Frequently Asked Questions about Estimated Flu Burden

Illnesses

What is the main lesson learned from annual estimates of annual influenza illness?

Seasonal influenza is associated with large numbers of illnesses, which can impact school attendance, worker absenteeism, and daily productivity. The results of our burden estimates demonstrate the substantial health impact of influenza and underscore the need to ensure vaccination of all people aged 6 months and older.

How many people get sick with flu every year?

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

CDC uses mathematical modeling in combination with data from traditional flu surveillance systems to estimate the numbers of flu illnesses in the United States. CDC estimates that flu has resulted in between 9.3 million and 49 million illnesses each year in the United States since 2010. For more information on these estimates see CDC’s Disease Burden of Influenza page. For more information on CDC surveillance systems, see CDC’s Overview of Influenza Surviellance in the United States.

Hospitalization

What is the main lesson learned from the updated estimates of influenza-associated hospitalization?

Seasonal influenza is associated with large numbers of hospitalizations. These estimates highlight that influenza can be severe, but can also make some health conditions worse (such as lung disease) or lead to other complications that require hospitalization. Influenza vaccination is the first and best way to prevent influenza virus infection and its serious complications. All people, aged 6 months and older, should get vaccinated against influenza, particularly people at increased risk for serious complications including young children, adults 65 years and older, and those with chronic medical conditions.

CDC’s current estimates of hospitalization only go back to 2010. What were the previous estimates of the number of people in the United States hospitalized as a result of seasonal influenza?

Before CDC had FluSurv-NET to gather data on laboratory-confirmed influenza hospitalizations, CDC periodically made estimates of flu hospitalizations using a statistical model of data on hospitalizations with flu-like illnesses.   A study conducted by CDC and published in the Journal of American Medical Association (JAMA) in September 2004 provided information on the number of people in the United States that were hospitalized from influenza-related complications each year. The study was based on records from 1979 to 2001 from about 500 hospitals across the United States. The study concluded that, on average, more than 200,000 people in the United States are hospitalized each year for respiratory and heart conditions illnesses associated with influenza virus infections. Prior to that, in a paper published in 2000, looking at records from 1970 – 1995,  CDC estimated that an average of 114,000 people were hospitalized as a result of influenza-associated infections each year (7). The current methodology to estimates influenza-associated hospitalizations is straightforward, uses directly observed hospitalization rates from FluSurv-NET surveillance data are, and can be updated annually. Previously it took longer to arrive at estimates because of delays in the availability of large administrative databases that were used in previous statistical models. It’s important to note that the results of both methods have similarly shown that hundreds of thousands of people are hospitalized with influenza each year.

Why are the current estimates of influenza-associated hospitalizations different from previous estimates?

The estimated number of Influenza hospitalizations has always varied from season to season. In the past, CDC had referred to an average number of hospitalizations that was estimated based on statistical models using hospital discharge records from 1979 to 2001 from selected hospitals across the United States. In addition to year-to-year variation, changes in health care practices and the increasing age of the US population, with a much greater number of adults over age 65, may result in different numbers of influenza hospitalizations now than in previous decades. Because of these differences, it is challenging to directly compare current estimates of disease burden to those from many years ago.

Why does CDC use a range to describe illnesses, hospitalizations and deaths?

CDC believe that using a range is a better way to represent the variability of flu. More recent estimates to calculate these estimates use surveillance data on patients hospitalized with laboratory-confirmed influenza, in geographically distributed areas in the United States.

Can you explain why some of the estimates on this website are different from previously published estimates using this methodology? (For example, total flu-related hospitalization during 2014-2015 was previously estimated to be 974,000, but the current estimate is 590,000 people)?

The estimate was updated based on recently available information about exact testing practices during that season. The surveillance system used to estimate influenza-related hospitalizations, FluSurv-NET, collects data on patients hospitalized who have laboratory-confirmed influenza. Influenza testing is done at the request of the clinician, but not everyone is tested. Also, influenza tests are not perfectly accurate. Thus, the reports of laboratory-confirmed influenza-related hospitalizations to FluSurv-NET are likely underestimates of the true number. To adjust for this, we collect data annually from participating FluSurv-NET sites on the amount of influenza testing and the type of test that is used at the site, and this information is used to correct for the underestimate. These testing data are often not available for up to two years after the influenza season, and thus more recent season’s estimates may need to be revised when testing data become available, as was the case for the 2014-2015 season.

How many people are hospitalized from flu every year?

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 13 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 allows CDC to calculate an overall hospitalization rate, as well as by age group, but this system does not provide the total number of flu hospitalizations that actually occur in the United States. Reported FluSurv-NET hospitalization rates are adjusted to correct for under-detection, which is calculated from the percent of persons hospitalized with respiratory illness who were tested for influenza and the average sensitivity of influenza tests used in the participating FluSurv-NET surveillance hospitals. CDC uses these methods to estimate the true burden of flu hospitalizations in the United States. Since 2010, CDC estimates that flu has resulted in between 140,000 and 960,000 hospitalizations each year. For more information on these estimates see CDC’s Disease Burden of Influenza page. For more information on CDC surveillance systems, see CDC’s Overview of Influenza Surviellance in the United States.

Deaths

What are seasonal influenza-related deaths?

Seasonal influenza-related deaths are deaths that occur in people for whom 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 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.

Flu deaths in children are slightly different though because these are nationally notifiable, which means that individual flu deaths must be reported to the Centers for Disease Control and Prevention. States report flu-related child deaths in the United States through the Influenza Associated Pediatric Mortality Surveillance System. However, even deaths in children may be underreported, for many of the same reasons listed above.

Why are the current estimates of influenza-associated deaths different from previous estimates?

