Estimated Flu-Related Illnesses, Medical Visits, Hospitalizations, and Deaths in the United States — 2017–2018 Flu Season

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.

2017–2018 Burden Estimates: Background and Results

Flu 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 (1). 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 flu-like illness, hospitalizations rates, and proportions of pneumonia and flu-related deaths.

CDC estimates that flu burden during the 2017–2018 flu season was high, with an estimated 41 million people getting sick with flu, 21 million people going to a health care provider for flu-related symptoms, 710,000 flu hospitalizations, and 52,000 deaths from flu (Table 1). The number of cases of flu-related illness that occurred during 2017-2018 was the highest since the 2009 H1N1 pandemic, when an estimated 60 million people were sick with flu (2).

The 2017–2018 flu season was additionally atypical in that it was severe for all ages (1). The burden of flu and the rates of flu-related 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 flu in children, 25 million cases of flu in working age adults (aged 18-64 years), and 5 million cases in adults aged 65 years and older.

Our estimates of hospitalizations and mortality associated with the 2017–2018 flu season continue to demonstrate how severe flu can be. More than 43,000 hospitalizations occurred in children (aged < 18 years); however, 66% 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 flu virus infection. An estimated 8,100 deaths occurred among working age adults (aged 18–64 years), an age group that often has low flu vaccination.

Deaths in children with laboratory-confirmed flu virus infection are reportable in the United States and 188 deaths were reported for the 2017–2018 season. However, flu-related pediatric deaths are likely under-reported as not all children whose death was related to flu virus infection may have been tested for flu (3,4). 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 526 deaths associated with flu in children.

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 approach (5,6). 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, 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 (7,9). 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.