FluView Summary ending on October 14, 2023
Updated October 20, 2023
Seasonal influenza activity remains low nationally although small increases were reported in some parts of the country.
Viruses
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.
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, but there are slight increases in some parts of the country.
- Nationally, outpatient respiratory illness is below baseline1, and all 10 HHS regions are below their respective baselines.
- The number of flu hospital admissions remains low.
- During week 41, of the 159 viruses reported by public health laboratories, 124 (78.0%) were influenza A, and 35 (22.0%) were influenza B. Of the 99 influenza A viruses subtyped during week 41,93 (93.9%) were influenza A(H1N1), and 6 (6.1%) were A(H3N2).
- One influenza-associated pediatric death that occurred during the 2022-2023 season was reported this week.
- CDC recommends that everyone ages 6 months and older get an annual flu vaccine, ideally by the end of October.2
- There are also prescription flu antiviral drugs that can be used to treat flu illness; those need to be started as early as possible.3
- Influenza virus is one of several viruses that contribute to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, influenza, and RSV activity on a weekly basis.
U.S. Virologic Surveillance
Nationally, the percentage of specimens testing positive for influenza in clinical laboratories remained stable (change of <0.5 percentage points) compared to the previous week. In Region 8, this week’s percentage increased compared to the previous week, and in all other regions this week’s percentage remained stable. For regional and state level data and age group distribution, please visit FluView Interactive. Viruses known to be associated with recent live attenuated influenza vaccine (LAIV) receipt or found upon further testing to be a vaccine virus are not included, as they are not circulating influenza viruses.
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 41 |
Data Cumulative since October 1, 2023 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 49,312 | 108,688 |
| No. of positive specimens (%) | 661 (1.3%) | 1,294 (1.2%) |
| Positive specimens by type | ||
| Influenza A | 457 (69.1%) | 853 (65.9%) |
| Influenza B | 204 (30.9%) | 441 (34.1%) |
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 41 |
Data Cumulative since October 1, 2023 (Week 40) |
|
|---|---|---|
| No. of specimens tested | 2,374 | 5,105 |
| No. of positive specimens | 159 | 343 |
| Positive specimens by type/subtype | ||
| Influenza A | 124 (78.0%) | 268 (78.1%) |
| Subtyping Performed | 99 (79.8%) | 233 (86.9%) |
| (H1N1)pdm09 | 93 (93.9%) | 214 (91.8%) |
| H3N2 | 6 (6.1%) | 19 (8.2%) |
| H3N2v | 0 (0%) | 0 (0%) |
| Subtyping not performed | 25 (20.2%) | 35 (13.1%) |
| Influenza B | 35 (22.0%) | 75 (21.9%) |
| Lineage testing performed | 26 (74.3%) | 56 (74.7%) |
| Yamagata lineage | 0 (0%) | 0 (0%) |
| Victoria lineage | 26 (100%) | 56 (100%) |
| Lineage not performed | 9 (25.7%) | 19 (25.3%) |
Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data
Influenza Virus Characterization
CDC performs genetic and antigenic characterization of viruses submitted from U.S. 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 477 influenza viruses collected since May 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 | 234 | ||||
| 6B.1A.5a | 234 (100%) | 2a | 83 (35.5%) | ||
| 2a.1 | 151 (64.5%) | ||||
| A/H3 | 35 | ||||
| 3C.2a1b.2a | 35 (100%) | 2a.3a | 4 (11.4%) | ||
| 2a.3a.1 | 30 (85.7%) | ||||
| 2b | 1 (2.9%) | ||||
| B/Victoria | 208 | ||||
| V1A | 208 (100%) | 3a.2 | 208 (100%) | ||
| B/Yamagata | 0 | ||||
| Y3 | 0 | Y3 | 0 (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: Sixty-eight A(H1N1)pdm09 viruses 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 A/Wisconsin/67/2022-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant-based influenza vaccines.
- A (H3N2): Fourteen A(H3N2) viruses were antigenically characterized by HI or HINT, and all were well-recognized (reacting at titers that were within 4-fold of the homologous virus) 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: Forty-five influenza B/Victoria-lineage viruses 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.
Antiviral susceptibility data will be reported later this season, when a sufficient number of viruses has been tested.
