2017-2018 Estimated Influenza Illnesses, Medical visits, Hospitalizations, and Deaths and Estimated Influenza Illnesses, Medical visits, Hospitalizations, and Deaths Averted by Vaccination in the United States

This web page provides estimates on the burden of influenza in the United States for the 2017–2018 influenza season. For the past several years, CDC has used a mathematical model to estimate the numbers of influenza illnesses, medical visits, hospitalizations, and deaths (1-4). The methods used to calculate the estimates have been described previously (1-2). CDC uses the estimates of the burden of influenza in the population to inform policy and communications related to influenza prevention and control.

2017–2018 Burden Estimates

Influenza activity in the United States during the 2017–2018 season began to increase in November and remained at high levels for several weeks during January and February (6). While influenza A(H3N2) viruses predominated through February, and were predominant overall for the season, influenza B viruses were more commonly reported starting in March 2018. The season had high severity with unusually high levels of outpatient influenza-like illness, hospitalizations rates, and proportions of pneumonia and influenza-associated deaths.

CDC estimates that the burden of illness during the 2017–2018 season was high with an estimated 45 million people getting sick with influenza, 21 million people going to a health care provider, 810,000 hospitalizations, and 61,000 deaths from influenza (Table 1). The number of cases of influenza-associated illness that occurred during 2017-2018 was the highest since the 2009 H1N1 pandemic, when an estimated 60 million people were sick with influenza (7).

The 2017–2018 influenza season was additionally atypical in that it was severe for all ages (6). The burden of influenza and the rates of influenza-associated hospitalization are generally higher for the very young and the very old, and while this was also true during the 2017–2018 season, rates of hospitalization in all age groups were the highest seasonal rates seen since hospital-based surveillance was expanded in 2005 to include all ages (Table 2). This translated into an estimated 11 million cases of influenza in children, 28 million cases of influenza in working age adults (aged 18-64 years), and 6 million cases in adults aged 65 years and older.

Our estimates of hospitalizations and mortality associated with the 2017–2018 influenza season continue to demonstrate how severe influenza virus infection can be. More than 46,000 hospitalizations occurred in children (aged < 18 years); however, 67% of hospitalizations occurred in older adults aged ≥65 years. Older adults also accounted for 83% of deaths, highlighting that older adults are particularly vulnerable to severe disease with influenza virus infection. An estimated 9,600 deaths occurred among working age adults (aged 18–64 years), an age group that often has low influenza vaccination.

Deaths in children with laboratory-confirmed influenza virus infection are reportable in the United States and 183 deaths were reported for the 2017–2018 season. However, influenza-associated pediatric deaths are likely under-reported as not all children whose death was related to an influenza virus infection may have been tested for influenza (8,9). 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 from death certificates, estimating that there were more than 640 deaths associated with influenza in children.

2017-18 Burden Averted Estimates

CDC additionally uses these estimates of disease burden, in a model with vaccine coverage and vaccine effectiveness (VE), to estimate the numbers of flu illnesses, medical visits and hospitalization prevented by vaccination. From the model, CDC estimates that influenza vaccination during the 2017–2018 influenza season prevented 6.2 million illnesses, 3.2 million medical visits, 91,000 hospitalizations and 5,700 deaths associated with influenza (Table 3).

Conclusion

CDC estimates that influenza was associated with 45 million illnesses, 21 million medical visits, 810,000 hospitalizations, and 61,000 deaths during the 2017–2018 influenza season. This burden was higher than any season since the 2009 pandemic and serves as a reminder of how severe seasonal influenza can be.

Specifically, CDC estimates that influenza vaccination during the 2017–2018 influenza season prevented 6.2 million illnesses, 3.2 million medical visits, 91,000 hospitalizations and 5,700 deaths associated with influenza. This report underscores the benefits of the current vaccination program, but also highlights areas where improvements in vaccine uptake and vaccine effectiveness could deliver even greater benefits to the public’s health.

Limitations

Burden Estimates Limitations

These estimates are subject to several limitations.

First, rates of influenza-associated hospitalizations are based on data reported to the Influenza Hospitalization Surveillance Network (FluSurv–NET) through October 1, 2019. Final case counts may differ slightly as further data cleaning from the 2017-2018 season are conducted by FluSurv–NET sites. The most updated crude rates of hospitalization for FluSurv-NET sites are available on FluView Interactive (7).

