U.S. Influenza Surveillance: Purpose and Methods

Overview

FluView, a weekly influenza surveillance report, and FluView Interactive, an online application that allows for more in-depth exploration of influenza surveillance data, are updated each week. The data presented each week are preliminary and may change as more data are received.

The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state, local, and territorial health departments, public health and clinical laboratories, vital statistics offices, health care providers, hospitals, clinics, and emergency departments. Information in four categories is collected from nine data sources to:

• Find out when and where influenza activity is occurring;
• Determine what influenza viruses are circulating;
• Detect changes in influenza viruses; and
• Measure the impact influenza is having on illness, hospitalizations, and deaths.

It is important to maintain a comprehensive system for influenza surveillance for the following reasons:

• Influenza viruses are constantly undergoing minor changes (this is commonly referred to as “antigenic drift”), and ongoing data collection and characterization of viruses helps to monitor these changes.
• Influenza viruses can also undergo an abrupt, major change (referred to as “antigenic shift”) that results in a virus that is much different than currently circulating influenza viruses. Antigenic shift can result in a pandemic if a new influenza A virus that is able to infect people easily and spread from person to person in an efficient and sustained way emerges. Surveillance of viruses can detect these changes and help inform the public health response.
• Annual influenza vaccination is recommended for person aged ≥6 months, and influenza vaccines are updated regularly based on surveillance findings.
• Antiviral treatment recommendations for influenza virus infection are informed by laboratory surveillance for antiviral resistance.
• Influenza surveillance and targeted research studies are used to monitor the impact of influenza on different segments of the population (e.g., people in certain age groups, people with underlying medical conditions) and the impact of influenza prevention measures such as vaccines.

Virologic Surveillance

U.S. Influenza Collaborating Laboratories System (ICLS) and the National Respiratory and Enteric Virus Surveillance System (NREVSS) – Approximately 100 public health and approximately 300 clinical laboratories located throughout all 50 states, Puerto Rico, Guam, and the District of Columbia participate in virologic surveillance for influenza through either the U.S. Influenza Collaborating Laboratories System or NREVSS. Influenza testing practices differ between public health and clinical laboratories, and each network provides valuable information for monitoring influenza activity. Clinical laboratories primarily test respiratory specimens for diagnostic purposes, and data from these laboratories provide useful information on the timing and intensity of influenza activity. Public health laboratories primarily test specimens for surveillance purposes to understand what influenza virus types, subtypes, and lineages are circulating and the ages of people that are infected.

All public health and clinical laboratories report each week to CDC the total number of respiratory specimens tested for influenza and the number positive for influenza viruses, along with age or age group of the person, if available. CDC presents data from clinical laboratories that include the weekly total number of specimens tested for influenza, the number of positive influenza tests, and the percent positive by influenza virus type. A change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza from clinical labs is considered increasing or decreasing compared to the previous week while a change of less than 0.5 percentage points is considered to be stable compared to the previous week. This threshold was determined by evaluating percentage positive fluctuations during periods of low influenza activity (defined as <2.0% positivity). Activity during this period is primarily attributable to random variation which does not signify a genuine increase in viral prevalence. The average percentage point change observed during these periods is 0.16%, while the cumulative effect of two standard deviations is 0.37%. When the average percentage point change is added to the value of two standard deviations, the resultant total is 0.53%. Upon rounding this value to one decimal place, it is expressed as 0.5%.

For public health laboratories, CDC presents the weekly total number of specimens tested and the number positive by influenza virus type and subtype/lineage. To obtain specimens in an efficient manner, public health laboratories often receive specimens that have already tested positive for influenza virus at a clinical laboratory. As a result, monitoring the percent of specimens testing positive for influenza in a public health laboratory is less useful for estimating the timing and intensity of influenza activity (i.e., we expect a higher percent positive than what is actually causing illness or infections in the community). In order to use each data source most appropriately and to avoid duplication, reports from public health and clinical laboratories are presented separately in both FluView and FluView Interactive.

