Influenza Hospitalization Surveillance Network (FluSurv-NET)
Population-based surveillance is the collection, analysis and interpretation of data on a population in a specified area.
FluSurv-NET is a population-based surveillance system.
The Influenza Hospitalization Surveillance Network (FluSurv-NET) is part of the Respiratory Virus Hospitalization Surveillance Network (RESP-NET), which is designed to conduct population-based surveillance for laboratory-confirmed influenza, COVID-19, and respiratory syncytial virus (RSV)-associated hospitalizations. RESP-NET also includes surveillance networks for COVID-19 and RSV (also known as COVID-NET and RSV-NET, respectively).
FluSurv-NET collects data on laboratory-confirmed influenza-associated hospitalizations among children and adults through a network of acute care hospitals in 14 states.
Why FluSurv-NET Data is Important
FluSurv-NET is CDC’s source for important data on hospitalization rates associated with flu. FluSurv-NET also provides demographic and clinical information including age, sex and underlying medical conditions among persons hospitalized with flu. Data gathered are used to estimate age-specific hospitalization rates on a weekly basis and to describe characteristics of persons hospitalized with influenza illness.
FluSurv-NET Case Definition
For FluSurv-NET, a case is defined as a person who is a resident 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 by a positive result of a viral culture, direct or indirect fluorescent antibody staining, rapid antigen test, or molecular assay.
How FluSurv-NET Calculates Hospitalization Rates
A minimum set of data are collected on all identified cases to produce weekly hospitalization rates:
- race and ethnicity
- surveillance site
- date of hospital admission
- positive influenza test result/date
Hospitalization rates are calculated as the number of residents of a defined area who are hospitalized with a positive influenza laboratory test divided by the total population within the defined area. NCHS bridged-race population estimates (https://www.cdc.gov/nchs/nvss/bridged_race.htm) are 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) are used as denominators.
FluSurv-NET coverage area includes 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, Ohio, Oregon, Tennessee and Utah.
FluSurv-NET covers more than 30 million people and includes an estimated 9 percent 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, the information might not be generalizable to the entire country.
*As of the 2023-2024 influenza season, Connecticut will use county equivalents instead of counties.
Accessing FluSurv-NET Data
Influenza-associated hospitalization rates are reported to CDC on a weekly basis each influenza season. During seasons with influenza activity outside of the typical influenza season of October 1 through April 30, sites will be given the option to extend influenza hospitalization surveillance beyond the established surveillance period. 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.
FluSurv-NET Sampling Methodology for Collection of Clinical Data
Starting with the 2017-18 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 upon entry into the surveillance database. Trained surveillance staff conducted medical chart abstractions on sampled cases using a standardized case report form.
The following types of clinical data are collected for sampled cases:
- current season influenza vaccination status
- clinical (medical) history (underlying health conditions)
- clinical course (admission to intensive care unit)
- medical interventions (e.g., receipt of antiviral treatment, mechanical ventilation)
- outcomes (e.g., discharged from the hospital, in-hospital death)
Sampling occurred beyond the 2017-18 season using a similar methodology. The table below summarizes the sampling strategies offered during influenza seasons where any sampling occurred at rates <100%.
FluSurv-NET Sampling Methodology for Collection of Clinical Data
FluSurv-NET Sampling Methodology for Collection of Clinical Data
*All other age groups were sampled at 100%, if applicable, and 100% of in-hospital and 30 day-post-discharge deaths were sampled. For the 2023-24 influenza season, sampling rates will be determined mid-season.
In seasons where sampling occurred at rates <100%, clinical estimates are weighted to reflect the probability of selection. The FluSurv-NET sampling methodology has also 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 sources to estimate annual and weekly disease burden of influenza in the United States. Estimates are made of symptomatic illnesses, medically attended illnesses, hospitalizations and deaths. Reported rates are adjusted in an attempt 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 flu burden averted by influenza vaccination are available online.
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