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Preliminary In-Season U.S. Influenza Burden Estimates
This web page provides weekly preliminary estimates of the cumulative in-season numbers of flu illnesses, medical visits and hospitalizations in the United States. CDC has estimated the burden of flu since 2010 using a mathematical model that is based on observed rates of laboratory-confirmed influenza-associated hospitalizations collected through a surveillance network that covers approximately 8.5% of the U.S. population (~27 million people). Estimates of flu-related deaths will be provided at a later time, when there is sufficient data to support a more precise estimate for that outcome.
The 2018-2019 flu season is the first season CDC has reported in-season burden estimates of flu in the U.S. These in-season estimates will be updated over the course of the flu season.
| Symptomatic Illnesses | Medical Visits | Hospitalizations | |
|---|---|---|---|
| Week 52 ending December 29, 2018 | 4,090,000 – 4,900,000 | 1,990,000 – 2,420,000 | 40,100 – 48,300 |
CDC publishes weekly surveillance reports summarizing key influenza activity indicators.
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 2018-2019 season are not available in real-time. CDC used data on testing practices from the 2014-2015 influenza season 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.
Frequently Asked Questions
What does the cumulative burden of influenza for the 2018-2019 season mean?
The cumulative burden of influenza is an estimate of the number of people who have been sick, seen a healthcare provider, been hospitalized, or died as a result of influenza since October 01, 2018. CDC does not know the exact number of people who have been sick and affected by influenza because influenza is not a reportable disease in most areas of the United States. However, these numbers are estimated using a mathematical model, based on observed rates of laboratory-confirmed influenza-associated hospitalizations.
How does CDC estimate the cumulative burden of seasonal influenza?
Preliminary estimates of the cumulative burden of seasonal influenza during the 2018-2019 season in the United States are based on crude rates of laboratory-confirmed influenza-associated hospitalizations, reported through the Influenza Hospitalization Surveillance Network (FluSurv-NET), which were adjusted for the frequency of influenza testing during recent prior seasons and the sensitivity of influenza diagnostic assays. Rates of hospitalization were then multiplied by previously estimated ratio of hospitalizations to symptomatic illnesses, and frequency of seeking medical care to calculate symptomatic illnesses, medical visits, and deaths associated with seasonal influenza, respectively.
Why does the estimate of cumulative burden change each week?
The estimates of cumulative burden of seasonal influenza are considered preliminary and may change each week as new laboratory-confirmed influenza-associated hospitalizations are reported to CDC. New reports include both new admissions that have occurred during the reporting week and also patients admitted in previous weeks that have been newly reported to CDC.
How does the number of flu hospitalizations estimated so far this season compare with previous end-of-season hospitalization estimates?
The number of hospitalizations estimated so far this season is lower than end-of-season total hospitalization estimates for any season since CDC began making these estimates. The graphic gives some comparisons with other seasons. This table also summarizes all estimated influenza disease burden, by season, in U.S. from 2010-11 through 2017-18.