The age distribution of people who have tested positive for influenza reported from public health laboratories can be visualized in FluView Interactive. The number and proportion of influenza virus-positive specimens by influenza A subtype and influenza B lineage are presented by age group (0-4 years, 5-24 years, 25-64 years, and ≥65 years) each week and cumulative totals are provided for the season.

Additional laboratory data for current and past seasons and by geographic level (national, Department of Health and Human Services (HHS) region, and state) are available on FluView Interactive.

Virus Characterization – This includes genetic and antigenic characterization. Most influenza viruses submitted for virus characterization in the United States come from state and local public health laboratories. Due to Right Size considerations, public health laboratories are asked to submit to CDC the following specimens, if available, every other week during the 2025-2026 season: 4 influenza A(H1N1)pdm09, 6 influenza A(H3N2), and 4 influenza B. Therefore, the number of viruses characterized will not reflect the actual proportion of circulating viruses. The goals of genetic and antigenic characterization are to: 1) assess how similar the currently circulating influenza viruses are relative to the reference viruses representing the current U.S. influenza vaccines, 2) monitor evolutionary changes that continually occur in influenza viruses circulating in humans. Specimens received from public health laboratories for virus characterization also serve as an important source of viruses to create candidate vaccine viruses for use in future influenza seasons.

For genetic characterization, all influenza-positive surveillance specimens received at CDC undergo next-generation sequencing to determine their genetic identity. This analysis identifies circulating influenza viruses and helps monitor the evolutionary trajectory of viruses circulating in the human population. Sequences of virus gene segments are used for phylogenetic analysis. Based on the similarity of the hemagglutinin (HA) gene and changes in the HA protein sequence, viruses are categorized by HA clades and subclades (Nextclade, https://clades.nextstrain.org). However, genetic changes that classify the clades/subclades do not always result in antigenic changes.

Antigenic characterization is performed on a subset of viruses selected from the recent genetically characterized viruses based on the identified genetic changes in their surface proteins. This subset of viruses may not be proportional to the number of such viruses circulating in the United States. Antigenic characterization includes hemagglutination inhibition (HI) assay (H1N1pdm09, H3N2, and B/Victoria viruses) and neutralization-based HINT (H3N2 viruses) using antisera from ferrets infected with reference viruses representing the current U.S. season influenza vaccines. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. Antigenic differences between viruses are determined by comparing how well the antibodies raised against the vaccine reference viruses recognize the circulating viruses, which were grown in cell culture. This allows for the detection of "antigenic drift", a term that describes the gradual antigenic change that occurs as viruses evolve to escape host immune pressure. In HI assays, viruses are deemed antigenically similar when their HI titer differences are less than or equal to 4-fold. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than 8-fold.

CDC also analyzes influenza viruses collected by public health laboratories for susceptibility to influenza antivirals, including neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and a polymerase acidic protein (PA) cap-dependent endonuclease inhibitor (baloxavir). Susceptibility to the neuraminidase inhibitors is assessed using next-generation sequencing analysis. Neuraminidase sequences of viruses are inspected to detect the presence of amino acid substitutions previously associated with reduced or highly reduced inhibition by any of the three neuraminidase inhibitors. In addition, a subset of viruses is tested using a neuraminidase inhibition assay. The level of neuraminidase activity inhibition by antivirals is reported according to the provisional thresholds utilized by domestic and international surveillance laboratories. Susceptibility to baloxavir is assessed using next-generation sequencing analysis to identify amino acid substitutions in the PA protein previously associated with reduced susceptibility to this antiviral. A subset of representative viruses is also tested phenotypically using a cell culture-based assay named IRINA (Influenza Replication Inhibition Neuraminidase-based Assay). For surveillance purposes, antiviral susceptibility is typically conducted on viruses that are collected from patients not treated with influenza antivirals or before initiation of treatment with antivirals.