Previously, CDC periodically made estimates of flu deaths using a statistical model of data on deaths with respiratory and circulatory causes.  An August 27, 2010 MMWR report entitled “Updated Estimates of Mortality Associated with Seasonal Influenza through the 1976-2007 Influenza Season” (MMWR 2010; 59(33): 1057-1062.), provided estimates of the range of flu-associated deaths that occurred in the United States during the three decades from the 1976-1977 season to the 2006-2007 flu season. In that era, CDC estimated that flu-associated deaths ranged from a low of about 3,000 to a high of about 49,000 deaths.

The previous range used to describe influenza-related deaths, from 3,000 to 49,000, was based on data from 30 influenza seasons from 1976 through 2007 used in a statistical model (1). The range described in the tables above, 12,000 to 79,000, is based on data from the 2010-2011 through 2017-2018 influenza seasons using a different mathematical model.

While there are differences in methods used to estimate the deaths in the two time periods, other factors may also contribute to why some seasons have different numbers of influenza deaths than seen in the past, including changes in the way that death certificates are filed, changes in the age structure of the population, or changes in the prevalence of chronic medical conditions that put people at high-risk of influenza complications.

Why did the death burden method change?

CDC has made periodic estimates of deaths associated with influenza virus infections for five decades, initially relying on direct counts of deaths with influenza listed as a cause of death and then focusing on deaths with pneumonia or influenza listed as a cause. Recognizing that direct counts of deaths was underestimating the true burden, CDC turned to using statistical models. Initially, these methods relied on setting specific weeks during the winter and spring when influenza viruses circulated as times ‘at risk’. Increases above periodic, regular variations in deaths during these weeks ‘at risk’ were attributed to the circulation of influenza viruses, and often termed excess deaths. These statistical methods were used to estimate excess deaths that occurred from 1976-2007 (8) and more recently from 2005-2016 (9). Current estimates of flu-related deaths are now based on the mathematical model described above. Each approach, the statistical and the mathematical model, has its own strengths as well as limitations.

The statistical methods require final US mortality records, which are not routinely available until 2-3 years after the date of an individual death. Thus, national vital statistics data cannot be used currently to estimate influenza-associated deaths in a near real-time manner.

Because of these reasons, we have chosen to use a mathematical model that is based on FluSurv-NET, a strong surveillance platform of laboratory-confirmed influenza hospitalizations. We find that the mathematical model is straightforward and can be used to compare one season to another in a way that is simple, yet comprehensive.

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 underreported on death certificates and patients aren’t always tested for seasonal influenza infection, particularly the elderly who are at greatest risk of seasonal influenza complications and death. Some deaths – particularly among the elderly – are associated with secondary complications of 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. Only counting deaths where influenza was included on a death certificate would be a gross underestimation of seasonal influenza’s true impact.

How many flu-associated deaths occur in people who were not vaccinated?

As previously explained, flu-associated deaths in adults are not a nationally notifiable condition, and so states are not required to report flu-associated deaths in adults to CDC. In contrast, flu-associated deaths in children are a nationally notifiable condition, and so jurisdictions (inclusive of state, city or local public health departments) do provide data to CDC on flu-associated deaths in children. These data generally include demographic information, flu laboratory test results, clinical information, and information on the child’s vaccination history, when it is available. During past seasons, approximately 80% of flu-associated deaths in children have occurred in children who were not vaccinated. Based on available data, this remains true for the 2017-2018 season, as well. The latest surveillance data on flu deaths in children is available.

Why are estimated pediatric deaths in this report different from the number of pediatric deaths reported through the Influenza-Associated Pediatric Mortality Surveillance System?

Deaths associated with laboratory-confirmed influenza in children aged <18 years 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 may be tested for flu or children may be tested later in their illness when seasonal influenza can no longer be detected from respiratory samples.

CDC estimates the numbers of flu-related deaths using mathematical models to account for likely under-reporting and deaths that may have occurred outside of a hospital. 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. (10)

How many adults die from flu each year?

Flu deaths in adults are not nationally notifiable. In order to monitor influenza 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 influenza 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. CDC estimates that from 2010-2011 to 2017-2018, influenza-associated deaths in the United States ranged from a low of 12,000 (during 2011-2012) to a high of 79,000 (during 2017-2018). The model used to estimate flu-associated deaths uses a ratio of deaths-to-hospitalizations in order to estimate the total flu-related deaths during a season. For more information: How CDC Estimates Burden.

For more information: Overview of Influenza Surveillance in the United States, “Mortality Surveillance.”

How many children die from flu each year?

Influenza-associated deaths in children (people younger than 18) became nationally reportable in 2004. Since that time the number of pediatric flu deaths reported to CDC each year has ranged from 37 (2011-2012 season) to 185 deaths (2017-2018 season). It’s important to note that the actual number of flu deaths in children is thought to be higher than what is reported by states to CDC because not all flu deaths in children are detected/reported. CDC also estimates the numbers of flu-related deaths using statistical models. Estimates of deaths in children since 2010 have ranged from 37 (2011-2012) to about 1,200 (2012-2013). CDC believes these estimated numbers of pediatric deaths are likely a better estimate of the number of pediatric flu deaths. For more information on these estimates see CDC’s Disease Burden of Influenza page.

Why is it 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 associated deaths include the following:

• the sheer volume of deaths to be counted;
• the lack of testing (not everyone that dies with an influenza-like illness is tested for influenza);
• and the different coding of deaths (influenza-associated deaths often are a result of complications secondary to underlying medical problems, and this may be difficult to sort out).
• For more information: Estimating Seasonal Influenza-Associated Deaths in the United States.