Estimated Influenza Illnesses, Medical visits, Hospitalizations, and Deaths in the United States — 2017–2018 influenza season

Note: The preliminary burden estimates on this page have been updated from those first reported in December 2018 based on additional data available from FluSurv-NET. Estimates for the 2017-2018 season are still preliminary and may change again once data on testing practices and deaths from the 2017-2018 season become available.

* All estimates from the 2017-2018 influenza season are preliminary and may change as data from the season are cleaned and finalized.

The overall burden of influenza for the 2017-2018 season was an estimated 45 million influenza illnesses, 21 million influenza-associated medical visits, 810,000 influenza-related hospitalizations, and 61,000 influenza-associated deaths (Table: Estimated Influenza Disease Burden, by Season — United States, 2010-11 through 2017-18 Influenza Seasons).

A full report of the 2017-2018 season burden and influenza illness and burden of influenza illness prevented by vaccination in the United States is available online.

Table 1: Estimated influenza disease burden, by age group — United States, 2017-2018 influenza season

Symptomatic Illnesses Medical Visits Hospitalizations Deaths
Age group Estimate 95% Cr UI Estimate 95% Cr UI Estimate 95% Cr UI Estimate 95% Cr UI
0-4 yrs 3,678,342 (2,563,438 -7,272,693) 2,464,489 (1,695,054 – 4,904,296) 25,644 (17,871 – 50,702) 115 (0 – 367)
5-17 yrs 7,512,601 (5,899,989 – 10,199,144) 3,906,553 (3,002,375 – 5,356,724) 20,599 (16,177 – 27,965) 528 (205 – 1,392)
18-49 yrs 14,428,065 (12,258,820 – 19,396,710) 5,338,384 (4,262,260 -7,333,716) 80,985 (68,809 – 108,874) 2,803 (1,610 – 6,936)
50-64 yrs 13,237,932 (9,400,614 -23,062,957) 5,692,311 (3,895,925 – 10,028,080) 140,385 (99,691 – 244,576) 6,751 (4,244 – 15,863)
65+ yrs 5,945,690 (3,907,025 – 11,786,777) 3,329,586 (2,139,716-  6,623,717) 540,517 (355,184 -1,071,525) 50,903 (35,989 – 83,230)
All ages 44,802,629 (39,322,959 – 57,928,172) 20,731,323 (17,978,392 – 27,248,302) 808,129 (620,768 -1,357,043) 61,099 (46,404 – 94,987)

* Uncertainty interval

Table 2: Estimated rates of influenza-associated disease outcomes, per 100,000, by age group — United States, 2017-2018 influenza season

Illness rate Medical visit rate Hospitalization rate Mortality rate
Age group Estimate 95% Cr UI Estimate 95% Cr UI Estimate 95% Cr UI Estimate 95% Cr UI
0-4 yrs 18,448.1 (12,856.5 – 36,475.0) 12,360.2 (8,501.3 -24,596.7) 128.6 (89.6 – 254.3) 0.6 (0.0, 1.8)
5-17 yrs 13,985.6 (10,983.6 – 18,987.0) 7,272.5 (5,589.3 -9,972.2) 38.3 (30.1 – 52.1) 1.0 (0.4, 2.6)
18-49 yrs 10,469.7 ( 8,895.6 – 14,075.1) 3,873.8 (3,092.9 – 5,321.7) 58.8 (49.9 – 79.0) 2.0 (1.2, 5.0)
50-64 yrs 20,881.1 (14,828.2 – 36,378.8) 8,978.9 (6,145.3 -15,818.0) 221.4 (157.2 – 385.8) 10.6 (6.7, 25.0)
65+ yrs 11,690.6 ( 7,682.1 -23,175.5) 6,546.7 (4,207.2 -13,023.8) 1,062.8 (698.4 – 2,106.9) 100.1 (70.8, 163.7)

* Uncertainty interval

All estimates from the 2017-2018 influenza season are preliminary and may change as data from the season are cleaned and finalized. The model to generate burden estimates uses data on influenza testing practices at FluSurv-NET hospitals to correct for known under-detection of influenza and mortality data from the National Center for Health Statistics for estimation of deaths. The most recent estimates for the burden of influenza during 2017-2018 in Tables 1-2 above use more recent data on testing practices and mortality and are lower than those previously reported.

Can you explain why the estimates on this page are different from previously published and reported estimates for 2017-2018? (For example, total flu-related deaths during 2017-2018 was previously estimated to be 79,000, but the current estimate is 61,000)?

The estimates on this page have been updated from an earlier report published in December 2018 based on more recently available information.  There is a trade-off between timeliness and accuracy of the burden estimates. To provide timely burden estimates to the public, clinicians, and public health decision-makers, we use preliminary data that may lead to over- or under-estimates of the true burden.  However, each season’s estimates will be finalized when data on testing practices and deaths for that season are available.

For the revised 2017-2018 estimates, we included additional information in our estimation regarding influenza testing practices. The surveillance system used to estimate influenza-related hospitalizations, FluSurv-NET, collects data on patients hospitalized with laboratory-confirmed influenza. Influenza testing is done at the request of the clinician, but not everyone is tested and 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 of hospitalizations. To adjust for this, CDC collects 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 possible underestimate of influenza-related hospitalizations. These testing data are often not available for up to two years after the end of an influenza season, and thus the estimates are revised when additional testing data become available. For the original preliminary 2017-2018 burden estimates, data on testing practices during the 2014-2015 season were used to make preliminary estimates because this season had the highest levels of testing among the prior seasons for which data were available and resulted in the most conservative (lowest) estimates of burden. More recent data from the 2016-17 season show that influenza testing has been increasing among most age groups.  The current estimates were made using the highest testing rate for each age-group during 2010-11 to 2016-17 and has resulted in some burden estimates being lower than previously estimated.

Additionally, the method we use to estimate influenza-associated deaths relies on additional data from FluSurv-NET and the National Center for Health Statistics (data on cause of deaths and numbers of deaths that occur in versus outside the hospital) that are also not available for up to two years after the end of the season being estimated.

The 2017-2018 estimates are still preliminary because not all of the required data are currently available.  When those data become available, these estimates will be updated again and the results may change.

More answers to frequently asked questions about CDC’s influenza burden estimates are available.