Results of genetic and antigenic characterization and antiviral susceptibility testing are presented in the virus characterization and antiviral susceptibility sections of the FluView report.

Surveillance for Novel Influenza A Viruses – In 2007, human infection with a novel influenza A virus became a nationally notifiable condition in the United States. Novel influenza A virus infections include all human infections with influenza A viruses that are different from currently circulating human seasonal influenza A(H1) and A(H3) viruses. These viruses include those that are subtyped as nonhuman in origin and those that cannot be subtyped with standard laboratory methods and reagents. Rapid detection and reporting of human infections with novel influenza A viruses – viruses against which there is often little to no pre-existing immunity in the population – is important to facilitate prompt awareness and characterization of influenza A viruses with pandemic potential and accelerate the implementation of public health responses to limit the transmission and impact of these viruses.

Newly identified cases of human infections with novel influenza A viruses in the United States are reported in FluView and additional information, including case counts by geographic location, virus subtype, and calendar year or season, are available on FluView Interactive.

Outpatient Illness Surveillance

U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) — Information on outpatient visits to health care providers for respiratory illness referred to as influenza-like illness [ILI) is collected through ILINet. ILINet consists of outpatient healthcare providers in all 50 states, Puerto Rico, the District of Columbia, and the U.S. Virgin Islands. More than 124 million patient visits were reported during the 2024-25 season. Each week, approximately 4,000 outpatient health care providers around the country report to CDC the number of patient visits for ILI by age group (0-4 years, 5-24 years, 25-49 years, 50-64 years, and ≥65 years) and the total number of visits for any reason. Approximately 70% of these providers also report total visits by age group. For this system, ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and a cough and/or a sore throat. Since the 2021-22 season, the case definition no longer includes "without a known cause other than influenza". Since ILINet monitors visits for ILI, not laboratory-confirmed respiratory viruses, it can capture visits due to any respiratory pathogen that presents with the symptoms of fever plus cough or sore throat. These data should be evaluated in the context of other surveillance data to obtain a complete and accurate picture of influenza activity.

Additional data on medically attended visits for ILI for current and past seasons and by geographic level (national, HHS region, and state) are available on FluView Interactive.

The national percentage of patient visits to healthcare providers for ILI reported each week is calculated by combining state-specific data weighted by state population. A change of greater than 0.1 percentage points in the percent of visits for ILI is considered increasing or decreasing compared to the previous week while a change of less than or equal to 0.1 percentage points is considered stable compared to the previous week. This threshold was determined by calculating the average weekly change in percent ILI during periods of low influenza activity for the past 10 years: 0.1% (range from 0% to 0.3%). A non-influenza time period (e.g., a "non-influenza week" or "low influenza activity period") is defined as two or more consecutive weeks in which each week accounted for less than 2% of the season's total number of specimens that tested positive for influenza in public health laboratories. Since fluctuation during periods of low influenza activity are likely to be random variation, a change greater than the average observed during this time is considered an increase or decrease.

In addition to tracking the ILI percent from week to week, the ILI percentage is also compared to the national baseline each week. During the 2025-2026 influenza season, the national baseline is 3.1%. The baseline is developed by calculating the mean percentage of patient visits for ILI during non-influenza weeks for the most recent three seasons (2022-2023, 2023-2024, 2024-2025) and adding two standard deviations. Region-specific baselines are calculated using the same methodology. Due to the wide variability in regional level data, it is not appropriate to apply the national baseline to regional data.