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 any pathogen that can present with similar symptoms, including influenza virus, 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 complete and accurate picture of influenza, SARS-CoV-2, and other respiratory 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
Nationwide during week 41, 2.3% of patient visits reported through ILINet were due to respiratory illness that included fever plus a cough or sore throat, also referred to as ILI. This has remained stable (change of ≤ 0.1 percentage points) compared to week 40 and is below the national baseline of 2.9%. All 10 HHS regions are below their respective baselines. Region 8 increased, Region 2 decreased, and all other regions remained stable compared to week 40. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.
* Effective October 3, 2021 (week 40), the ILI definition (fever plus cough or sore throat) no longer includes “without a known cause other than influenza.”
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 increased for the 0-4 years and 25-49 years age groups and remained stable (change of ≤ 0.1 percentage point) for all other age groups (4-24 years, 50-64 years, and 65+ years) in week 41 compared to week 40.
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 41 (Week ending Oct. 14, 2023) |
Week 40 (Week ending Oct. 7, 2023) |
Week 41 (Week ending Oct. 14, 2023) |
Week 40 (Week ending Oct. 7, 2023) |
|
| Very High | 0 | 0 | 0 | 1 |
| High | 2 | 1 | 7 | 3 |
| Moderate | 0 | 2 | 14 | 11 |
| Low | 4 | 6 | 58 | 57 |
| Minimal | 49 | 46 | 615 | 632 |
| Insufficient Data | 0 | 0 | 235 | 225 |
*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:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or ILI Activity Map
Hospitalization Surveillance
FluSurv-NET
The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in selected 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 103 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and October 14, 2023. The weekly hospitalization rate observed in week 41 was 0.2 per 100,000 population. The overall cumulative hospitalization rate was 0.3 per 100,000 population.
When examining rates by age, the cumulative hospitalization rate per 100,000 population among adults aged 18 years and older was 0.4, while among children aged 0-17 years, the cumulative hospitalization rate per 100,000 population was 0.2.
In this figure, cumulative rates for all seasons prior to the 2023-2024 season reflect end-of-season rates. For the 2023-2024 season, rates for recent hospitals admissions are subject to reporting delays. As hospitalization data are reviewed 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:
Surveillance Methods |FluView Interactive: Rates by Age, Sex, and Race/Ethnicity or Data on Patient Characteristics | RESP-NET Interactive
National Healthcare Safety Network (NHSN) Hospitalization Surveillance
Hospitals report to NHSN the number of patients admitted with laboratory-confirmed influenza. During week 41, 1,228 patients with laboratory-confirmed influenza were admitted to a hospital. The number of patients admitted to a hospital with laboratory-confirmed influenza slightly increased compared to week 40 (change of >5%). Regions 5, 6, 8, and 9 slightly increased and all other regions remained stable or decreased.
Additional NHSN Hospitalization Surveillance information:
Surveillance Methods | Additional Data | FluView Interactive
Mortality Surveillance
National Center for Health Statistics (NCHS) Mortality Surveillance
Based on NCHS mortality surveillance data available on October 19, 2023, 0.05% of the deaths that occurred during the week ending October 14, 2023 (week 41), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to week 40. The data presented are preliminary and may change as more data are received and processed.
The percentages of deaths due to pneumonia and influenza (P&I) and due to pneumonia, influenza, or COVID-19 (PIC) will no longer be displayed in FluView but are available in FluView Interactive.
Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive
Influenza-Associated Pediatric Mortality
No influenza-associated pediatric deaths occurring during the 2023-2024 season have been reported to CDC.
One influenza-associated pediatric death occurring during the 2022-2023 season was reported to CDC during week 41. The death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 39 (the week ending September 30, 2023). A total of 179 influenza-associated pediatric deaths that occurred during the 2022-2023 season have been reported to CDC.
Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive
Trend Indicators
Increasing: 
Decreasing: 
Stable: 
Indicators Status by System
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 0.3 percentage points of the percent of deaths due to PIC compared to the previous week.
Reference Footnotes
1U.S. Influenza Surveillance: Purpose and Methods (2023 Oct). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/overview.htm#ILINet.
2Grohskopf LA, Blanton LH, Ferdinands JM, Chung JR, Broder KR, Talbot HK. Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season. MMWR Recomm Rep 2023;72(No. RR-2):1–25. DOI: http://dx.doi.org/10.15585/mmwr.rr7202a1
3Influenza Antiviral Medications: Summary for Clinicians (2023 Sept). Centers for Disease Control and Prevention. https://www.cdc.gov/flu/professionals/antivirals/summary-clinicians.htm.
Additional National and International Influenza Surveillance Information
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.
