http://www.cdc.gov/flu/about/disease/us ... deaths.htmQuestions and Answers Regarding Estimating Deaths from Influenza in the United States
How many people die from flu each year in the United States?
The number of influenza-associated (i.e., flu-related) deaths varies from year to year because flu seasons often fluctuate in length and severity. CDC estimated that about 36,000 people died of flu-related causes each year, on average, during the 1990s in the United States. This figure includes people dying from complications of flu. This estimate came from a 2003 study published in the Journal of the American Medication Association (JAMA), which looked at the 1990-91 through the 1998-99 flu seasons . Statistical modeling was used to estimate how many flu-related deaths occurred among people whose underlying cause of death on their death certificate was listed as a respiratory or circulatory disease. During these years, the number of estimated deaths ranged from 17,000 to 52,000.
In 2009, CDC published additional estimates of flu-related deaths comparing different methods, including the methods used in the 2003 JAMA study. The seasons studied included the 1993-94 through the 2002-03 flu seasons . Results from this study showed that during this time period, 36,171 flu-related deaths occurred per year, on average.
How did CDC estimate that an average of 36,000 people die in the U.S. each year from flu?
This statistic came from a 2003 JAMA study by CDC scientists . The study used statistical modeling to estimate that during 9 influenza seasons from 1990-91 through 1998-99, an annual average of 36,000 flu-related deaths occurred among people whose underlying cause of death on their death certificate was listed as a respiratory or circulatory disease. A 2009 study that appeared in the journal Influenza and Other Respiratory Viruses made a similar estimate for the 10 influenza seasons from 1993 to 2003 .
What are flu-related deaths?
Flu-related deaths are deaths that occur in people for whom influenza infection was likely a contributor to the cause of death, but not necessarily the primary cause of death.
Does CDC know the exact number of people who die from flu each year?
CDC does not know exactly how many people die from flu each year. There are several reasons for this: First, states are not required to report individual flu cases or deaths of people older than 18 years of age to CDC. Second, influenza is infrequently listed on death certificates of people who die from flu-related complications . Third, many flu-related deaths occur one or two weeks after a person’s initial infection, either because the person may develop a secondary bacterial co-infection (such as a staph infection) [1,8,11] or because influenza can aggravate an existing chronic illness (such as congestive heart failure or chronic obstructive pulmonary disease) . Also, most people who die from flu-related complications are not tested for flu, or they seek medical care later in their illness when influenza can no longer be detected from respiratory samples. Influenza tests are only likely to detect influenza if performed within a week after onset of illness. For these reasons, many flu-related deaths may not be recorded on death certificates. These are some of the reasons that CDC and other public health agencies in the United States and other countries use statistical models to estimate the annual number of flu-related deaths. (Flu deaths in children were made a nationally notifiable condition in 2004, and since then, states have been required to report flu-related child deaths in the United States through the Influenza Associated Pediatric Mortality Surveillance System).
Why does CDC estimate deaths associated with flu?
CDC feels it is important to convey the full burden of flu to the public. Flu is a serious disease that causes illness and deaths nearly every year in the United States. CDC estimates of annual influenza-associated deaths in the United States are made using well-established scientific methods that have been reviewed by scientists outside of CDC . CDC feels that these estimates are a timely representation of the current burden of flu on the United States.
Why does CDC model flu-related deaths using death certificates with the underlying cause of death listed as a respiratory or circulatory disease?
CDC uses underlying respiratory and circulatory (R&C) deaths in its mortality modeling because R&C deaths provide an estimate of deaths associated with respiratory infections that is more sensitive than underlying pneumonia and influenza (P&I) deaths and more specific than all-cause deaths.
What proportion of pneumonia and influenza deaths, respiratory and circulatory deaths and all-cause deaths are attributed to influenza?
For pneumonia and influenza (P&I) deaths, CDC estimates approximately 8,000 deaths are associated with flu. This represents 9.8% of P&I deaths. For respiratory and circulatory (R&C) deaths, CDC estimates approximately 36,000 deaths are associated with flu. This represents 3.1% percent of those deaths. For all-cause deaths, CDC estimates that approximately 51,000 deaths are associated with flu. This represents 2.2% of all deaths.
What proportion of acute respiratory disease hospitalizations is associated with influenza?
In one recent study of children younger than 5 years of age (Poehling et al, 2006), 6% of children hospitalized during the year with an acute respiratory tract infection or fever tested positive for influenza .
Why doesn’t CDC base its flu mortality estimates only on death certificates that specifically list influenza?
Influenza may lead to death from other causes, such as from congestive heart failure or chronic obstructive pulmonary disease, as was observed during the 1957-1958 pandemic.  It has been recognized for many years that influenza is infrequently listed on death certificates  and testing for influenza infections usually not done, particularly among the elderly who are at greatest risk of influenza complications and death. Some deaths – particularly in the elderly – are associated with secondary complications of influenza (including bacterial pneumonias). Influenza virus infection may not be identified in many instances because influenza virus is only detectable for a short period of time and many people don’t seek medical care until after the first few days of acute illness. For these reasons, statistical modeling strategies have been used to estimate flu-related deaths for many decades, both in the United States and the United Kingdom. Only counting deaths where influenza was included on a death certificate would be a gross underestimation of influenza’s true impact. [1,4,6-8,10,11].
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