Received and accepted 17 February 2011. Potential conflicts of interest: none reported. Correspondence: Lyn Finelli, DrPH, MS, Influenza Division, Centers for Disease Control and Prevention, 1600 Clifton Rd NE, MS A 32, Atlanta, GA 30333 (lyf8@cdc.gov). The Journal of Infectious Diseases 2011;203:1701–4 Published by Oxford University Press on behalf of the Infectious Diseases Society of America 2011. 0022-1899 (print)/1537-6613 (online)/2011/20312-0001$14.00 DOI: 10.1093/infdis/jir175