Archived: Estimated Influenza Illnesses, Medical visits, Hospitalizations, and Deaths in the United States — 2018–2019 influenza season

Introduction

This web page provides estimates on the burden of influenza in the United States for the 2018–2019 influenza season. For the past several years, CDC has estimated the numbers of influenza illnesses, medical visits, hospitalizations, and deaths^1–4. The methods used to calculate the estimates have been described previously³. CDC uses the estimates of the burden of influenza in the population to inform policy and communications related to influenza.

2018-2019 Estimates

Influenza 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 influenza-like illness, hospitalizations rates, and proportions of pneumonia and influenza-associated deaths.

CDC estimates that the burden of illness during the 2018–2019 season included an estimated 35.5 million people getting sick with influenza, 16.5 million people going to a health care provider for their illness, 490,600 hospitalizations, and 34,200 deaths from influenza (Table 1). The number of influenza-associated illnesses that occurred last season was similar to the estimated number of influenza-associated illnesses during the 2012–2013 influenza season when an estimated 34 million people had symptomatic influenza illness⁶.

Peak activity during the 2018–2019 influenza 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 influenza were much lower during the 2018–2019 season (Table 2). Among children, however, rates of influenza 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, 136 deaths in children with laboratory–confirmed influenza virus infection were reported in the United States⁸. 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^9,10. By combining data on hospitalization rates, influenza testing practices, and the frequency of death in and out of the hospital from death certificates, we estimate that there were approximately 480  deaths associated with influenza in children during 2018–2019.

Our estimates of hospitalizations and mortality associated with the 2018–2019 influenza season continue to demonstrate how serious influenza virus infection can be. We estimate, overall, there were 490,600 hospitalizations and 34,200 deaths during the 2018–2019 season. More than 46,000 hospitalizations occurred in children (aged <18 years); however, 57% of hospitalizations occurred in older adults aged ≥65 years. Older adults also accounted for 75% of influenza-associated deaths, highlighting that older adults are particularly vulnerable to severe outcomes resulting from an influenza virus infection. An estimated 8,100 deaths occurred among working age adults (aged 18–64 years), an age group that often has low influenza vaccination uptake¹¹.

Conclusion

CDC estimates that influenza was associated with more than 35.5 million illnesses, more than 16.5 million medical visits, 490,600 hospitalizations, and 34,200 deaths during the 2018–2019 influenza season. This burden was similar to estimated burden during the 2012–2013 influenza season¹.

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 2018–2019 season are conducted by FluSurv–NET sites. The most updated crude rates of hospitalization for FluSurv-NET sites from the 2018–2019 season are available on FluView Interactive⁷.

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 approach³. However, data on testing practices during the 2018–2019 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 2018–2019 season will be updated at a later date when data on contemporary testing practices become available.

Third, estimates of influenza-associated 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.

Fourth, our estimate of influenza-associated deaths relies on information about location of death from death certificates. However, death certificate data during the 2018–2019 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-related 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 2018–2019 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 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 influenza-associated 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 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.

References:

1. Centers for Disease C, Prevention. Estimated influenza illnesses and hospitalizations averted by influenza vaccination – United States, 2012–13 influenza season. MMWR Morb Mortal Wkly Rep 2013; 62(49): 997–1000.
2. Reed C, Kim IK, Singleton JA, et al. Estimated influenza illnesses and hospitalizations averted by vaccination––United States, 2013–14 influenza season. MMWR Morb Mortal Wkly Rep 2014; 63(49): 1151–4.
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.
4. 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.
5. Xu X, Blanton L, Elal AIA, et al. Update: Influenza Activity in the United States During the 2018–19 Season and Composition of the 2019–20 Influenza Vaccine. MMWR Morb Mortal Wkly Rep 2019; 68(24): 544–51.
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).
9. Lees CH, Avery C, Asherin R, et al. Pandemic (H1N1) 2009–associated deaths detected by unexplained death and medical examiner surveillance. Emerg Infect Dis 2011; 17(8): 1479–83.
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.