Influenza Hospitalization Surveillance Network (FluSurv-NET)

Purpose and Methods

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.

About FluSurv-NET

The Influenza Hospitalization Surveillance Network (FluSurv-NET) is a population-based surveillance system that 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.

Hospitalization Rates

Patient Characteristics

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:

age
sex
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 (http://www.cy118119.com/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

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%.

Influenza Season