What to know
Summary
Viruses
Illness
All data are preliminary and may change as more reports are received.
Directional arrows indicate changes between the current week and the previous week. Additional information on the arrows can be found at the bottom of this page.
A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.1
Additional information on the current and previous influenza seasons for each surveillance component are available on FluView Interactive.
Key Points
• Seasonal influenza activity remains low nationally
• Two human infections with influenza A(H3N2) variant viruses were reported by the Minnesota Department of Health.
• CDC estimates that there have been at least 35 million illnesses, 400,000 hospitalizations, and 25,000 deaths from flu so far this season.
• There are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for higher risk patients.1
• Seasonal flu viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and RSV activity on a weekly basis.
U.S. virologic surveillance
Nationally, the percentage of respiratory specimens testing positive for influenza in clinical laboratories remained stable (change of ≤0.5 percentage points) compared to the previous week. Nationally, influenza A(H1N1)pdm09, A(H3N2), and B/Victoria viruses are all co-circulating. However, the distribution of circulating viruses varies by region. For regional and state level data and age group distribution, please visit FluView Interactive.
Clinical Laboratories
The results of tests performed by clinical laboratories nationwide are summarized below. Data from clinical laboratories (the percentage of specimens tested that are positive for influenza virus) are used to monitor whether influenza activity is increasing or decreasing.
| Week 37 | Data Cumulative since October 1, 2023 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 47,214 | 4,043,470 |
| No. of positive specimens (%) | 245 (0.5%) | 352,287 (8.7%) |
| Positive specimens by type | ||
| Influenza A | 224 (91.4%) | 243,734 (69.2%) |
| Influenza B | 21 (8.6%) | 108,542 (30.8%) |
Public Health Laboratories
The results of tests performed by public health laboratories nationwide are summarized below. Data from public health laboratories are used to monitor the proportion of circulating influenza viruses that belong to each influenza subtype/lineage.
| Week 37 |
Data Cumulative since October 1, 2023 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 1,061 | 132,608 |
| No. of positive specimens | 55 | 40,249 |
| Positive specimens by type/subtype | ||
| Influenza A | 54 (98.2%) | 31,012 (77.1%) |
| Subtyping Performed | 49 (90.7%) | 26,211 (84.5%) |
| (H1N1)pdm09 | 25 (51.0%) | 17,029 (65.0%) |
| H3N2 | 24 (49.0%) | 9,166 (35.0%) |
| H3N2v | 0 | 3 (<0.1%) |
| H5* | 0 | 13* (<0.1%) |
| Subtyping not performed | 5 (9.3%) | 4,801 (15.5%) |
| Influenza B | 1 (1.8%) | 9,237 (22.9%) |
| Lineage testing performed | 1 (100.0%) | 8,035 (87.0%) |
| Yamagata lineage | 0 | 0 |
| Victoria lineage | 1 (100.0%) | 8,035 (100.0%) |
| Lineage not performed | 0 | 1,202 (13.0%) |
*This data reflects specimens tested and the number determined to be positive for influenza viruses at the public health labs (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. The guidance for influenza A/H5 testing recommends testing both a conjunctival and respiratory swab for people with conjunctivitis which has resulted in more specimens testing positive for influenza A/H5 than the number of human H5 cases. For more information on the number of people infected with A/H5, please visit the "How CDC is monitoring influenza data among people to better understand the current avian influenza A (H5N1) situation"
This graph reflects the number of specimens tested and the number determined to be positive for influenza viruses at the public health lab (specimens tested is not the same as cases). It does not reflect specimens tested only at CDC and could include more than one specimen tested per person. Specimens tested as part of routine influenza surveillance as well as those tested as part of targeted testing for people exposed to influenza A(H5) are included.
Additional virologic surveillance information for current and past seasons:
Novel Influenza A Virus
Two human infections with novel influenza A viruses were reported by the Minnesota Department of Health. The patients were each infected with an influenza A(H3N2) variant (A(H3N2)v) virus.
Both patients are <18 years of age, sought healthcare during the week ending September 7, 2024 (Week 36), were not hospitalized, and have recovered from their illnesses. An investigation by state public health officials found that both patients, who are not contacts with one another, attended the same agricultural fair prior to their illness onset, where the first child had indirect swine contact and the second child had direct swine contact.
