FluView Summary ending on May 18, 2024

Updated May 24, 2024

FluView a weekly influenza surveillance report prepared by the Influenza Division
Key Updates for Week 20, ending May 18, 2024

Seasonal influenza activity is low nationally.

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.

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.
  • Nationally, outpatient respiratory illness remained stable and is below baseline for the seventh week in a row.1 All 10 HHS regions are below their region-specific baselines.
  • One human infection with an influenza A(H5N1) virus was reported by the Michigan Department of Health.
  • During Week 20, of the 64 viruses reported by public health laboratories, 49 (76.6%) were influenza A and 15 (23.4%) were influenza B. Of the 38 influenza A viruses subtyped during Week 20, 15 (39.5%) were influenza A(H1N1)pdm09 and 23 (60.5%) were A(H3N2).
  • Two influenza-associated pediatric deaths were reported to CDC during Week 20, bringing the season total to 169 pediatric deaths.
  • CDC estimates that there have been at least 35 million illnesses, 390,000 hospitalizations, and 25,000 deaths from flu so far this season.
  • CDC recommends that everyone 6 months and older get an annual flu vaccine as long as flu activity continues.2
  • 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
  • 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. Regions 1, 2, 7, and 9 decreased in percent positivity while regions 3, 4, 5, 6, 8, and 10 remained stable during Week 20 compared to Week 19. The regions with the highest percent positivity were regions 10 (4.8%), 9 (2.9%), and 8 (2.0%). 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 20
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 35,946 3,234,435
No. of positive specimens (%) 731 (2.0%) 344,458 (10.6%)
Positive specimens by type
Influenza A 462 (63.2%) 237,465 (68.9%)
Influenza B 269 (36.8%) 106,982 (31.1%)

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.

results of tests from Public Health Laboratories
Week 20
Data Cumulative since
October 1, 2023
(Week 40)
No. of specimens tested 594 108,684
No. of positive specimens 64 36,220
Positive specimens by type/subtype    
         Influenza A 49 (76.6%) 27,548 (76.1%)
Subtyping Performed 38 (77.6%) 23,228 (84.3%)
            (H1N1)pdm09 15 (39.5%) 15,763 (67.9%)
             H3N2 23 (60.5%) 7,465 (32.1%)
             H3N2v 0 (0.0%) 0 (0.0%)
Subtyping not performed 11 (22.4%) 4,320 (15.7%)
        Influenza B 15 (23.4%) 8,672 (23.9%)
Lineage testing performed 5 (33.3%) 7,553 (87.1%)
            Yamagata lineage 0 (0.0%) 0 (0.0%)
            Victoria lineage 5 (100.0%) 7,553 (100.0%)
Lineage not performed 10 (66.7%) 1,119 (12.9%)

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 highly pathogenic avian influenza (HPAI) A(H5N1) virus was reported by the Michigan Department of Health and Human Services.

A patient aged >18 years in Michigan developed conjunctivitis while working at a commercial dairy farm where HPAI A(H5N1) virus had been detected in cows. The patient reported their symptom onset to the Michigan Department of Health and Human Services via a text-based symptom monitoring system. Respiratory and conjunctival swab specimens were collected from the patient. The respiratory specimen (a nasopharyngeal swab) tested negative for influenza A virus at the Michigan Public Health Laboratory using a Centers for Disease Control and Prevention (CDC) assay. Both specimens were sent to CDC for further testing, where they were received and tested on May 21, 2024. The conjunctival specimen was positive for A(H5N1) virus using diagnostic RT-PCR and sequencing, and the respiratory specimen tested negative for influenza A and A(H5) virus. The patient did not report symptoms other than eye redness, discharge, and discomfort consistent with conjunctivitis, was not hospitalized, and has fully recovered. (https://www.cdc.gov/flu/avianflu/clinicians-evaluating-patients.htm).

In response to this detection, public health officials are conducting surveillance in the area and additional case investigation activities, including contact tracing. Additional symptomatic persons among workers exposed to infected cattle at the same farm have not been identified through monitoring of workers. Additional cases of human infection with A(H5) virus associated with this case and human-to-human transmission of A(H5) virus have not been identified to date.

This is the third person to test positive for A(H5) virus in the United States. The first was reported in April 2022 in Colorado in a person who reported fatigue during culling of poultry infected with HPAI A(H5N1) virus, and the second was reported in April 2024 in a dairy farm worker with conjunctivitis in Texas. This is the second case associated with an ongoing multistate outbreak of HPAI A(H5N1) in dairy cows.

Currently in the United States, HPAI A(H5N1) virus detections among wild birds are widespread. There are outbreaks among animals including poultry, backyard flocks, and dairy cows. Sporadic infections in wild mammals also have been reported by United States Department of Agriculture (USDA) Animal Plant Health Inspection Service (APHIS).

