Weekly U.S. Influenza Surveillance Report

Updated March 29, 2024

FluView a weekly influenza surveillance report prepared by the Influenza Division
Key Updates for Week 12, ending March 23, 2024

Seasonal influenza activity remains elevated nationally but is decreasing.

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 elevated but is decreasing nationally.
  • Nationally, percent positivity for both influenza A and B decreased compared to last week.
  • One human infection with an influenza A(H1N2) variant virus was reported by the Pennsylvania Department of Health.
  • Nationally, outpatient respiratory illness declined but remains above baseline.1 Regions 2, 4, 6, and 8 are below their baselines, while all other HHS regions remain at or above their region-specific baselines.
  • Nationally, the number of weekly flu hospital admissions has been decreasing since January.
  • During Week 12, of the 403 viruses reported by public health laboratories, 239 (59.3%) were influenza A and 164 (40.7%) were influenza B. Of the 172 influenza A viruses subtyped during Week 12, 76 (44.2%) were influenza A(H1N1)pdm09 and 96 (55.8%) were A(H3N2).
  • Five influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 11, bringing the season total to 126 pediatric deaths.
  • CDC estimates that there have been at least 31 million illnesses, 350,000 hospitalizations, and 22,000 deaths from flu so far this season.
  • CDC recommends that everyone 6 months and older get an annual flu vaccine as long as influenza viruses are spreading.2 Vaccination can still provide benefit this season.
  • There also 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.3
  • 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 decreased (change of >0.5 percentage points) compared to the previous week. Regions 3, 5, 6, 7, 8, and 10 reported a decrease; regions 1, 4, and 9 remained stable; and region 2 reported an increase (likely a reporting artifact) during Week 12 compared to Week 11. The regions with the highest percent positivity were regions 7 (20.1%), 5 (16.4%), and 1 (10.4%). 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. Viruses known to be associated with recent receipt of live attenuated influenza vaccine (LAIV) 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.

results of tests from Clinical Laboratories
Week 12
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 77,183 2,663,679
No. of positive specimens (%) 8,088 (10.5%) 311,936 (11.7%)
Positive specimens by type
Influenza A 4,130 (51.1%) 220,085 (70.6%)
Influenza B 3,958 (48.9%) 91,841 (29.4%)

Clinical Laboratory Data

INFLUENZA Virus Isolated
View Chart Data | View Full Screen
 
 

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.

results of tests from Public Health Laboratories
Week 12
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 1,917 93,464
No. of positive specimens 403 30,309
Positive specimens by type/subtype    
         Influenza A 239 (59.3%) 23,805 (78.5%)
Subtyping Performed 172 (72.0%) 19,659 (82.6%)
            (H1N1)pdm09 76 (44.2%) 14,150 (72.0%)
             H3N2 96 (55.8%) 5,509 (28.0%)
             H3N2v 0 (0.0%) 0 (0.0%)
Subtyping not performed 67 (28.0%) 4,146 (17.4%)
        Influenza B 164 (40.7%) 6,504 (21.5%)
Lineage testing performed 116 (70.7%) 5,517 (84.8%)
            Yamagata lineage 0 (0.0%) 0 (0.0%)
            Victoria lineage 116 (100.0%) 5,517 (100.0%)
Lineage not performed 48 (29.3%) 987 (15.2%)

Clinical Laboratory Data

INFLUENZA Virus Isolated
View Chart Data | View Full Screen


Additional virologic surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive: National, Regional, and State Data or Age Data

Novel Influenza A Virus:

A human infection with a novel influenza A virus was reported by the Pennsylvania Department of Health. The patient was infected with an influenza A(H1N2) variant (A(H1N2)v) virus. The patient is < 18 years of age, sought healthcare during the week ending March 9, 2024 (week 10), was hospitalized, and has since recovered. An investigation by local public health officials found that the patient had swine contact prior to their illness onset. Additional investigation identified mild illness in two of the patient’s close contacts who also had contact with swine, that began prior to the patient’s onset of symptoms. No person-to-person transmission of A(H1N2)v virus associated with this patient has been identified. The investigation is ongoing. This is the first human infection with a variant influenza A virus reported in the United States in 2024.

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.

Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be understood, and appropriate public health measures can be taken.

Additional information on influenza in swine, variant influenza virus infection in humans, and guidance to interact safely with swine can be found at www.cdc.gov/flu/swineflu/index.htm.

Additional information regarding human infections with novel influenza A viruses:

Surveillance Methods  |  FluView Interactive

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 3,211 influenza viruses collected since October 1, 2023.

Influenza Virus Characterization from viruses collected in the U.S. from September 29, 2019
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 1,342
6B.1A.5a 1,342 (100%) 2a 331 (24.7%)
2a.1 1,011 (75.3%)
A/H3 989
3C.2a1b.2a 989 (100%) 2a.1b 1 (0.1%)
2a.3a 1 (0.1%)
2a.3a.1 986 (99.7%)
2b 1 (0.1%)
B/Victoria 880
V1A 880 (100%) 3a.2 880 (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: 200 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): 223 A(H3N2) viruses were antigenically characterized by HI or HINT, and 221 (99.1%) 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: 149 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 | CDC.

