Why CDC Estimates the Burden of Seasonal Flu in the US

Overview

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 surveillance systems. Together, these surveillance systems are designed to provide a national picture of flu activity, including how many flu-like-illnesses, flu-related hospitalizations and flu-related 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-related hospitalizations, and flu-related deaths that occur each year.

CDC does not know the exact number of flu illnesses, hospitalizations, and deaths that occur annually

CDC does not know the exact number of flu illnesses, flu-associated hospitalizations, or flu-associated deaths that occur each year in U.S. 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. States are only required to report deaths only for children younger than 18 years. 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 United States. To gain a better understanding of the true burden of flu, CDC and other public health agencies in the United States and other countries use statistical or mathematical models to estimate the annual number of seasonal flu-related cases, hospitalizations, and deaths.

Learnings from seasonal estimates of flu burden in the US

Estimates of the burden of flu in the United States help to demonstrate the true burden of disease in the country. 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. There are tens of thousands of flu-related deaths and hundreds of thousands of hospitalizations every year, and there can be large year-to-year variations.