CDC recommends that state and local public health departments monitor people who are exposed to birds or other animals (including livestock) infected or suspected to be infected with avian influenza A viruses for onset of signs and symptoms for 10 days after their last exposure and that people who develop signs or symptoms of respiratory illness and/or conjunctivitis be tested for influenza. Between February 9, 2022 and May 21, 2024, more than 9,000 people were actively monitored following HPAI exposure.

Information about avian influenza is available at https://www.cdc.gov/flu/avianflu/index.htm.

Interim recommendations for Prevention, Monitoring, and Public Health Investigations are available at https://www.cdc.gov/flu/avianflu/hpai/hpai-interim-recommendations.html.

The latest case reports on avian influenza outbreaks in wild birds, commercial poultry, backyard or hobbyist flocks, and mammals in the U.S. are available from the USDA at https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/avian/avian-influenza/2022-hpai.

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 4,540 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 1,725
6B.1A.5a 1,725 (100%) 2a 398 (23.1%)
2a.1 1,327 (76.9%)
A/H3 1,529
3C.2a1b.2a 1,529 (100%) 2a.1b 1 (0.1%)
2a.3a 1 (0.1%)
2a.3a.1 1,526 (99.8%)
2b 1 (0.1%)
B/Victoria 1,286
V1A 1,286 (100%) 3a.2 1,286 (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: 398 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): 405 A(H3N2) viruses were antigenically characterized by HI or HINT, and 394 (97.3%) 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: 281 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 1, 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 4,530 1,726 1,527 1,277
Reduced Inhibition 1 (0.02%) 1 (0.1%) 0 (0.00%) 0 (0.00%)
Highly Reduced Inhibition 2 (0.04%) 2 (0.1%) 0 (0.00%) 0 (0.00%)
Peramivir Viruses Tested 4,530 1,726 1,527 1,277
Reduced Inhibition 1 (0.02%) 0 (0.00%) 0 (0.00%) 1 (0.1%)
Highly Reduced Inhibition 2 (0.04%) 2 (0.1%) 0 (0.00%) 0 (0.00%)
Zanamivir Viruses Tested 4,530 1,726 1,527 1,277
Reduced Inhibition 0 (0.00%) 0 (0.00%) 0 (0.00%) 0 (0.00%)
Highly Reduced Inhibition 0 (0.00%) 0 (0.00%) 0 (0.00%) 0 (0.00%)
PA Cap-Dependent Endonuclease Inhibitor Baloxavir Viruses Tested 4,397 1,670 1,486 1,241
Decreased Susceptibility 1 (0.02%) 0 (0.0%) 1 (0.1%) 0 (0.0%)

Two A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution 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. One B virus had NA-A245G amino acid substitution and showed 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 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 20, 2.0% 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) since Week 19 and is below the national baseline. The percentage of visits for ILI decreased in regions 8 and 10, increased slightly in Region 2, and remained stable in all other regions in Week 20 compared to Week 19. All 10 regions are below 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 remained stable in all age groups in Week 20 compared to Week 19.

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 20
(Week ending
May 18, 2024)
Week 19
(Week ending 
May 11, 2024)
Week 20
(Week ending
May 18, 2024)
Week 19
(Week ending
May 11, 2024)
Very High 0 0 0 0
High 0 0 1 1
Moderate 2 0 3 1
Low 1 5 19 36
Minimal 52 50 644 662
Insufficient Data 0 0 262 229

 

*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 24,763 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2023, and May 18, 2024. The weekly hospitalization rate observed in Week 20 was 0.2 per 100,000 population. The peak weekly hospitalization rate observed this season was 9.0 per 100,000 population and occurred during Week 52.

Among 24,763 hospitalizations, 20,954 (84.6%) were associated with influenza A virus, 3,631 (14.7%) with influenza B virus, 50 (0.2%) with influenza A virus and influenza B virus co-infection, and 127 (0.5%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 3,671 (68.6%) were A(H1N1) pdm09 and 1,683 (31.4%) were A(H3N2).

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

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:
Surveillance Methods | Additional Data | FluView Interactive

Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on May 23, 2024, 0.1% of the deaths that occurred during the week ending May 18, 2024 (Week 20), were due to influenza. This percentage slightly decreased (≥ 0.1 percentage point change) compared to Week 19. 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

Two influenza-associated pediatric deaths occurring during the 2023-2024 season were reported to CDC during Week 20, bringing the total pediatric deaths for this season to 169. One death was associated with an influenza A virus with no subtyping performed and occurred during Week 7 (the week ending February 17, 2024). The other death was associated with an influenza A(H3) virus and occurred during Week 17 (the week ending April 27, 2024).


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.