Influenza Hospitalization Surveillance Network (FluSurv-NET)

Jan. 30, 2026

The Influenza Hospitalization Surveillance Network (FluSurv-NET) monitors laboratory-confirmed influenza-associated hospitalizations among children and adults in the United States.

Population-based surveillance is the collection, analysis and interpretation of data on a population in a specified area.

FluSurv-NET

FluSurv-NET data are used to estimate influenza hospitalization rates on a weekly basis and to describe characteristics of people hospitalized with influenza illness on FluView and FluView Interactive.

The Influenza Hospitalization Surveillance Network (FluSurv-NET) is part of the Respiratory Virus Hospitalization Surveillance Network (RESP-NET), which conducts population-based surveillance for laboratory-confirmed hospitalizations associated with influenza, COVID-19 (COVID-NET), and respiratory syncytial virus (RSV) (RSV-NET).

FluSurv-NET was established for children younger than 18 years of age during the 2003-2004 season and was expanded to include adults during the 2005-2006 season.

Why FluSurv-NET Data is Important

FluSurv-NET: Monitoring Influenza Hospitalizations

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FluSurv-NET is CDC's source for important data on hospitalization rates associated with influenza. Influenza associated hospitalization rates are used to understand trends in influenza virus circulation, estimate disease burden, and respond to outbreaks. Demographic and detailed clinical information including underlying conditions, allow CDC to better understand influenza- associated trends, determine who is most at risk, and describe characteristics of people hospitalized with influenza illness. These data guide public health planning and decision making.

FluSurv-NET Case Definition

A case is defined as laboratory-confirmed influenza in a person who:

Lives in a defined FluSurv-NET catchment area AND
Tests positive for influenza by a laboratory test ordered by a health care professional within 14 days prior to or during hospitalization. Laboratory confirmation is defined as a positive influenza result from a viral culture, direct or indirect fluorescent antibody staining, rapid antigen test, or molecular assay.

FluSurv-NET Surveillance Area

FluSurv-NET currently comprises more than 90 counties and county equivalents in 14 states that participate in the Emerging Infections Program (EIP) and the Influenza Hospitalization Surveillance Program (IHSP). Participating states include: California, Colorado, Connecticut, Georgia, Maryland, Michigan, Minnesota, New Mexico, New York, North Carolina, Oregon, Tennessee, Utah, and Washington.

FluSurv-NET covers more than 34 million people and includes an estimated 10% of the U.S. population. The counties covered are located in 9 Health and Human Services (HHS) regions. The designated FluSurv-NET surveillance area is generally similar to the U.S. population by demographics; however, FluSurv-NET data might not be generalizable to the entire country.

*Since the 2023-2024 influenza season, Connecticut has used county equivalents instead of counties.

How FluSurv-NET Calculates Hospitalization Rates

A minimum set of data are collected on all identified cases to produce weekly hospitalization rates, including:

Age
Sex
Race and ethnicity
Surveillance site
Date of hospital admission
Date and type of positive test result

Hospitalization rates are calculated by dividing the number of residents in a surveillance area who are hospitalized with laboratory-confirmed influenza by the total population for that area. The National Center for Health Statistics (NCHS) bridged-race population estimates were used as denominators prior to the 2020-2021 season. Beginning with the 2020-2021 season, unbridged census population estimates (U.S. Census Bureau, Population Division, Vintage 2020-2022 Special Tabulation) have been used as denominators.

Accessing FluSurv-NET Data

Influenza-associated hospitalization rates are reported to CDC on a weekly basis during each influenza season. During seasons when influenza activity occurs outside of the typical influenza season of October 1 through April 30, sites are given the option to extend influenza hospitalization surveillance beyond the established surveillance period. Starting with the 2025-26 influenza season, FluSurv-NET will collect data year-round. FluSurv-NET data, including hospitalization rates for different age groups and data on patient characteristics, are available on FluView and FluView Interactive.

FluSurv-NET hospitalization data are preliminary during each season; data presented may change as more reports are received. In particular, case counts for recent hospital admissions are subject to reporting lags. As data are received each week during the influenza season, prior case counts and rates may be updated.

Clinical data collected in FluSurv-NET and Sampling Methodology

Data collected to describe clinical characteristics of sampled patients hospitalized with influenza include:

Medical history (e.g., underlying health conditions)
Clinical course (e.g., admission to intensive care unit)
Medical interventions (e.g., receipt of antiviral treatment, mechanical ventilation)
Outcomes (e.g., discharged from the hospital or in-hospital death)
Current season influenza vaccination status

Starting with the 2017-2018 influenza season, FluSurv-NET implemented a sampling strategy for collection of clinical data. Clinical data were collected for a random sample of cases ≥50 years of age stratified by age and surveillance site; 100% of cases <50 years were sampled during this season and some sites elected to sample 100% of cases ≥50 years. Additionally, 100% of cases who died in-hospital or within 30 days after hospital discharge were also sampled. Random numbers were auto-generated and assigned to each case patient upon entry into the surveillance database. Trained surveillance staff conduct medical chart abstractions on sampled cases using a standardized case report form.

Sampling occurred beyond the 2017-2018 season using a similar methodology. The table below summarizes the sampling strategies offered during influenza seasons when any sampling occurred at rates <100%.

Influenza Season

Age groups sampled at rates <100%*

2017-18

≥50 years

2018-19

≥65 years

2019-20

≥18 years

2022-23

All ages

2023-24

All ages

2024-25

≥5 years**

*All other age groups were sampled at 100%, if applicable, and 100% of in-hospital and 60 day-post-discharge deaths were sampled. Starting with the 2024-2025 influenza season, sampling rates were determined mid-season.

**For these age groups, depending on month and site, sampling rates ranged from 1%-100%.

During seasons when sampling occurs at rates <100%, clinical estimates are weighted to reflect the probability of selection. The FluSurv-NET sampling methodology also has been described in the below studies:

Chow EJ, Rolfes MA, O'Halloran A, et al. Respiratory and nonrespiratory diagnoses associated with influenza in hospitalized adults. JAMA Netw Open. 2020; 3(3):e201323. doi:10.1001/jamanetworkopen.2020.1323

Chow EJ, Rolfes MA, O'Halloran A, et al. Acute cardiovascular events associated with influenza in hospitalized adults: a cross-sectional study. Ann Intern Med. 2020; 173(8): 605- 613. doi:10.7326/M20-1509

How FluSurv-NET contributes to CDC’s Influenza Burden Estimates

CDC uses FluSurv-NET data in combination with other data to estimate annual and weekly disease burden of influenza in the United States. Estimates are calculated for numbers of influenza symptomatic illnesses, medically attended illnesses, hospitalizations and deaths. Reported rates are to correct for the under-detection of influenza. This adjustment is done by using the percent of people hospitalized with respiratory illnesses who were tested for influenza and the average sensitivity of influenza tests used in the participating FluSurv-NET hospitals. Weekly estimates of influenza burden are available during each influenza season. Annual estimates of US influenza burden and influenza burden averted by influenza vaccination are available online.