Estimated Flu-Related Illnesses, Medical visits, Hospitalizations, and Deaths in the United States — 2019–2020 Flu Season

Note: The burden estimates on this page have been updated from the preliminary estimates reported in November 2020 based on the availability of additional data. Burden estimates for the 2019-2020 flu season are still considered preliminary and are subject to change as more data become available. More information on why preliminary flu burden estimates change is available below.

The overall burden of influenza (flu) for the 2019-2020 was an estimated 35 million flu-related illnesses, 16 million flu-related medical visits, 380,000 flu-related hospitalizations, and 20,000 flu-related deaths (Table 1).

For the past several years, CDC has estimated the numbers of flu-related illnesses, medical visits, hospitalizations, and deaths1–4. More information on How CDC Estimates Flu Burden and Why CDC Estimates Flu Burden is available.3

Table 1: Estimated flu disease burden, by age group — United States, 2019-2020 flu season
Symptomatic Illnesses Medical Visits Hospitalizations Deaths
Age group Estimate 95% UI Estimate 95% UI Estimate 95% UI Estimate 95%UI
0-4 yrs 3,878,588 (2,979,865,
6,554,798)
2,598,654 (1,954,987,
4,376,793)
27,040 (20,774,
45,697)
347 (90,
571)
5-17 yrs 7,752,967 (5,836,198,
12,157,092)
4,031,543 (2,970,975,
6,408,299)
21,258 (16,002,
33,334)
139 (31,
406)
18-49 yrs 13,311,444 (9,807,734,
23,970,578)
4,925,234 (3,494,425,
8,948,555)
74,717 (55,051,
134,547)
2,184 (1,688,
4,770)
50-64 yrs 8,125,732 (6,132,930,
14,699,555)
3,494,065 (2,503,300,
6,327,545)
86,171 (65,038,
155,885)
5,727 (3,845,
10,222)
65+ yrs 1,881,249 (1,299,641,
4,394,897)
1,053,499 (701,664,
2,473,436)
171,023 (118,149,
399,536)
11,945 (10,229,
70,284)
All ages 34,949,979 (29,761,825,
49,176,022)
16,102,995 (13,673,370,
22,307,300)
380,209 (312,272,
631,906)
20,342 (18,061,
79,906)
Table 2: Estimated rates of flu-related disease outcomes, per 100,000, by age group — United States, 2019-2020 flu season
Illness rate Medical visit rate Hospitalization rate Mortality rate
Age group Estimate 95% UI Estimate 95% UI Estimate 95% UI Estimate 95% UI
0-4 yrs 19,812.3 (15,221.5,
33,482.7)
13,274.2 (9,986.3,
22,357.2)
138.1 (106.1,
233.4)
1.8 (0.5,
2.9)
5-17 yrs 14,501.7 (10,916.4,
22,739.5)
7,540.9 (5,557.1,
11,986.5)
39.8 (29.9,
62.3)
0.3 (0.1,
0.8)
18-49 yrs 9,630.9 ( 7,095.9,
17,342.8)
3,563.4 (2,528.2,
6,474.3)
54.1 (39.8,
97.3)
1.6 (1.2,
3.5)
50-64 yrs 12,913.2 ( 9,746.3,
23,360.2)
5,552.7 (3,978.2,
10,055.6)
136.9 (103.4,
247.7)
9.1 (6.1,
16.2)
65+ yrs 3,480.0 ( 2,404.1,
8,129.9)
1,948.8 (1,298.0,
4,575.5)
316.4 (218.6,
739.1)
22.1 (18.9,
130.0)

 

*Some of the data used to calculate burden estimates are incomplete or not yet available.  These estimates will change as those data become available and the estimates are updated.

Uncertainty interval: Adjusted estimates are presented in two parts: an uncertainty interval [UI] and a point estimate. The uncertainty interval provides a range in which the true number or rate of flu illnesses, medical visits, hospitalizations, or deaths would be expected to fall if the same study was repeated many times, and it gives an idea of the precision of the point estimate. A 95% uncertainty interval means that if the study were repeated 100 times, then 95 out of 100 times the uncertainty interval would contain the true point estimate. Conversely, in only 5 times out of a 100 would the uncertainty interval not contain the true point estimate.

woman getting vacine

CDC estimates the flu-related illnesses, hospitalizations, and deaths prevented by seasonal flu vaccination.

Why did the estimates for the 2017-2018, 2018-2019, and 2019-2020 decrease compared with previous estimates for these seasons?

CDC’s model used to estimate the burden of flu includes information collected about flu testing practices. Because current testing data was not available at the time of estimation (it takes approximately two years to finalize information on flu testing practices), the estimates that were previously published on the CDC website were made using testing information from prior flu seasons.