Viruses collected in the U.S. since October 01, 2023, were tested for antiviral susceptibility as follows:

Viruses collected in the U.S. tested for antiviral susceptibility
Antiviral Medication Total Viruses A/H1 A/H3 B/Victoria
Neuraminidase Inhibitors Oseltamivir Viruses Tested 3,106 1,314 937 855
Reduced Inhibition 1 (0.03%) 1 (0.1%) 0 (0.0%) 0 (0.0%)
Highly Reduced Inhibition 2 (0.1%) 2 (0.2%) 0 (0.0%) 0 (0.0%)
Peramivir Viruses Tested 3,106 1,314 937 855
Reduced Inhibition 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%)
Highly Reduced Inhibition 2 (0.1%) 2 (0.2%) 0 (0.0%) 0 (0.0%)
Zanamivir Viruses Tested 3,106 1,314 937 855
Reduced Inhibition 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%)
Highly Reduced Inhibition 0 (0.0%) 0 (0.0%) 0 (0.0%) 0 (0.0%)
PA Cap-Dependent Endonuclease Inhibitor Baloxavir Viruses Tested 3,018 1,275 915 828
Decreased Susceptibility 1 (0.05%) 0 (0.0%) 1 (0.1%) 0 (0.0%)

Two A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution and showed 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. 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 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, during Week 12, 3.1% 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 decreased (change of >0.1 percentage points) for the fourth consecutive week but remains above the national baseline. The percentage of visits for ILI remained stable in regions 2 and 9 and decreased in all other regions in Week 12 compared to Week 11. Regions 2, 4, 6 and 8 are below their baseline, and regions 1, 3, 5, 7, 9, and 10 are at or above their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infection to ILI varies by location.

national levels of ILI and ARI

* 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.”

View Chart Data (current season only) | View Full Screen

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 0-4 years, 5-24 years, and 25-49 years age groups, and remained stable in the 50-64 years and 65+ years age groups in Week 12 compared to Week 11.

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).

ILI Activity by State/Jurisdiction and Core Based Statistical Area
Activity Level Number of Jurisdictions Number of CBSAs
Week 12
(Week ending
Mar. 23, 2024)
Week 11
(Week ending 
Mar. 16, 2024)
Week 12
(Week ending
Mar. 23, 2024)
Week 11
(Week ending
Mar. 16, 2024)
Very High 0 1 2 4
High 9 9 25 44
Moderate 4 12 53 72
Low 20 19 158 188
Minimal 22 14 463 393
Insufficient Data 0 0 228 228

 

*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 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 22,080 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and March 23, 2024. The weekly hospitalization rate observed in Week 12 was 1.4 per 100,000 population. The peak weekly hospitalization rate was observed during Week 52 and is the third highest peak weekly rate observed during all seasons going back to 2010-2011 following the 2014-2015 and 2017-2018 seasons. The overall cumulative hospitalization rate for the season was 72.2 per 100,000 population. This cumulative hospitalization rate is the second highest cumulative hospitalization rate when compared against previous end-of-season rates for Week 12, and it is the second highest cumulative in-season hospitalization rate observed in Week 12, following the 2017-2018 season (96.1). Cumulative in-season hospitalization rates observed in Week 12 from 2010-2011 through 2022-2023 ranged from 0.7 to 67.3.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (196.9), followed by adults aged 50-64 years (86.1) and children aged 0-4 years (75.3). When examining age-adjusted rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (135.0), followed by non-Hispanic American Indian or Alaska Native persons (101.5), Hispanic persons (65.6), non-Hispanic White persons (55.6), and non-Hispanic Asian/Pacific Islander persons (37.8).

Among 22,080 hospitalizations, 18,898 (85.6%) were associated with influenza A virus, 3,028 (13.7%) with influenza B virus, 45 (0.2%) with influenza A virus and influenza B virus co-infection, and 109 (0.5%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 3,173 (71.5%) were A(H1N1) pdm09 and 1,258 (28.4%) were A(H3N2).

Among 2,638 hospitalized adults with information on underlying medical conditions, 95.4% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, obesity, and metabolic disease. Among 1,276 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 22.6% were pregnant. Among 810 hospitalized children with information on underlying medical conditions, 69.0% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by obesity and neurologic disease.

In these figures, cumulative and weekly 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 weekly number of patients admitted with laboratory-confirmed influenza. During Week 12, 5,738 patients with laboratory-confirmed influenza were admitted to a hospital. Nationally and in regions 1, 2, 3, 4, 5, 6, 7, 8, and 9, the number of patients admitted to a hospital with laboratory-confirmed influenza for Week 12 decreased (change of >5%) compared to Week 11. The number of hospitalizations with laboratory-confirmed influenza increased slightly in region 10.


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 March 28, 2024, 0.5% of the deaths that occurred during the week ending March 23, 2024 (Week 12), were due to influenza. This percentage remained stable (< 0.1 percentage point change) compared to Week 11. 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:
Surveillance Methods | FluView Interactive

Influenza-Associated Pediatric Mortality

Five influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 12. The deaths occurred during week 52 of 2023 (the week ending December 30, 2023) and during weeks 2 and 10 of 2024 (the weeks ending January 13, 2024, and March 9, 2024). Three deaths were associated with influenza A viruses and two deaths were associated with influenza B viruses. Two of the influenza A viruses had subtyping performed; one was an A(H1N1) virus and one was an A(H3) virus. Neither of the influenza B viruses had lineage determined.


Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Trend Indicators

Increasing: Increasing
Decreasing: Decreasing
Stable: 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 0.1 percentage points of the percent of deaths due to influenza 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.