During the investigation into the first patient, public health officials identified a separate, and likely unrelated illness had occurred in the household. The day after attending the agricultural fair, all household members developed symptoms. One household member of the first patient was tested and was positive for SARS-CoV-2. All household members, including the case, recovered from their illness. Subsequently, the case patient developed new symptoms and tested positive for influenza A, which was further identified as A(H3N2)v. Other household members remained well. During the investigation into the second case, no illness was identified among contacts. No human-to-human transmission of influenza A(H3N2)v associated with either case was identified.
Including these reports, nine variant influenza virus cases have been reported during the 2023-2024 season (four A(H1N2)v, four A(H3N2)v, and one A(H1N1)v virus).
When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a "variant" influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.
Fourteen cases of human infection with influenza A (H5) virus have also been reported in the United States during the 2023-2024 season, for a total of 23 novel influenza A virus cases reported this season. Four of these influenza A (H5) virus infections occurred in individuals working with dairy cows, nine in individuals associated with poultry depopulation and disposal, and one in an individual with an unknown source of exposure. An ongoing outbreak of H5N1 continues in domestic dairy cows and poultry, and monitoring for additional human cases is ongoing.
Additional information on influenza in swine, variant influenza virus infection in humans, and guidance to interact safely with swine can be found at Swine Flu.
Additional information regarding human infections with novel influenza A viruses:
Influenza Virus Characterization
CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories according to the Right Size Roadmap submission guidance. These data are used to compare how similar the currently circulating influenza viruses are to the reference viruses representing viruses contained in the current influenza vaccines. The data are also used to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. CDC also tests susceptibility of circulating influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.
CDC has genetically characterized 5,444 influenza viruses collected since October 1, 2023.
| Virus Subtype or Lineage | Genetic Characterization | ||||
|---|---|---|---|---|---|
| Total No. of Subtype/Lineage Tested |
HA Clade |
Number (% of subtype/lineage tested) |
HA Subclade |
Number (% of subtype/lineage tested) |
|
| A/H1 | 2,008 | ||||
| 6B.1A.5a | 2,008 (100%) | 2a | 508 (25.3%) | ||
| 2a.1 | 1,500 (74.7%) | ||||
| A/H3 | 1,949 | ||||
| 3C.2a1b.2a | 1,949 (100%) | 2a.1b | 1 (0.1%) | ||
| 2a.3a | 1 (0.1%) | ||||
| 2a.3a.1 | 1,946 (99.8%) | ||||
| 2b | 1 (0.1%) | ||||
| B/Victoria | 1,487 | ||||
| V1A | 1,487 (100%) | 3a.2 | 1,487 (100%) | ||
| B/Yamagata | 0 | ||||
| Y3 | 0 | Y3 | 0 | ||
CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, H3N2, B/Victoria, and B/Yamagata viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2023-2024 Northern Hemisphere recommended cell or recombinant-based vaccine viruses. Antigenic differences between viruses are determined by comparing how well the antibodies made against the vaccine reference viruses recognize the circulating viruses that have been grown in cell culture. Ferret antisera are useful because antibodies raised against a particular virus can often recognize small changes in the surface proteins of other viruses. In HI assays, viruses with similar antigenic properties have antibody titer differences of less than or equal to 4-fold when compared to the reference (vaccine) virus. In HINT, viruses with similar antigenic properties have antibody neutralization titer differences of less than or equal to 8-fold. Viruses selected for antigenic characterization are a subset representing the genetic changes in the surface proteins seen in genetically characterized viruses.
Influenza A Viruses
- A (H1N1)pdm09: 582 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 581 (99.8%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
- A (H3N2): 677 A(H3N2) viruses were antigenically characterized by HI or HINT, and 641 (94.7%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer in HI or reacting at titers that were less than or equal to 8-fold of the homologous virus in HINT) by ferret antisera to cell-grown A/Darwin/6/2021-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.
Influenza B Viruses
- B/Victoria: 454 influenza B/Victoria-lineage virus were antigenically characterized by HI, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Austria/1359417/2021-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines.
- B/Yamagata: No influenza B/Yamagata-lineage viruses were available for antigenic characterization.
Assessment of Virus Susceptibility to Antiviral Medications
CDC assesses susceptibility of influenza viruses to the antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir using next generation sequence analysis supplemented by laboratory assays. Information about antiviral susceptibility test methods can be found at U.S. Influenza Surveillance: Purpose and Methods.
