Can you explain why the estimates on this page are different from previously published and reported estimates for 2017-2018? (For example, total flu-related deaths during 2017-2018 was previously estimated to be 79,000, but the current estimate is 61,000)?

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.

More answers to frequently asked questions about CDC’s influenza burden estimates are available.