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 flu illnesses or deaths among 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 bacterial pneumonia) 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. 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. they 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 and mathematical models to estimate the annual number of flu-related deaths.