Since then, complete information to estimate the burden of the 2017-2018 and 2018-2019 flu seasons has become available. Final testing information from the 2017-2018 seasons indicated an increase in testing for flu across all age groups and the FluSurv-NET sites.  Because the percent of individuals who were tested for flu was high in all age groups, the adjustment for under-detection of flu was lower and our burden estimates decreased.  The estimates for the 2019-2020 season pull information from all past seasons including the 2017-2018 and 2018-2019 seasons and because our methods use the most conservative estimates of under-detection of flu, the 2019-2020 burden estimates also decreased.  The 2017-2018 and 2018-2019 season estimates are now considered final, however the 2019-2020 burden estimates are still preliminary and may change as more information becomes available.

More information on How CDC Estimates Flu Burden is available, as well as answers to frequently asked questions about CDC’s flu burden estimates.

Background and Results: 2019–2020 Burden Estimates

Flu activity in the United States during the 2019–2020 season began to increase in November and was consistently high through January and February.  The season was characterized by two consecutive waves of activity, beginning with influenza B viruses and followed by A(H1N1)pdm09 viruses. Overall, influenza A(H1N1)pdm09 viruses were the most commonly reported influenza viruses this season. Activity began to decline in March, perhaps associated with community prevention measures for COVID-19 (5-6). The 2019-20 season is described as having moderate severity; however, the effect of flu differed by age group and the severity of the season in some age groups was higher. Hospitalization rates among children 0-4 years old and adults 18-49 years old were higher than observed during the 2009 H1N1 pandemic (6).

CDC estimates that the burden of illness during the 2019–2020 season was moderate with an estimated 35 million people sick with flu, 16 million visits to a health care provider for flu, 380,000 hospitalizations for flu, and 20,000 flu deaths  (Table 1). The number of cases of flu-related illness, medically attended illnesses, hospitalizations, and deaths were lower than some more recent seasons and similar to other seasons where influenza A(H1N1)pdm09 viruses dominated (7, 8).

The 2019–2020 flu season was atypical in that it was severe for children aged 0-4 years where rates of infections, medically attended illnesses, hospitalizations, and deaths were higher than those observed during the 2017-2018 season, a recent season with high severity (7). The burden of flu and the rates of flu-related hospitalization are usually higher for the very young and the very old, and while this was observed during the 2019–2020 season, rates of hospitalization in adults aged 18-49 years were the highest seasonal rates seen since the 2017-2018 season (Table 2). These rates mean that an estimated 13 million cases of flu in younger adults (aged 18-49 years), which is the second highest number of infections for this age group since CDC began reporting flu burden estimates in the 2010-11 season.

CDC’s estimates of hospitalizations and mortality associated with the 2019–2020 flu season show the effects that flu virus infections can have on society. Nearly 50,000 hospitalizations occurred in children aged < 18 years and 75,000 hospitalizations among adults aged 18-49 years. Forty-five percent of hospitalizations occurred in older adults aged ≥65 years. Older adults also accounted for 59% of deaths, which is lower than recent previous seasons.  These findings continue to highlight that older adults are particularly vulnerable to severe disease with flu virus infection. An estimated 8,000 deaths (39% of all deaths) occurred among working age adults (aged 18–64 years), an age group for which flu vaccine coverage is often low (9). This also underscores that flu viruses can affect individuals of any age and prevention measures such as vaccination are important to reducing the impact of the seasonal epidemics on the population and healthcare system.

Deaths in children with laboratory-confirmed flu virus infection have been a reportable disease in the United States since 2004; 199 deaths were reported for the 2019-20 season as of May 27, 2021. However, flu-related pediatric deaths are likely under-reported, as not all children whose death was related to a flu virus infection may have been tested for flu (10,11). Therefore, we used a mathematical model to estimate the total number of pediatric deaths based on hospitalization rates and the frequency of death in and out of the hospital using death certificates. We estimate that at least 486 deaths associated with flu occurred during the 2019-2020 season among children aged <18 years.

Limitations

These estimates are subject to several limitations.

First, national rates of flu-related hospitalizations and in-hospital death were adjusted for the frequency of flu testing and the sensitivity of flu diagnostic assays, using a multiplier approach (3).

However, data on testing practices during the 2019–2020 season were not available at the time of estimation. We adjusted rates using the lowest multiplier from any season between 2010–2011 and 2018–2019. Burden estimates from the 2019–2020 season will be updated at a later date when data on contemporary testing practices become available.

Second, estimates of flu-related illness are made by multiplying the number of hospitalizations by the ratio of illnesses to hospitalizations; estimates of medical visits are made by a similar process.  These multipliers are based on data from a prior season, which may not be accurate if patterns of care-seeking have changed.