Second, national rates of influenza-associated hospitalizations and in-hospital death were adjusted for the frequency of influenza testing and the sensitivity of influenza diagnostic assays, using a multiplier approach (3). However, data on testing practices during the 2017–2018 season were not available at the time of estimation. We adjusted rates using the most conservative multiplier from any season between 2010–2011 and 2016–2017. Burden estimates from the 2017–2018 season will be updated at a later date when data on contemporary testing practices become available.

Third, estimates of influenza-associated 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.

Fourth, our estimate of influenza-associated deaths relies on information about location of death from death certificates.  However, death certificate data during the 2017–2018 season were not available at the time of estimation. We have used death certification data from all influenza seasons between 2010-2011 and 2016–2017 where these data were available from the National Center for Health Statistics. Furthermore, our model uses the frequency of influenza-associated deaths that have cause of death related to pneumonia or influenza (P&I), other respiratory or cardiovascular (other R&C), or other non-respiratory, non-cardiovascular (non-R&C) to account for deaths occurring outside of a hospital by cause of death. These frequencies were not available from the 2017-2018 season at the time of estimation, so we used the average frequencies of each cause from previous seasons, 2010–2011 to 2016–2017.

Fifth, estimates of burden were derived from rates of influenza-associated 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 influenza 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.

Burden Averted Estimates Limitations

These averted burden estimates are subject to several limitations. First, influenza 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. 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 averted by vaccination. The model of averted illness calculates outcomes directly prevented among persons who were vaccinated. If there is also indirect protection 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 influenza 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 influenza vaccination. Finally, because the data and methods used to make these calculations are continually updated, future estimates may differ from those presented here.

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

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.

2017-2018 Tables for Influenza Burden and Burden-Averted Estimates

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% UI* Estimate 95% UI Estimate 95% UI Estimate 95% 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: Table 2: Estimated rates of influenza-associated disease outcomes, per 100,000, by age group — United States, 2017-2018 influenza season
Illnesses 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

18,448.10 (12,856.5, 36,475.0) 12,360.20 (8,501.3, 24,596.7) 128.6 (89.6, 254.3) 0.6 (0.0, 1.8)

5-17 yrs

13,985.60 (10,983.6, 18,987.0) 7,272.50 (5,589.3, 9,972.2) 38.3 (30.1, 52.1) 1 (0.4, 2.6)

18-49 yrs

10,469.70 (8,895.6, 14,075.1) 3,873.80 (3,092.9,  5,321.7) 58.8 (49.9, 79.0) 2 (1.2, 5.0)

50-64 yrs

20,881.10 (14,828.2, 36,378.8) 8,978.90 (6,145.3, 15,818.0) 221.4 (157.2, 385.8) 10.6 (6.7, 25.0)

65+ yrs

11,690.60 (7,682.1, 23,175.5) 6,546.70 (4,207.2, 13,023.8) 1,062.80 (698.4, 2,106.9) 100.1 (70.8, 163.7)

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 3: Estimated Number of Influenza Illnesses, Medical Visits, Hospitalizations, and Deaths Averted by Vaccination, by Age Group — United States, 2017–2018 Influenza
Averted Illnesses Averted Medical Visits Averted Hospitalizations Averted Deaths
Age Group No. 95% UI* No. 95% UI* No. 95% UI* No. 95% UI*

0-4 yrs

1,721,215 (823,798, 3,655,558) 1,153,214 (570,764, 2,391,130) 15,139 (7,332, 31,900) 68 (0, 269)

5-17 yrs

1,151,025 (268,849, 2,246,430) 598,533 (146,717, 1,107,901) 4,275 (1,361, 7,706) 110 (25, 335)

18-49 yrs

1,044,837 (454,076, 1,782,353) 386,590 (179,669, 613,839) 6,534 (2,846, 10,932) 226 (25, 335)

50-64 yrs

1,647,176 (384,460, 3,637,231) 708,286 (175,929, 1,481,055) 16,792 (3,329, 37,941) 808 (84, 605)

65+ yrs

595,961 (0, 2,439,161) 333,738 (0, 1,291,842) 48,163 (0, 202,692) 4,536 (0, 21,015)

All ages

6,160,213 (2,076,700, 12,315,081) 3,180,360 (1,157,485, 6,071,355) 90,904 (15,986, 271,632) 5,747 (341, 23,499)

* Uncertainty interval

References

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