Regional baselines for the 2025-2026 influenza season are:

Region 1 —2.2% Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont

Region 2 — 4.5% New Jersey, New York, Puerto Rico, and the U.S. Virgin Islands

Region 3 — 2.7% Delaware, District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia

Region 4 — 3.4% Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee

Region 5 — 2.3% Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin

Region 6 — 4.1% Arkansas, Louisiana, New Mexico, Oklahoma, and Texas

Region 7 — 2.0% Iowa, Kansas, Missouri, and Nebraska

Region 8 — 2.7% Colorado, Montana, North Dakota, South Dakota, Utah, and Wyoming

Region 9 — 3.9% Arizona, California, Hawaii, Nevada, and the Northern Mariana Islands

Region 10 — 2.1% Alaska, Idaho, Oregon, and Washington

ILI Activity Indicator Map: — Activity levels in a jurisdiction are based on the percent of outpatient visits due to ILI compared to the average percent of ILI visits that occur during weeks with little to no influenza virus circulation (non-influenza weeks) in that jurisdiction. The number of sites reporting each week is variable; therefore, baselines are adjusted each week based on the sites within each jurisdiction who provided data. To perform this adjustment, provider level baseline ILI ratios are calculated for providers that have a sufficient reporting history. Providers that do not have the required reporting history to calculate a provider-specific baseline are assigned the baseline ratio for their practice type (e.g., pediatrics, emergency department, urgent care, family medicine, etc.). The jurisdiction level baseline is then calculated using a weighted sum of the baseline ratios for each contributing provider.

The activity levels compare the mean reported percent of visits due to ILI during the current week to the mean reported percent of visits due to ILI during non-influenza weeks. The 13 activity levels correspond to the number of standard deviations below, at, or above the mean for the current week compared to the mean during non-influenza weeks. Activity levels are classified as minimal (levels 1-3), low (levels 4-5), moderate (levels 6-7), high (levels 8-10), and very high (levels 11-13). An activity level of 1 corresponds to an ILI percentage below the mean, level 2 corresponds to an ILI percentage less than 1 standard deviation above the mean, level 3 corresponds to an ILI percentage more than 1 but less than 2 standard deviations above the mean, and so on, with an activity level of 10 corresponding to an ILI percentage 8 to 11 standard deviations above the mean. The very high levels correspond to an ILI percentage 12 to 15 standard deviations above the mean for level 11, 16 to 19 standard deviations above the mean for level 12, and 20 or more standard deviations above the mean for level 13.

The ILI Activity Indicator map reflects the intensity of ILI activity, not the extent of geographic spread of ILI, within a jurisdiction. Therefore, outbreaks occurring in a single area could cause the entire jurisdiction to display high or very high activity levels. In addition, data collected in ILINet may disproportionally represent certain populations within a jurisdiction and therefore may not accurately depict the full picture of respiratory illness activity for the entire jurisdiction. Differences in the data presented by CDC and independently by some health departments are likely to represent differing levels of data completeness with data presented by the health department likely being more complete.

The ILI Activity Indicator Map displays state-specific and core-based statistical area (CBSA) specific activity levels by week for multiple seasons and allows a visual representation of relative levels of ILI activity from state to state. More information is available on FluView Interactive.

National Syndromic Surveillance System (NSSP) - The National Syndromic Surveillance System (NSSP) uses emergency department (ED) data to monitor respiratory illness diagnoses. About 83% of U.S. EDs send data to NSSP, often within 24 hours. The percentage of ED visits with a discharge diagnosis of influenza overall, and by age group, is presented in FluView. More information on how NSSP data are monitored for respiratory illnesscan be found in the Companion Guide: NSSP Emergency Department Data on Respiratory Illness.

Non-Influenza Respiratory Viruses - CDC also provides integrated information about COVID-19, influenza, and respiratory syncytial virus (RSV) activity from NSSP on a website that is updated weekly. Information about other respiratory virus activity can be found here.