Third, our estimate of flu-related deaths relies on information about location of death from death certificates to calculate ratios of deaths occurring in the hospital to deaths occurring outside of the hospital by categories of causes of death.  However, death certificate data during the 2019–2020 season were not available at the time of estimation. We used death certification data from all flu seasons from 2010–2011 through 2018–2019 where these data were available from the National Center for Health Statistics. To calculate these ratios, first we calculate the frequency of flu-related deaths reported from our FluSurv-NET surveillance system that have cause of death identified as pneumonia or influenza (P&I), other respiratory or cardiovascular (other R&C), or other non-respiratory, non-cardiovascular (non-R&C).  Next, to account for deaths occurring outside of a hospital, we use information from national death certificates to calculate the proportion of deaths from these causes that occur in and out of the hospital. Data to generate these frequencies were not available from the 2019–2020 season at the time of estimation, so we used the average frequencies of location of death for each of the cause categories from previous seasons, 2010–2011 through 2018–2019.

Fourth, estimates of burden were derived from rates of flu-related hospitalization, which is a different approach than the statistical models used in older published reports. This makes it difficult to directly compare our estimates for seasons since 2009 to those older reports, though the estimates from our current method are largely consistent with estimates produced with statistical models for similar years (12–13). However, it is useful to keep in mind that direct comparisons to flu disease burden decades ago are complicated by large differences in the age of the US population and the increasing number of adults aged ≥65 years.

References:

  1. Reed C, Chaves SS, Daily Kirley P, Emerson R, Aragon D, Hancock EB, et al. Estimating influenza disease burden from population-based surveillance data in the United States. PLoS One. 2015;10(3):e0118369.
  2. Rolfes MA, Foppa IM, Garg S, et al. Annual estimates of the burden of seasonal influenza in the United States: A tool for strengthening influenza surveillance and preparedness. Influenza Other Respir Viruses 2018; 12(1): 132–7.
  3. Centers for Disease Control and Prevention. Estimated influenza illnesses and hospitalizations averted by influenza vaccination – United States, 2012-13 influenza season. MMWR Morb Mortal Wkly Rep. 2013 Dec 13;62(49):997-1000.
  4. Reed C, Kim IK, Singleton JA, Chaves SS, Flannery B, Finelli L, et al. Estimated influenza illnesses and hospitalizations averted by vaccination–United States, 2013-14 influenza season. MMWR Morb Mortal Wkly Rep. 2014 Dec 12;63(49):1151-4.
  5. Olsen SJ, Azziz-Baumgartner E, Budd AP, et al. Decreased Influenza Activity During the COVID-19 Pandemic — United States, Australia, Chile, and South Africa, 2020. MMWR Morb Mortal Wkly Rep 2020;69:1305–1309. https://www.cdc.gov/mmwr/volumes/69/wr/mm6937a6.htm
  6. Control, C.f.D. Flu Activity & Surveillance. 7/8/2020 [cited 2020 9/22/2020]; Available from: https://www.cdc.gov/flu/weekly/fluactivitysurv.htm.Garten R,
  7. Blanton L, Elal AIA, Alabi N, Barnes J, Biggerstaff M, et al. Update: Influenza Activity in the United States During the 2017-18 Season and Composition of the 2018-19 Influenza Vaccine. MMWR Morb Mortal Wkly Rep. 2018 Jun 8;67(22):634-42.
  8. Past Seasons Estimated Influenza Disease Burden. 2020. Available at: https://www.cdc.gov/flu/about/burden/past-seasons.html. Accessed September 30, 2020.
  9. Centers for Disease Control and Prevention. Flu Vaccination Coverage, United States, 2018-19 Influenza Season. September 26, 2019 [cited 2020 September 30]; Available from: https://www.cdc.gov/flu/fluvaxview/coverage-1819estimates.htm
  10. Shrestha SS, Swerdlow DL, Borse RH, Prabhu VS, Finelli L, Atkins CY, et al. Estimating the Burden of 2009 Pandemic Influenza A (H1N1) in the United States (April 2009–April 2010). Clin Infect Dis. 2011;52(suppl_1):S75-S82.
  11. Lees CH, Avery C, Asherin R, Rainbow J, Danila R, Smelser C, et al. Pandemic (H1N1) 2009–associated Deaths Detected by Unexplained Death and Medical Examiner Surveillance. Emerg Infect Dis. 2011;17(8):1479-83.
  12. Martin K, Strain A, Reagan-Steiner S, Lynfield R, DeVries A, Lees C, et al. Influenza-associated Pediatr Deaths Identified Through Minnesota’s Unexplained Critical Illness and Death Project – Minnesota, 2004-2017; Abstract 9836.  Council of State and Territorial Epidemiologist. West Palm Beach, FL; 2018.
  13. Centers for Disease Control and Prevention. Estimates of deaths associated with seasonal influenza — United States, 1976-2007. MMWR Morb Mortal Wkly Rep. 2010 Aug 27;59(33):1057-62.
  14. Thompson WW, Shay DK, Weintraub E, Brammer L, Bridges CB, Cox NJ, et al. Influenza-associated hospitalizations in the United States. JAMA. 2004 Sep 15;292(11):1333-40.
  15. Thompson WW, Shay DK, Weintraub E, Brammer L, Cox N, Anderson LJ, et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA. 2003 Jan 8;289(2):179-86.