Estimating Seasonal Influenza-Associated Deaths in the United States: CDC Study Confirms Variability of Flu

Questions and Answers

What are seasonal influenza-related deaths?

Seasonal influenza-related deaths are deaths that occur in people for whom seasonal 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 seasonal flu each year?

CDC does not know exactly how many people die from seasonal flu each year. There are several reasons for this. First, states are not required to report individual seasonal flu cases or deaths of people older than 18 years of age to CDC. Second, seasonal influenza is infrequently listed on death certificates of people who die from flu-related complications. Third, many seasonal 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 bacterial pneumonia) or because seasonal influenza can aggravate an existing chronic illness (such as congestive heart failure or chronic obstructive pulmonary disease). Also, most people who die from seasonal flu-related complications are not tested for flu, or they seek medical care later in their illness when seasonal influenza can no longer be detected from respiratory samples. Sensitive influenza tests are only likely to detect influenza if performed within a week after onset of illness. In addition, some commonly used tests to diagnose influenza in clinical settings are not highly sensitive and can provide false negative results (i.e. the misdiagnose flu illness as not being flu.) 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 seasonal 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 seasonal flu?

CDC feels it is important to convey the full burden of seasonal flu to the public. Seasonal flu is a serious disease that causes illness, hospitalizations, and deaths 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.

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What categories does CDC use to estimate flu-associated deaths?

CDC uses two categories of underlying cause of death information listed on death certificates: pneumonia and influenza (P&I) causes and respiratory and circulatory (R&C) causes. CDC uses statistical models with records from these two categories to make estimates of influenza-associated mortality. CDC uses underlying R&C deaths (which include P&I deaths) as the primary outcome in its mortality modeling because R&C deaths provide an estimate of deaths that include secondary respiratory or cardiac complications that follow influenza. R&C causes of death are more sensitive than underlying P&I deaths and more specific than all-cause deaths.

How many people die from seasonal flu each year in the United States?

The number of seasonal influenza-associated (i.e., seasonal flu-related) deaths varies from year to year because flu seasons are unpredictable and often fluctuate in length and severity. Therefore, a single estimate cannot be used to summarize influenza-associated deaths. Instead, a range of estimated deaths is a better way to represent the variability and unpredictability of flu. An August 27, 2010 MMWR report entitled “Thompson MG et al. Updated Estimates of Mortality Associated with Seasonal Influenza through the 2006-2007 Influenza Season. MMWR 2010; 59(33): 1057-1062.," provides updated estimates of the range of flu-associated deaths that occurred in the United States during the three decades prior to 2007. CDC estimates that from the 1976-1977 season to the 2006-2007 flu season, flu-associated deaths ranged from a low of about 3,000 to a high of about 49,000 people. Death certificate data and weekly influenza virus surveillance information was used to estimate how many flu-related deaths occurred among people whose underlying cause of death was listed as respiratory or circulatory disease on their death certificate.

How do the new estimates compare with the 36,000 figure that is often cited as an estimate of annual flu-associated deaths?

The 36,000 estimate was presented in a 2003 study by CDC scientists published in the Journal of the American Medical Association (JAMA), using similar statistical modeling techniques, but only refers to a period from 1990-91 through 1998-99. During those years, the number of estimated deaths ranged from 17,000 to 52,000, with an average of about 36,000. The JAMA study also looked at seasonal influenza-associated deaths over a 23 year period, from 1976-1977 and 1998-1999. During that period, estimates of respiratory and circulatory influenza-associated deaths ranged from about 5,000 to about 52,000, with an average of about 25,000. While the 36,000 number is often cited, it's important to note that during that decade, influenza A (H3N2) was the predominant virus during most of the seasons, and H3N2 influenza viruses are typically associated with higher death rates. CDC believes that the range of deaths over the past 31 years (~3,000 to ~49,000) is a more accurate representation of the unpredictability and variability of flu-associated deaths.

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What else does this study tell us?

The MMWR study also found that during seasons when influenza A (H3N2) viruses were prominent death rates were more than double what they were during seasons when influenza A (H1N1) or influenza B viruses predominated. In addition, the study confirmed previous findings that about 90% of influenza associated deaths occur among adults 65 years and older.

Why doesn’t CDC base its seasonal flu mortality estimates only on death certificates that specifically list influenza?

Seasonal influenza may lead to death from other causes, such as pneumonia, congestive heart failure, or chronic obstructive pulmonary disease. It has been recognized for many years that influenza is infrequently listed on death certificates and testing for seasonal influenza infections is usually not done, particularly among the elderly who are at greatest risk of seasonal influenza complications and death. Some deaths — particularly in the elderly — are associated with secondary complications of seasonal 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/or many people don’t seek medical care until after the first few days of acute illness. For these and other reasons, statistical modeling strategies have been used to estimate seasonal 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 seasonal influenza’s true impact.

Does CDC think that influenza causes most P&I deaths?

No, only a small proportion of deaths in either of these two categories are estimated to be influenza-related. CDC estimated that only 8.5% of all pneumonia and influenza deaths and only 2.1% of all respiratory and circulatory deaths were influenza-related.

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