Kenneth Stoller, M.D.
International Hyperbaric Medical Association
Dear Dr. Stoller:
On behalf of the National Center for Immunization and Respiratory Disease, Centers for Disease Control and Prevention (CDC), I am responding to your October 23, 2008, Information Quality Request for Correction regarding CDC estimates of annual influenza-associated deaths in the United States. CDC staff who are experts in influenza disease have carefully reviewed your request for correction and provide the following response.
It has been recognized for many years that influenza is infrequently listed on death certificates of persons who die with an influenza-related complication 1 and testing for influenza infections has been infrequent, particularly among the elderly, who are at greatest risk of serious complications from influenza. In addition, many influenza-associated deaths occur one or two weeks after the initial infection, either because of the development of secondary bacterial infections 3-5 or because influenza exacerbates chronic illnesses (e.g., congestive heart failure or chronic obstructive pulmonary disease) 6. These patients’ influenza diagnosis might have been missed because influenza tests are only likely to detect influenza if performed during the first 5 days after onset of illness when virus shedding is most likely to occur. Therefore, direct counting of a specific International Classification of Diseases (ICD) code for influenza on death certificates (the source of the National Center for Health Statistic counts to which you refer in your letter) likely substantially underestimates influenza-associated mortality.
Because direct counting from death certificates coded specifically with the code for influenza provides an underestimate, statistical modeling strategies have been used to estimate influenza-associated deaths for many decades, both in the United States and the United Kingdom, and such methods are now being used all over the world 2;3-5;7-10. CDC estimates of annual influenza-associated deaths in the United States are made using well-established scientific methods that have been peer reviewed 2. This method was used to generate the statistics to which you refer: CDC confirms that during the influenza seasons 1990–1991 through the 1998–1999, an estimated average of 36,000 influenza-associated deaths occurred annually in the United States, when deaths with an underlying respiratory or cardiac cause was the outcome of interest 2. The annual average of 36,000 influenza-associated deaths for the 1990-91 influenza season through the 1998-99 influenza season refers to respiratory and circulatory deaths, which we estimate were related to influenza circulation. It is also important to recognize the variability in mortality among influenza seasons. During the period for which CDC estimated 36,000 annual influenza-associated deaths, on average, the annual estimates ranged from 17,000 to 51,000 2.
CDC thinks it is important to convey the full burden of influenza to the public. Thus, we do not agree that it is misleading to combine direct deaths associated with influenza with those likely associated with secondary pneumonia and estimates of those likely associated exacerbations of underlying chronic conditions by influenza into a single number. However, we have made editorial changes to our website to ensure that we consistently communicate that the 36,000 figure is a) an estimate, b) an average, and, c) that these are "influenza-associated" deaths. (Specific language used may include: "influenza-associated"; "flu-associated"; "influenza-related", or "flu-related".)
One example of this would be, "On average, in the United States, about 36,000 people die from flu-related causes each year."
In addition, we have added a link on our website from the 36,000 figure at http://www.cdc.gov/flu/about/disease/us_flu-related_deaths.htm, to a Question & Answer related to this mortality statistic and how it is derived.
We hope that this information and the clarifications we have made to the language on our website answers your concerns. If you wish to appeal this response, you may submit a written appeal or an electronic request for reconsideration within 30 days of receipt of this letter. The appeal must state the reasons why this CDC response is insufficient or inadequate. You must attach a copy of your original request and this CDC response to it. Also, clearly mark the appeal with the words "Information Quality Appeal." Please send the appeal to the following address:
Centers for Disease Control and Prevention
Management Analysis and Services Office
1600 Clifton Road, N.E., Mailstop E-11
Atlanta, Georgia 30333
Facsimile: (404) 929-2781
E-mail: InfoQuality@cdc.gov Electronic
Beth P. Bell MD, MPH
National Center for Immunization and
Letter to Dr. K. Stoller: References
1Wiselka M. Influenza: diagnosis, management, and prophlaxis. BMJ 1994; 308:1341–1345.
2 Thompson WW, Shay DK, Weintraub E, Brammer L, Cox N, Anderson LJ et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA 2003; 289(2):179–186.
3 Tillett HE, Smith JW, Clifford RE. Excess morbidity and mortality associated with influenza in England and Wales. Lancet 1980; 1(8172):793–795.
4 Dushoff J, Plotkin JB, Viboud C, Earn DJ, Simonsen L. Mortality due to Influenza in the United States—An Annualized Regression Approach Using Multiple-Cause Mortality Data. Am J Epidemiol. 2006 Jan 15;163(2):181–7.
5 Simonsen L, Reichert TA, Viboud C, Blackwelder WC, Taylor RJ, Miller MA. Impact of Influenza Vaccination on Seasonal Mortality in the US Elderly Population. Arch Intern Med 2005; 165(3):265–272.
6 Falsey AR, Hennessey PA, Formica MA, Cox C, Walsh EE. Respiratory syncytial virus infection in elderly and high-risk adults. N Engl J Med 2005; 352(17):1749–1759.
7 Simonsen L, Clarke MJ, Williamson GD, Stroup DF, Arden NH, Schonberger LB. The impact of influenza epidemics on mortality: introducing a severity index. Am J Public Health 1997; 87(12):1944–1950.
8 Serfling RE. Methods for Current Statistical Analysis of Excess Pneumonia-Influenza Deaths. Public Health Rep 1963;78 (6):494–505.
9 Nicholson KG. Impact of influenza and respiratory syncytial virus on mortality in England and Wales from January 1975 to December 1990. Epidemiol Infect 1996;116 (1):51–63.