Hospitalization Surveillance

FluSurv-NET - Laboratory-confirmed influenza-associated hospitalizations are monitored through the Influenza Hospitalization Surveillance Network (FluSurv-NET). FluSurv-NET has conducted population-based surveillance for laboratory-confirmed influenza-related hospitalizations in children younger than 18 years of age since the 2003-2004 influenza season and in adults since the 2005-2006 influenza season. The current network covers over 90 counties or county equivalents in the 12 Emerging Infections Program (EIP) states (CA, CO, CT, GA, MD, MI, MN, NM, NY, OR, TN, and WA) and two additional states through the Influenza Hospitalization Surveillance Project (NC and UT). The network represents approximately 10% of the U.S. population (~34 million people).

A full description of this system is available at Influenza Hospitalization Surveillance Network (FluSurv-NET). FluSurv-NET data including hospitalization rates for multiple seasons and different age groups and data on patient characteristics (such as virus type, demographic, and clinical information) are available on FluView Interactive.

National Healthcare Safety Network (NHSN) Hospital Respiratory Data (HRD) Module – CDC's NHSN is the cornerstone of U.S. infectious disease tracking in healthcare facilities. NHSN provides facilities, health departments, tracking systems, states, regions, and the nation with data needed to identify problem areas, measure progress of prevention efforts, and ultimately eliminate healthcare-associated infections.

All acute care and critical access hospitals in the United States are required as a CMS Condition of Participation to report weekly data regarding respiratory viruses and hospital bed capacity and occupancy to NHSN's Hospital Respiratory Data (HRD) Module. These data provide situational awareness of the impact of COVID-19, influenza and RSV on patients and healthcare systems. Facility-level aggregated data are reported electronically via API, upload, or NHSN application webform and include the following:

• Hospital bed capacity and occupancy information, overall and by bed type (inpatient, ICU), designation (adult, pediatric), and condition (COVID-19, influenza, and RSV).

• Number of patients currently hospitalized with laboratory-confirmed COVID-19, influenza, and RSV, by age group.

• Number of new hospital admissions of patients with laboratory-confirmed COVID-19, influenza, and RSV, by age group.

• Additionally, hospital personal protective equipment (PPE) and supply data are available for voluntary reporting to NHSN in conjunction with the required data.

The numbers of new hospital admissions with laboratory confirmed influenza virus infection, intensive care unit (ICU ) hospitalizations, and hospitalization rates reported to NHSN are aggregated by week at the national and HHS region level and are available in FluView and/or on FluView Interactive.

New hospital admissions are defined as patients who were admitted to an inpatient bed and had a positive influenza test at admission or during the 14 days prior. Laboratory confirmation includes detection of influenza virus infection through molecular tests (e.g., polymerase chain reaction, nucleic acid amplification), antigen detection tests, immunofluorescence tests, and virus culture. For hospital reporting, laboratory-confirmed influenza is categorized as influenza A or B. These datafiles are available to the public here.

For the HRD module, the number of patients hospitalized with a positive influenza test and the rate of hospitalization per 100,000 population are monitored. A change of greater than or equal to 0.2 in the rate of hospital admissions or greater than or equal to 691 for the number of hospital admissions is considered increasing or decreasing compared to the previous week. A change of less than 0.2 or 691 for the rate or number of hospitalizations, respectively, is considered stable compared to the previous week. This threshold was determined by evaluating the rate and number fluctuations during periods of low influenza activity (defined as <2.0% positivity) during recent seasons and adding one standard deviation. Changes during this period is primarily attributable to random variation and does not signify a genuine change in influenza-associated hospitalizations. The average weekly change for influenza hospitalization rates during these periods is 0.07 per 100,000 population and the standard deviation is 0.13, resulting in a change threshold of 0.2 per 100,000. The average weekly change for the number of influenza- associated hospitalizations during these periods of low activity is 252.8 and the standard deviation is 438.3, which results in a change threshold of 691.

National Healthcare Safety Network (NHSN) Long- Term Care Respiratory Pathogens and Vaccination Module – CDC's NHSN provides healthcare facilities, such as long-term care facilities [e.g., Nursing homes/skilled nursing facilities], with a secure platform to systematically report patient outcomes and process measures. Reported data are used to strengthen local and national surveillance, monitor trends in infection rates, assist in identifying resource insecurities, and inform progress toward infection prevention goals.

