2021-2022 U.S. Flu Season: Preliminary In-Season Burden Estimates
CDC estimates* that, from October 1, 2021 through January 8, 2022, there have been:
1,600,000 – 2,700,000
730,000 – 1,200,000
flu medical visits
16,000 – 33,000
970 – 2,900
*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 data collected through CDC’s Influenza Hospitalization Surveillance Network (FluSurv-NET) and are preliminary.
**Influenza testing across the United States may be higher than normal because of the COVID-19 pandemic. These estimates may partly reflect increases in testing in recent weeks and may be adjusted downward once the season is complete and final data are available.
This web page provides weekly, preliminary estimates of the cumulative in-season numbers of flu illnesses, medical visits, hospitalizations, and deaths in the United States. 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, CDC has estimated the burden of flu since 2010 using a mathematical model that is based on data collected through the Influenza Hospitalization Surveillance Network (FluSurv-NET), a network that covers approximately 9% of the U.S. population.
The estimates of the preliminary, cumulative burden of seasonal influenza are subject to several limitations.
First, the cumulative rate of lab-confirmed flu-related hospitalizations reported during the season may be an underestimate of the rate at the end of the season because of identification and reporting delays.
Second, rates of lab-confirmed flu-related hospitalizations were adjusted for the frequency of flu testing and the sensitivity of flu diagnostic assays. However, data on testing practices during the current flu season are not available in real-time. To make these estimates, CDC uses data on testing practices from the past flu seasons as a proxy. If more testing is being done compared to past flu seasons, these estimates may be inflated. Preliminary in-season burden estimates are finalized when data on contemporary testing practices become available, and the estimates may decrease if testing has increased. During the ongoing COVID-19 pandemic, testing may be higher and it is possible these estimates will be higher than our final estimates.
Third, estimates medical visits for flu-like illnesses are based on data from prior seasons, which may not be accurate if the severity of illness or patterns of care-seeking have changed.
What is cumulative burden of flu?
CDC does not know the exact number of people who have been sick and affected by flu because flu 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 lab-confirmed The cumulative burden of flu is an estimate of the number of people who have been sick, seen a healthcare provider, been hospitalized, or died as a result of flu within a certain timeframe. The in-season preliminary burden estimates are provided weekly during flu season when sufficient data are available to make an estimate, and end-of-season preliminary estimates are given at the end of each flu season. End-of-season preliminary estimates will be updated year-to-year and are considered final when all data are available ( used to calculate these estimates are on a two-year delay).
How does CDC estimate the cumulative burden of seasonal flu?
Preliminary estimates of the cumulative burden of seasonal flu are based on crude rates of lab-confirmed flu-related hospitalizations, reported through the Influenza Hospitalization Surveillance Network (FluSurv-NET), which are adjusted for the frequency of flu testing during recent prior seasons and the sensitivity of flu diagnostic tests. Rates of hospitalization are then multiplied by previously estimated ratio of hospitalizations to symptomatic illnesses, and frequency of seeking medical care to calculate symptomatic illnesses, medical visits, hospitalizations, and deaths associated with seasonal flu, respectively.
Why does the preliminary, in-season estimate of flu burden change each week?
The in-season estimates of flu burden are preliminary and change week-by-week as new flu 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 estimates on this page are preliminary, cumulative and will increase as the season progresses. Past end-of-season estimates (2010-2020) have ranged from 140,000-710,000 and are available here.
What was the estimated burden for 2020-2021?
CDC was not able to calculate the cumulative burden of flu for the 2020-2021 flu season, due to historically low numbers of flu.
More information on the 2020-2021 flu season burden estimates is available here.
For information about the estimated burden of COVID-19, visit: Estimated COVID-19 Burden | CDC.