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. It is not possible to know the exact number of people who have experienced flu illness in the United States because not everyone who gets sick with flu will seek medical care or be tested for influenza. Given this, not all flu illnesses will be identified through our surveillance systems. This is why we use mathematical models to estimate the impact of flu on the population. When sharing in-season estimates of flu disease burden, CDC believes that using a range is a better way to represent the weekly variability of the estimates. These real-time burden estimates use surveillance data from patients hospitalized with laboratory-confirmed flu in geographically distributed areas in the United States, which may fluctuate week to week during the season. 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.
Each week CDC calculates a lower estimate and an upper estimate of flu-related hospitalizations that have occurred since the beginning of the season (October 1, 2024). These are compared to end-of-season estimates of flu-related hospitalizations from previous flu seasons. These calculations allow for a preliminary comparison of how in-season flu-related hospitalizations are tracking in relation to past seasons. This can help health officials to continue to manage impacts on health care systems and promote prevention measures.
The estimates of the preliminary, cumulative burden of seasonal influenza are subject to several limitations:
The cumulative burden of flu is an estimate of the number of people who have been sick, seen a health care provider for, been hospitalized with, or died as a result of flu within a certain timeframe. The preliminary burden estimates are provided weekly during flu season when sufficient data are available to make an estimate, and preliminary estimates are given after the end of each flu season. End-of-season preliminary estimates will be updated year-to-year and are considered final when all data for that season are available (usually within two years of the preliminary estimate).
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 the previously estimated ratio of hospitalizations to symptomatic illnesses, and the frequency of patients seeking medical care to calculate symptomatic illnesses, medical visits, hospitalizations, and deaths associated with seasonal flu, respectively.
Preliminary, in-season estimates 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.
Comparing the number of flu hospitalizations estimated so far this season 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 of flu-related hospitalizations (2010-2024) have ranged from 120,000-710,000.