Viruses collected in the U.S. since October 1, 2023, were tested for antiviral susceptibility as follows:
| Antiviral Medication | Total Viruses | A/H1 | A/H3 | B/Victoria | ||
|---|---|---|---|---|---|---|
| Neuraminidase Inhibitors | Oseltamivir | Viruses Tested | 5,366 | 1,989 | 1,923 | 1,454 |
| Reduced Inhibition | 1 (0.02%) | 1 (0.1%) | 0 | 0 | ||
| Highly Reduced Inhibition | 6 (0.1%) | 6 (0.3%) | 0 | 0 | ||
| Peramivir | Viruses Tested | 5,366 | 1,989 | 1,923 | 1,454 | |
| Reduced Inhibition | 3 (0.1%) | 0 | 0 | 3 (0.2%) | ||
| Highly Reduced Inhibition | 7 (0.1%) | 6 (0.3%) | 0 | 1 (0.1%) | ||
| Zanamivir | Viruses Tested | 5,366 | 1,989 | 1,923 | 1,454 | |
| Reduced Inhibition | 1 (0.02%) | 0 | 0 | 1 (0.07%) | ||
| Highly Reduced Inhibition | 0 | 0 | 0 | 0 | ||
| PA Cap-Dependent Endonuclease Inhibitor | Baloxavir | Viruses Tested | 5,280 | 1,936 | 1,901 | 1,443 |
| Decreased Susceptibility | 1 (0.02%) | 0 | 1 (0.1%) | 0 | ||
Five A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution and one A(H1N1)pdm09 virus had NA-H275Y/H, conferring highly reduced inhibition by oseltamivir and peramivir. One (H1N1)pdm09 virus had NA-I223V and NA-S247N amino acid substitutions and showed reduced inhibition by oseltamivir. Two B viruses had NA-A245G amino acid substitution and showed reduced inhibition by peramivir. One B virus had NA-D197N amino acid substitution and showed reduced inhibition by zanamivir and peramivir. One B virus had NA-H273Y amino acid substitution and showed highly reduced inhibition by peramivir. One A(H3N2) virus had PA-I38T amino acid substitution and showed reduced susceptibility to baloxavir.
High levels of resistance to the adamantanes (amantadine and rimantadine) persist among influenza A(H1N1)pdm09 and influenza A(H3N2) viruses (the adamantanes are not effective against influenza B viruses). Therefore, use of these antivirals for treatment and prevention of influenza A virus infection is not recommended and data from adamantane resistance testing are not presented.
Outpatient respiratory illness surveillance
The U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) monitors outpatient visits for respiratory illness referred to as influenza-like illness [ILI (fever plus cough or sore throat)], not laboratory-confirmed influenza and will therefore capture respiratory illness visits due to infection with pathogens that can present with similar symptoms, including influenza viruses, SARS-CoV-2, and RSV. It is important to evaluate syndromic surveillance data, including that from ILINet, in the context of other sources of surveillance data to obtain a more complete and accurate picture of influenza, SARS-CoV-2, and other respiratory virus virus activity. CDC is providing integrated information about COVID-19, influenza, and RSV activity on a website that is updated weekly. Information about other respiratory virus activity can be found on CDC's National Respiratory and Enteric Virus Surveillance System (NREVSS) website.
Outpatient respiratory illness visits
Nationally, the percentage of visits for respiratory illness that were reported through ILINet remained stable (change of ≤ 0.1 percentage points) compared to the previous week and remains below the national baseline. All 10 regions are below their region-specific baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.
Outpatient respiratory illness visits by age group
About 70% of ILINet participants provide both the number of patient visits for respiratory illness and the total number of patient visits for the week broken out by age group. Data from this subset of providers are used to calculate the percentages of patient visits for respiratory illness by age group.
The percentage of visits for respiratory illness reported in ILINet decreased in the 5-24 years age group and remained stable in all other age groups in Week 37 compared to Week 36.
Outpatient respiratory illness activity map
Data collected in ILINet are used to produce a measure of ILI activity* by state/jurisdiction and Core Based Statistical Areas (CBSA).
| Activity Level | Number of Jurisdictions | Number of CBSAs | ||
|---|---|---|---|---|
| Week 37 (Week ending Sep. 14, 2024) |
Week 36 (Week ending Sep. 7, 2024) |
Week 37 (Week ending Sep. 14, 2024) |
Week 36 (Week ending Sep. 7, 2024) |
|
| Very High | 0 | 0 | 0 | 0 |
| High | 0 | 0 | 3 | 2 |
| Moderate | 0 | 0 | 5 | 9 |
| Low | 2 | 1 | 42 | 35 |
| Minimal | 52 | 54 | 611 | 629 |
| Insufficient Data | 1 | 0 | 268 | 254 |
*Data collected in ILINet may disproportionally represent certain populations within a jurisdiction or CBSA, and therefore, may not accurately depict the full picture of influenza activity for the entire jurisdiction or CBSA. Differences in the data presented here by CDC and independently by some health departments likely represent differing levels of data completeness with data presented by the health department likely being the more complete.