The NHSN Long-Term Care Respiratory Pathogens and Vaccination Module was launched to collect weekly case, hospitalization, and vaccination data for respiratory pathogens (e.g., COVID-19, influenza, RSV). This module is designed to ensure that reporting of respiratory pathogen cases, hospitalizations, and vaccination data is standardized and comparable across facilities over time. Data may be submitted via the NHSN web-based platform.

LTCFs certified by Centers for Medicare and Medicare Services (CMS) are required to comply with the CMS's reporting requirements to report COVID-19 data beginning September 2024 and influenza and RSV data beginning January 2025. These data must be reported according to the NHSN RPV protocol, and the accompanying table of instructions, to ensure that data are uniformly reported by participating facilities.

Hospitalization rates are calculated as the number of residents in long term care facilities who were hospitalized during the week of data collection and had a positive influenza test within 10 days prior to hospitalization divided by the total number of residents in those facilities. A change in hospitalization rates of greater than or equal to 5% is considered increasing or decreasing compared to the previous week while a change of less than 5% is considered to be stable compared to the previous week. By quantifying percentage change, this approach normalizes data, making it comparable across different populations and time periods, regardless of absolute case counts.

These data are reported each week at the national and HHS regional level. Of note, the data collection week for this module is defined as Monday through Sunday. This differs slightly from other national influenza surveillance system components as the collection week is Sunday through Saturday. This will not affect the monitoring of trends over time.

Additional information about CDC's NHSN LTCF Respiratory Pathogens and Vaccination (RPV) Module (e.g., collection forms, protocol and instructions, trainings, and future updates) are available here.

In previous seasons (2020–21 to 2022–23), CDC's FluView included LTCF influenza data, such as the percentage of facilities reporting at least one positive influenza test among residents. For more information, see the Archived COVID-19/Respiratory Pathogens and Vaccination Materials section on the NHSN long-term care webpage.

Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance Data – NCHS collects death certificate data from state vital statistics offices for all deaths occurring in the United States. Deaths included in this component of the U.S. Influenza Surveillance System are those which are classified based on ICD-10 multiple cause of death codes as associated with influenza (ICD-10 codes J09-J11), COVID-19 (ICD-10 code U07.1), or pneumonia (ICD-10 codes J12-J18). Data are aggregated by the week of death occurrence. NCHS surveillance data are included in FluView one week after the week of death and percentages for earlier weeks are continually revised and may increase or decrease as new and updated death certificate data are received by NCHS.

Three measures have been used to monitor influenza mortality: pneumonia and/or influenza (P&I); pneumonia, influenza, and/or COVID-19 (PIC); and influenza (I).

Prior to the 2020-2021 influenza season, the NCHS surveillance data were used to calculate the percent of all deaths occurring each week that had pneumonia and/or influenza (P&I) listed as a cause of death. Since many influenza deaths and many COVID-19 deaths have pneumonia included on the death certificate, P&I does not measure the impact of influenza in the same way that it had prior to the COVID-19 pandemic. This is because the proportion of pneumonia deaths associated with influenza is now influenced by COVID-19-related pneumonia. Beginning in the 2020-2021 influenza season, COVID-19 coded deaths were added to P&I to create the PIC (pneumonia, influenza, and/or COVID-19) classification. PIC includes all deaths with pneumonia, influenza, and/or COVID-19 listed on the death certificate.

Starting with the 2023-2024 influenza season, the percent of deaths with influenza listed on the death certificate began being displayed in FluView. P&I no longer measures the impact of influenza in the same way it had prior to the COVID-19 pandemic, and the PIC measure is often driven by COVID-19 activity making it difficult to monitor the impact of influenza using that measure. Although monitoring influenza-only coded deaths will underestimate the full impact of influenza mortality, this measure allows for tracking trends in the impact of influenza on mortality and is not as influenced by COVID-19 as the other two measures. Additionally, influenza burden estimates are calculated that account for underreporting of influenza on death certificates.

