From October 1, 2023, through December 30, 2023, the CDC tracked the number of flu-related cases that have impacted public health:
- There have been an estimated 10 to 19 million flu illnesses.
- Between 4.6 to 9.1 million medical visits were related to the flu.
- Approximately 110,000 to 230,000 individuals required hospitalization due to the flu.
- Tragically, there have been between 6,500 to 19,000 deaths attributed to the flu.
The data used for these calculations is gathered through CDC’s Influenza Hospitalization Surveillance Network (FluSurv-NET) and should be regarded as preliminary.
This network provides weekly, preliminary estimates of the cumulative number of flu illnesses, medical visits, hospitalizations, and deaths in the United States during the flu season. Each week, the CDC calculates both a lower estimate and an upper estimate of flu-related hospitalizations that have occurred since the beginning of the season (October 1, 2023). These estimates are updated weekly and compared to end-of-season estimates from previous flu seasons. In total, they calculate the overall flu disease burden to society.
The cumulative burden of flu refers to an approximation of the total number of individuals who have experienced illness, sought medical attention, been hospitalized, or passed away due to influenza within a specific period. Preliminary estimates of the in-season burden are released on a weekly basis during flu season whenever enough data is accessible for estimation.
At the conclusion of each flu season, an additional set of estimates are also provided. These end-of-season estimates are periodically revised from year to year and are considered final once all the data is collected, which usually takes around two years following the initial estimate.
The CDC uses a systematic approach to estimate the cumulative burden of seasonal flu. They start by gathering data on lab-confirmed flu-related hospitalizations through the Influenza Hospitalization Surveillance Network (FluSurv-NET). These rates are adjusted to account for variations in flu testing and the sensitivity of diagnostic tests.
Next, they multiply the rates of hospitalization by an estimated ratio of hospitalizations to symptomatic illnesses and factor in the frequency of seeking medical care. This calculation allows them to determine the number of symptomatic illnesses, medical visits, hospitalizations, and deaths associated with seasonal flu.
The preliminary in-season estimate of flu burden changes each week due to new cases of flu hospitalizations being reported to the CDC. These include new admissions that occurred during the reporting week as well as patients admitted in previous weeks that have been newly reported. It is important to note that these estimates are preliminary and will continue to increase as the season progresses.
In terms of comparing the number of flu hospitalizations estimated so far this season with previous end-of-season estimates, it’s important to consider that the estimates on this page are cumulative and will also increase as the season progresses. Previous end-of-season estimates from 2010-2023 have ranged from 100,000-710,000.
When interpreting the estimates of the preliminary cumulative burden of seasonal influenza, it is important to take into account several limitations. Firstly, there may be delays in identification and reporting, which could result in an underestimation of the reported rate of lab-confirmed flu-related hospitalizations during the season. And that the true rate may be higher by the end of the season.
Secondly, ongoing adjustments are made in order to calculate rates of lab-confirmed flu-related hospitalizations, taking into consideration testing frequency and assay sensitivity. However, real-time data on testing practices for the current flu season are not available. Therefore, past flu seasons’ testing practices are used as a proxy.
If more testing has been conducted compared to previous seasons, these estimates may be inflated. Once contemporary testing data becomes available, final burden estimates are adjusted accordingly, potentially decreasing them if there has been an increase in testing.
Lastly, estimates regarding medical visits for flu-like illnesses are based on data from prior seasons and may not accurately reflect current illness severity or patterns of care-seeking behavior.
It is important to bear these limitations in mind when considering the preliminary burden estimates. While they provide valuable information about the impact of seasonal influenza, they should be viewed as imperfect due to the potential biases and uncertainties.
