Why CDC Estimates the Burden of Season Influenza in the U.S.

Each year CDC estimates the burden of influenza (flu) in the U.S. CDC uses modeling to estimate the number of flu-associated illnesses, medical visits, hospitalizations, and deaths that occur in the U.S. in a given season.

CDC uses the estimates of the burden of flu in the population and the impact of flu vaccination to inform policy and communications related to flu.

Why does CDC estimate illness, hospitalization, and deaths associated with seasonal flu?

Seasonal flu is a serious disease that causes millions of illnesses, hundreds of thousands of hospitalizations, and tens of thousands of deaths every year in the United States. CDC tracks flu activity in the United States using multiple systems in five different surveillance categories. Together, these surveillance systems are designed to provide a national picture of flu activity, including how much flu-like-illness there is and the magnitude of flu-related hospitalizations and deaths.

For a number of reasons, however, these surveillance systems do not capture every flu-related illness, medical visit, hospitalization or even death in the United States. CDC feels it is important to convey the full burden of seasonal flu and thus uses well-established scientific methods that have been reviewed by scientists outside of CDC to make comprehensive estimates of the total numbers of annual flu illnesses, flu-associated hospitalizations, and flu-associated deaths that occur each year.

Why does CDC not know the exact number of flu illness, hospitalizations, and deaths in the U.S.?

CDC does not know the exact number of flu illnesses, flu-associated hospitalizations, or flu-associated deaths in adults for several reasons:

First, many people who become sick with flu will recover in a few days to less than two weeks and may not seek medical care.  For people who do seek medical care, most are not tested because the test results usually do not change how you are treated.

Second, people may seek medical care later in their illness when seasonal flu can no longer be detected from respiratory samples. Sensitive flu tests are only likely to detect flu if performed within a week after onset of illness. In addition, some commonly used tests to diagnose flu in clinical settings are not highly sensitive and can provide false negative results (i.e. they miss true flu infections.)

Third, states are not required to report to CDC individual seasonal flu cases or hospitalizations for people of any age; and are only required to report deaths only for children less than 18 years of age. Additionally 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 flu can aggravate an existing chronic illness (such as congestive heart failure or chronic obstructive pulmonary disease).  Therefore, seasonal flu is infrequently listed on death certificates of people who die from flu-related complications.

For these reasons, surveillance data may drastically underrepresent the true burden of flu in the U.S. To gain a better understanding of the actual burden of flu CDC and other public health agencies in the U.S. and other countries use statistical models to estimate the annual number of seasonal flu-related cases, hospitalizations, and deaths.

What can be learned from seasonal estimates of flu burden in the U.S.?

Estimates of the burden of flu in the U.S. help to demonstrate the true burden of disease in the U.S. Burden estimates also provide an idea of how the burden of flu varies from season to season, which can be based on a number of factors including the characteristics of circulating viruses, the timing of the season, how well the vaccine is working to protect against illness, and how many people have been vaccinated. Additionally, burden estimates provide insight into the populations most impacted by flu illness, which can inform important guidance and communication efforts to help ensure those at highest risk for flu illness and severe flu outcomes are protected. There are several different methods that have been used to estimate burden. The exact estimates may differ with different methods, however, they all give similar conclusions – that there are tens of thousands of flu-related deaths and hundreds of thousands of hospitalizations every year, and that there can be big year-to-year variations.