All three measures will be available on FluView Interactive, including baselines and thresholds for P&I and PIC. The PIC and P&I percentages are compared to a seasonal baseline of P&I deaths that is calculated using a periodic regression model incorporating a robust regression procedure applied to data from the two years prior to the COVID-19 pandemic (2018 week 40 through 2020 week 9) and the 2022-2023 through 2024-25 influenza seasons. An increase of 1.645 standard deviations above the seasonal baseline of P&I deaths is considered the "epidemic threshold," (i.e., the point at which the observed proportion of deaths attributed to pneumonia, influenza, or COVID-19 was significantly higher than would be expected at that time of the year in the absence of substantial influenza- or COVID-19-related mortality).

Additional influenza, P&I and PIC mortality data for current and past seasons and by geographic level (national, HHS region, and state) are available on FluView Interactive. Data displayed on the regional and state-level are aggregated by the state of residence of the decedent.

Influenza-Associated Pediatric Mortality Surveillance System — Influenza-associated pediatric mortality became a nationally notifiable condition in 2004. For surveillance purposes, an influenza-associated pediatric death is defined as a death in a person less than 18 years of age, resulting from a clinically compatible illness that was confirmed to be influenza by an appropriate laboratory diagnostic test. There should be no period of complete recovery between the illness and death. Demographic and clinical information is collected on each case and reported to CDC.

Information on influenza-associated pediatric deaths including basic demographics, underlying medical conditions, bacterial co-infections, and place of death, is available on FluView Interactive for the current and past seasons.

Influenza Surveillance Considerations

It is important to remember the following about influenza surveillance in the United States.

• While most influenza activity reporting by public health partners and health care providers to CDC is voluntary, hospitals and long-term care facilities are required to submit certain influenza data to NHSN under CMS' condition of participation reporting requirements.
• Surveillance information answers the questions of where, when, and what influenza viruses are circulating. It can be used to determine if influenza activity is increasing or decreasing but it does not directly provide the number of influenza illnesses. For more information regarding how CDC classifies influenza severity and the disease burden (number of illnesses, hospitalizations, and deaths) of influenza, please see Disease Burden of Influenza.
• The national influenza surveillance system comprises ten complementary components across four key categories. It utilizes data gathered through a nationwide network of reporting entities, including approximately 400 laboratories, 4,000 outpatient healthcare providers, vital statistics offices and the National Center for Health Statistics, research and healthcare personnel at FluSurv-NET sites, approximately 5,500 acute care hospitals, 15,000 long-term care facilities, and influenza surveillance coordinators and state epidemiologists from state, local, and territorial health departments.
• Influenza surveillance data are aggregated according to the week the event (e.g., positive laboratory test, outpatient visit, hospitalization, death) occurred. The week starts on Sunday and ends on Saturday for all, but the LTCF component which uses a Monday – Sunday week. Each surveillance participant/reporter is requested to report data to CDC by Tuesday afternoon of the following week. The data are then downloaded, compiled, and analyzed at CDC. FluView and FluView Interactive are updated weekly on Fridays after 11 AM.
• The reporting period for each influenza season begins during epidemiologic week 40 and ends week 39 of the following year. Epidemiologic weeks refer to the sequential numbering of weeks (Sunday through Saturday) during a calendar year. This means that the exact start of the new influenza surveillance season varies slightly from season to season. The 2025-2026 influenza season begins September 29, 2025, and ends on September 26, 2026.
• The timing of the annual “Influenza season” — as determined by elevated influenza activity – also varies from season to season. During most seasons, activity begins to increase in October, most often peaks between December and February and can remain elevated into May. The influenza season is said to have started after consecutive weeks of elevated influenza activity are registered in the various CDC influenza surveillance systems.