2017-2018 Estimated Flu Illnesses, Medical Visits, Hospitalizations, and Deaths and Estimated Flu-Related Prevented by Vaccination in the United States

This web page provides estimates on the burden of influenza (flu) prevented by vaccination in the United States for the 2017–2018 flu season (2017-2018 flu season burden estimates are also available). For the past several years, CDC has used  mathematical models to estimate the numbers of flu illnesses, medical visits, hospitalizations, and deaths, as well as the numbers of flu illnesses, medical visits, hospitalizations, and deaths prevented by vaccination. The methods used to calculate these estimates have been described previously (1-4). CDC uses the estimates of flu burden and of flu burden prevented by vaccination to inform policy and communications related to flu prevention and control. More information on why CDC estimates flu burden and flu burden prevented by vaccination is available.

2017-18 Burden Prevented by Vaccination Estimates

CDC uses the estimates of flu burden in another model that includes vaccine coverage and vaccine effectiveness (VE), to estimate the numbers of flu illnesses, medical visits and hospitalizations prevented by vaccination. Using this model, CDC estimates that flu vaccination during the 2017–2018 flu season prevented 6.9 million illnesses, 3.6 million medical visits, 94,000 hospitalizations and 6,000 deaths associated with flu. (Table 1).

These estimates underscore the benefits of the flu vaccination program, but also highlight areas where improvements in vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health.

Table 1: Estimated Number of flu Illnesses, Medical Visits, Hospitalizations, and Deaths Averted by Vaccination, by Age Group — United States, 2017–2018 flu
Averted Illnesses Averted Medical Visits Averted Hospitalizations Averted Deaths
Age Group No. 95% UI* No. 95% UI* No. 95% UI* No. 95% UI*
6 months-4 yrs 2,158,825 (1,477,310, 2,984,291) 1,446,413 (988,784, 2,001,358) 15,050 (10,299, 20,805) 69 (0, 186)
5-17 yrs 1,456,306 (664,387, 2,303,233) 757,279 (346,607, 1,203,190) 3,993 (1,822, 6,315) 85 (24, 187)
18-49 yrs 1,203,411 (707,530, 1,717,666) 445,262 (261,287, 638,993) 6,755 (3,971, 9,641) 198 (90, 354)
50-64 yrs 1,509,986 (583,253, 2,403,692) 649,294 (248,172, 1,037,580) 16,013 (6,185, 25,490) 760 (280, 1,353)
65+ yrs 579,427 (0, 1,568,261) 324,479 (0, 878,028) 52,675 (0, 142,569) 4,853 (0, 16,952)
All ages 6,907,956 (5,258,433, 8,768,791) 3,622,728 (2,768,528, 4,629,747) 94,486 (36,403, 185,335) 5,965 (880, 18,179)

*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 times would the uncertainty interval not contain the true point estimate.

Why are estimates on this page different from previously published and reported estimates for 2017-2018?

The estimates on this page have been updated from an earlier report and include final data from the 2017-2018 flu season. 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, CDC uses preliminary data from the current or past seasons that may lead to over- or under-estimates of the true burden. However, each season’s estimates are finalized when data on testing practices and deaths for that season are available, usually within two years. The updated estimates of the burden prevented by flu vaccine include data from the 2017-2018 season and are considered the final estimates for this season. More information on why preliminary burden and burden averted estimates may change is available.

Limitations

These prevented burden estimates are subject to several limitations. First, flu vaccination coverage estimates were derived from reports by survey respondents, not vaccination records, and are subject to recall bias. These reports are based on telephone surveys with relatively low response rates and nonresponse bias may remain after weighting for the survey design (weighting of the data is done to make the responses representative of the population). Estimates of the number of persons vaccinated based on these survey data have often exceeded the actual number of doses distributed, indicating that coverage estimates used in this report may overestimate the numbers of illnesses and hospitalizations prevented by vaccination. The model of prevented illness calculates outcomes directly prevented among persons who were vaccinated. If there is also indirect protection in the population from fewer infectious persons as a result of vaccination (i.e., herd immunity), the model would underestimate the number of illnesses and hospitalizations prevented by vaccination. Estimates of the averted burden in older adults, aged ≥65 years, do not reflect the increasing use of high-dose or adjuvanted flu vaccines, which may have higher effectiveness compared with standard vaccines; nor does the estimate reflect that vaccine effectiveness might continue to decrease with age, reaching very low levels among the oldest adults who also have the highest rates of flu vaccination. Finally, because the data and methods used to make these calculations are continually updated, future estimates may differ from those presented here.

References
  1. Centers for Disease Control and Prevention. Estimated flu illnesses and hospitalizations averted by flu vaccination – United States, 2012-13 flu season. MMWR Morb Mortal Wkly Rep. 2013 Dec 13;62(49):997-1000.
  2. Reed C, Kim IK, Singleton JA, Chaves SS, Flannery B, Finelli L, et al. Estimated flu illnesses and hospitalizations averted by vaccination–United States, 2013-14 flu season. MMWR Morb Mortal Wkly Rep. 2014 Dec 12;63(49):1151-4.
  3. Centers for Disease Control and Prevention. Estimated flu Illnesses and Hospitalizations Averted by Vaccination — United States, 2014–15 flu Season.  2015 December 10, 2015 [cited 2016 October 27]; Available from: Estimated flu Illnesses and Hospitalizations Averted by Vaccination — United States, 2014–15 flu Season
  4. Centers for Disease Control and Prevention. Flu Vaccination Coverage, United States, 2016-17 flu Season. September 28, 2017 [cited 2017 August 29]; Available from: https://www.cdc.gov/flu/fluvaxview/coverage-1617estimates.htm