Facts About Estimated Flu Burden

Flu illness

Millions of people get sick with flu each year

Seasonal flu is associated with large numbers of illnesses, which can impact school attendance, worker absenteeism, and daily productivity. CDC conducts surveillance for people who see their health care provider for flu-like illness through the Outpatient Influenza-like Illness Surveillance Network (ILINet); a network of thousands of health care providers who report the proportion of patients seeking care for flu-like illness weekly to CDC. This system allows CDC to track levels of medically attended flu-like illness over the course of the flu season.

However, CDC does not know exactly how many people get sick or seek medical care with seasonal flu each year. There are several reasons for this, including that ILINet does not include every health care provider in the United States and that it monitors flu-like illness, not laboratory-confirmed flu cases. Also, seasonal flu illness is not a reportable disease, and not everyone who gets sick with flu seeks medical care or gets tested for flu.

To estimate the number of flu illnesses and medically-attended flu illnesses that occur in the United States each year, CDC uses mathematical modeling in combination with data from traditional flu surveillance systems. CDC estimates that since 2010, except for the 2020-2021 flu season, flu has resulted in between 9 million and 41 million illnesses annually in the United States.

Flu-related hospitalization

Flu can lead to hospitalization

Seasonal flu is associated with large numbers of hospitalizations. These estimates of flu-related hospitalizations highlight flu's potential to cause serious illness. Being sick with flu can also make some health conditions worse, such as lung or heart disease, or lead to other complications that require hospitalization. Flu vaccination is the first and best way to prevent flu and its serious complications. Everyone 6 months and older should get vaccinated against flu yearly, particularly people at increased risk for serious complications including young children, adults 65 years and older, and people with certain chronic medical conditions.

Hundreds of thousands of people are hospitalized with flu every year

Using an estimation model, CDC estimates that since 2010, except for the 2020-2021 flu season, flu has resulted in between 140,000 and 710,000 hospitalizations each year.

To generate this estimate, CDC conducts surveillance for flu-related hospitalizations through the Influenza Hospitalization Surveillance Network (FluSurv-NET), a collaboration between CDC, the Emerging Infections Program, and additional Influenza Hospitalization Surveillance Project (IHSP) states in 14 geographically distributed areas in the United States (the network includes hospitals that serve roughly 9 percent of the U.S. population). The data collected through FluSurv-NET allow CDC to calculate an overall hospitalization rate, as well as by age group, but this system does not capture every flu hospitalization that occurs annually in the United States. To estimate the actual number of flu hospitalizations, reported FluSurv-NET hospitalization rates are adjusted to correct for under-detection and under-reporting.

Flu burden estimates change as more data become available

CDC releases in-season and preliminary flu burden estimates during each flu season and at the conclusion of the season. However, receipt of some of the data used in the burden estimation model are delayed by as many as 2 years. Providing in-season and preliminary estimates allows for the release of timely information about U.S. flu burden, but also means that these in-season and preliminary estimates may change as more data become available. Flu burden estimates are considered preliminary and subject to change until all data contributing to the estimates for that season are complete. Once all data are complete for a particular season, that flu season's estimates are considered final.

Flu-related deaths

How CDC defines “seasonal flu-related deaths”

Seasonal flu-related deaths are deaths that occur in people for whom flu was likely a contributor to the cause of death, but not necessarily the primary cause of death.

It is difficult to know exactly how many people die from flu

There are several factors that make it difficult to determine accurate numbers of deaths caused by flu regardless of reporting. Some of the challenges in counting flu-related deaths include the following:

• the large number of deaths to be counted.
• the lack of testing (not everyone that dies with an influenza-like illness is tested for flu);
• and deaths are coded differently (flu-related deaths often are a result of complications secondary to underlying medical problems, and this may be difficult to sort out).

CDC estimates the number of people who die from seasonal flu each year

CDC estimates that, since 2010, except for the 2020-2021 flu season, flu-related deaths in the United States ranged from a low of 12,000 to 51,000.

CDC does not know exactly how many people die from seasonal flu each year. In order to monitor flu-related deaths in all age groups, CDC tracks pneumonia and influenza (P&I)–attributed deaths through the National Center for Health Statistics (NCHS) Mortality Reporting System. This system tracks the proportion of death certificates processed that list pneumonia or flu as the underlying or contributing cause of death. This system provides an overall indication of whether flu-associated deaths are elevated but does not provide an exact number of how many people died from flu. As it does for the numbers of flu cases, doctor's visits and hospitalizations, CDC also estimates deaths in the United States using mathematical modeling. The model used to estimate flu-related deaths uses a ratio of deaths-to-hospitalizations in order to estimate the total flu-related deaths during a season.

Flu deaths in children are slightly different though because these are nationally notifiable, which means that individual flu deaths should be reported to CDC. However, even deaths in children are under-reported

Flu-related deaths in children are underreported

Deaths associated with laboratory-confirmed flu in children younger than 18 years old became nationally notifiable in 2004 and are reported to CDC through the Influenza-Associated Pediatric Mortality Surveillance System. The number of reported deaths is published each week in FluView. However, the number of reported deaths is likely an underestimate of the total number of flu-related pediatric deaths because not all children are tested for flu or children might be tested later in their illness when seasonal flu can no longer be detected from respiratory samples.

CDC estimates the number of flu-related deaths using mathematical models to account for likely under-reporting. The estimates of deaths associated with flu that we report are from one such model. Previously published reports have found that the estimated numbers of flu-related deaths in children from statistical models may be two to three times higher than the number of reported deaths.