Additional information about medically attended visits for ILI for current and past seasons:
Hospitalization surveillance
FluSurv-Net
The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in 14 states and represents approximately 9% of the U.S. population. FluSurv-NET hospitalization data are preliminary. As data are received each week, prior case counts and rates are updated accordingly.
A total of 25,441 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and September 14, 2024. The weekly hospitalization rate observed in Week 37 was 0.0 per 100,000 population. The peak weekly hospitalization rate observed this season was 8.9 per 100,000 population and occurred during Week 52.
Among 25,441 hospitalizations, 21,535 (84.6%) were associated with influenza A virus, 3,773 (14.8%) with influenza B virus, 55 (0.2%) with influenza A virus and influenza B virus co-infection, and 78 (0.3%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 4,485 (67.5%) were A(H1N1) pdm09 and 2,158 (32.5%) were A(H3N2).
**In this figure, weekly rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospital admissions are subject to reporting delays and are shown as a dashed line for the current season. As hospitalization data are received each week, prior case counts and rates are updated accordingly.
Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
National healthcare safety network (NHSN) hospitalization surveillance
Effective May 1, 2024, hospitals are no longer required to report hospital admissions, hospital capacity, or hospital occupancy data to HHS through NHSN. Voluntarily reported NHSN hospital data can found at Weekly United States Hospitalization Metrics by Jurisdiction.
Additional NHSN Hospitalization Surveillance information:
Mortality surveillance
Based on NCHS mortality surveillance data available on September 19, 2024, the percentage of deaths that were due to influenza remained stable (<0.1 percentage point change) compared to the previous week. The data presented are preliminary and may change as more data are received and processed.
Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Influenza-Associated Pediatric Mortality
No influenza-associated pediatric deaths were reported to CDC during Week 37.
A total of 199 influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.
Additional pediatric mortality surveillance information for current and past seasons:
Additional National and International Influenza Surveillance Information
Increasing: 
Decreasing: 
Stable: 
Clinical Labs: Up or down arrows indicate a change of greater than or equal to 0.5 percentage points in the percent of specimens positive for influenza compared to the previous week.
Outpatient Respiratory Illness (ILINet): Up or down arrows indicate a change of greater than 0.1 percentage points in the percent of visits due to respiratory illness (ILI) compared to the previous week.
NHSN Hospitalizations: Up or down arrows indicate change of greater than or equal to 5% of the number of patients admitted with laboratory-confirmed influenza compared to the previous week.
NCHS Mortality: Up or down arrows indicate change of greater than 0.1 percentage points of the percent of deaths due to influenza compared to the previous week.
FluView Interactive: FluView includes enhanced web-based interactive applications that can provide dynamic visuals of the influenza data collected and analyzed by CDC. These FluView Interactive applications allow people to create customized, visual interpretations of influenza data, as well as make comparisons across flu seasons, regions, age groups and a variety of other demographics.
National Institute for Occupational Safety and Health: Monthly surveillance data on the prevalence of health-related workplace absenteeism among full-time workers in the United States are available from NIOSH.
U.S. State and local influenza surveillance: Select a jurisdiction below to access the latest local influenza information.
World Health Organization:
Additional influenza surveillance information from participating WHO member nations is available through FluNet and the Global Epidemiology Reports.
WHO Collaborating Centers for Influenza:
Australia, China, Japan, the United Kingdom, and the United States (CDC in Atlanta, Georgia)
Europe:
The most up-to-date influenza information from Europe is available from WHO/Europe and the European Centre for Disease Prevention and Control.
Public Health Agency of Canada:
The most up-to-date influenza information from Canada is available in Canada's weekly FluWatch report.
Public Health England:
The most up-to-date influenza information from the United Kingdom is available from Public Health England.
Any links provided to non-Federal organizations are provided solely as a service to our users. These links do not constitute an endorsement of these organizations or their programs by CDC or the Federal Government, and none should be inferred. CDC is not responsible for the content of the individual organization web pages found at these links.
A description of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component is available on the surveillance methods page.
- U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.
- Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm.