April 2020
*Because influenza surveillance does not capture all cases of flu that occur in the U.S., CDC provides these estimated ranges to better reflect the larger burden of influenza. These estimates are calculated based on CDC’s weekly influenza surveillance data and are preliminary.
**Influenza testing across the United States may be higher than normal at this time of year because of the COVID-19 pandemic. These estimates may partly reflect increases in testing in recent weeks and maybe adjusted downward once the season is complete and final data for the 2019/20 season are available.
Limitations
The estimates of the cumulative burden of seasonal influenza are subject to several limitations.
First, the cumulative rate of laboratory-confirmed influenza-associated hospitalizations reported during the season may be an under-estimate of the rate at the end of the season because of identification and reporting delays.
Second, rates of laboratory-confirmed influenza-associated hospitalizations were adjusted for the frequency of influenza testing and the sensitivity of influenza diagnostic assays. However, data on testing practices during the 2019-2020 season are not available in real-time. CDC used data on testing practices from the past influenza seasons as a proxy. Burden estimates 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 data from prior seasons, which may not be accurate if the seriousness of illness or patterns of care-seeking have changed.
