Estimated Flu-Related Illnesses, Medical visits, Hospitalizations, and Deaths in the United States — 2018–2019 Flu Season

The overall burden of influenza (flu) for the 2018-2019 season was an estimated 29 million flu illnesses, 13 million flu-related medical visits, 380,000 flu-related hospitalizations, and 28,000 flu deaths (Table 1).

For the past several years, CDC has estimated the numbers of flu illnesses, medical visits, hospitalizations, and deaths^1–4. More information on How CDC Estimates Flu Burden and Why CDC Estimates Flu Burden is available.

*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.

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: 2018-2019 Flu Burden Estimates

CDC estimates that the burden of illness during the 2018–2019 season included an estimated 29 million people getting sick with flu, 13 million people going to a health care provider for their illness, 380,000 hospitalizations, and 28,000 deaths from flu (Table 1).

Flu activity in the United States during the 2018–2019 season began to increase in November and remained at high levels for several weeks during January–February⁵. Influenza A viruses were the predominant circulating viruses last year. While influenza A(H1N1pdm09) viruses predominated from October 2018 – mid February 2019, influenza A(H3N2) viruses were more commonly reported starting in late February 2019. Influenza B viruses were not commonly reported among circulating viruses during the 2018–2019 season. The season had moderate severity based on levels of outpatient flu-like illness, hospitalizations rates, and proportions of pneumonia and flu-related deaths.

Peak activity during the 2018–2019 flu season was classified as having moderate severity across ages in the population. Compared with the 2017–2018 season, which was classified as high severity, the overall rates and burden of flu were much lower during the 2018–2019 season (Table 2). Among children, however, rates of flu during the 2018–2019 season were similar to the 2017–2018 season. In addition, the 2018–2019 season had two waves of activity, including a wave predominated by influenza A(H1N1)pdm09 viruses and another wave of similar magnitude attributable to influenza A(H3N2) viruses⁵. The dual waves resulted in a protracted season during 2018–2019 that was less severe when compared with peak activity in 2017–2018 but resulted in a similar burden of illness in children by the end of the season.

During the 2018–2019 season, 144 deaths in children with laboratory–confirmed flu virus infection were reported in the United States⁸. However, flu-related pediatric deaths are likely under-reported as not all children whose death was related to an flu virus infection may have been tested for flu^9,10. By combining data on hospitalization rates, flu testing practices, and the frequency of death in and out of the hospital from death certificates, we estimate that there were approximately 372 deaths associated with flu in children during 2018–2019.

Our estimates of hospitalizations and mortality associated with the 2018–2019 flu season continue to demonstrate how serious flu virus infection can be. We estimate, overall, there were 380,000 hospitalizations and 28,000 deaths during the 2018–2019 season. More than 39,000 hospitalizations occurred in children (aged <18 years); however, 55% of hospitalizations occurred in older adults aged ≥65 years. Older adults also accounted for 75% of flu-related deaths, highlighting that older adults are particularly vulnerable to severe outcomes resulting from a flu virus infection. An estimated 6,000 deaths occurred among working age adults (aged 18–64 years), an age group that often has low flu vaccination uptake¹¹.

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 approach³. However, flu testing practices at sites that contribute to our FluSurv-NET surveillance system may not be representative of flu testing practices for the entire United States.

Second, estimates of flu-related illness and medical visits are based on a ratio of illnesses to hospitalizations determined in a prior study.  This ratio is based on data from prior seasons, 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 since 2009 to those older reports, though the estimates from our current method are largely consistent for similar years^12–15. Furthermore, some of the previous published models have estimated flu-related hospitalizations and deaths back as far as the 1970s, and that level of historic data is not available for this current method.  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.

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

References:

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3. Reed C, Chaves SS, Daily Kirley P, et al. Estimating influenza disease burden from population–based surveillance data in the United States. PLoS One 2015; 10(3): e0118369.
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6. Prevention CfDCa. Past Seasons Estimated Influenza Disease Burden. https://www.cdc.gov/flu/about/burden/past–seasons.html (accessed 10/9/2019 2019).
7. Centers for Disease C, Prevention.  How CDC Classifies Flu Severity. https://www.cdc.gov/flu/about/classifies-flu-severity.htm
8. Centers for Disease C, Prevention. FluView Interactive. https://www.cdc.gov/flu/weekly/fluviewinteractive.htm (accessed 10/9/2019 2019).
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10. Martin K SA, 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. Council of State and Territorial Epidemiologist. West Palm Beach, FL; 2018.
11. Prevention CfDCa. Flu Vaccination Coverage, United States, 2018–19 Influenza Season. https://www.cdc.gov/flu/fluvaxview/coverage–1819estimates.htm (accessed 10/9/2019 2019).
12. Centers for Disease C, Prevention. Estimates of deaths associated with seasonal influenza ––– United States, 1976–2007. MMWR Morb Mortal Wkly Rep 2010; 59(33): 1057–62.
13. Foppa IM, Cheng PY, Reynolds SB, et al. Deaths averted by influenza vaccination in the U.S. during the seasons 2005/06 through 2013/14. Vaccine 2015; 33(26): 3003–9.
14. Thompson WW, Shay DK, Weintraub E, et al. Influenza–associated hospitalizations in the United States. JAMA 2004; 292(11): 1333–40.
15. Thompson WW, Shay DK, Weintraub E, et al. Mortality associated with influenza and respiratory syncytial virus in the United States. JAMA 2003; 289(2): 179–86.