Weekly US Influenza Surveillance Report: Key Updates for Week 52, ending December 28, 2024

What to know

Seasonal influenza activity continues to increase and is elevated across most of the country.

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 continues to increase and is elevated across most of the country.

• Nationally, percent positivity for influenza, the percentage of emergency department visits for influenza, and influenza-associated hospitalizations increased this week compared to last week.

• During Week 52, of the 1,254 viruses reported by public health laboratories, 1,234 were influenza A and 20 were influenza B. Of the 823 influenza A viruses subtyped during Week 52, 335 (40.7%) were influenza A(H1N1)pdm09 and 488 (59.3%) were A(H3N2).

• Outpatient respiratory illness is above baseline nationally for the fifth consecutive week and is above baseline in all 10 HHS regions.

• No influenza A(H5) cases were reported to CDC this week. To date, human-to-human transmission of influenza A(H5) virus has not been identified in the United States.

• Two pediatric deaths associated with seasonal influenza virus infection were reported this week, bringing the 2024-2025 season total to 11 pediatric deaths.

• CDC estimates that there have been at least 5.3 million illnesses, 63,000 hospitalizations, and 2,700 deaths from flu so far this season.

• CDC recommends that everyone ages 6 months and older get an annual influenza (flu) vaccine.1

• There are prescription flu antiviral drugs that can treat flu illness; those should be started as early as possible and are especially important for patients at higher risk for severe illness.2

• Influenza viruses are among several viruses contributing to respiratory disease activity. CDC is providing updated, integrated information about COVID-19, flu, and respiratory syncytial virus (RSV) activity on a weekly basis.

U.S. virologic surveillance

Nationally, the percentage of respiratory specimens testing positive for influenza virus in clinical laboratories increased (change of ≥ 0.5 percentage points) compared to the previous week. The regions with the highest percent positivity were regions 8 (28.1), 9 (32.7), and 10 (26.0). Influenza A(H1N1)pdm09 and A(H3N2) were the predominant viruses reported this week. 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 52 Data Cumulative since
September 29, 2024
(Week 40)
No. of specimens tested 63,682 1,047,154
No. of positive specimens (%) 11,912 (18.7%) 52,596 (5.0%)
Positive specimens by type
Influenza A 11,674 (98.0%) 50,497 (96.0%)
Influenza B 238 (2.0%) 2,099 (4.0%)

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 52
Data Cumulative since
September 29, 2024
(Week 40)
No. of specimens tested 1,731 25,214
No. of positive specimens 1,254 11,292
Positive specimens by type/subtype    
         Influenza A 1,234 (98.4%) 10,906 (96.6%)
Subtyping Performed 823 (66.7%) 9,374 (86.0%)
            (H1N1)pdm09 335 (40.7%) 4,137 (44.2%)
             H3N2 488 (59.3%) 5,160 (55.0%)
             H3N2v 0 0
             H5* 0 77 (0.8%)
Subtyping not performed 411 (33.3%) 1,532 (14.0%)
        Influenza B 20 (1.6%) 386 (3.4%)
Lineage testing performed 2 (10.0%) 183 (47.4%)
            Yamagata lineage 0 0
            Victoria lineage 2 (100.0%) 183 (100.0%)
Lineage not performed 18 (90.0%) 203 (52.6%)

*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"

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 human-to-human.

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

No cases of influenza A(H5) were reported to CDC this week. To date, human-to-human transmission of influenza A(H5) virus has not been identified in the United States.

The CSTE position statement, which includes updated case definitions for confirmed, probable, and suspected cases is available at http://www.cste.org/resource/resmgr/position_statements_files_2023/24-ID-09_Novel_Influenza_A.pdf

An up-to-date human case summary during the 2024 outbreak by state and exposure source is available at www.cdc.gov/bird-flu/situation-summary/index.html

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/bird-flu/prevention/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 United States 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:

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 relative to the reference viruses representing 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 polymerase acidic protein (PA) endonuclease inhibitor baloxavir. The HA clade and subclades were assigned using Nextclade (https://clades.nextstrain.org).

CDC has genetically characterized 698 influenza viruses collected since September 29, 2024.

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 270
5a.2a 149 (55.2%) C.1.9 149 (55.2%)
5a.2a.1 121 (44.8%) D 4 (1.5%)
D.1 1 (0.4%)
D.3 13 (4.8%)
D.5 103 (38.1%)
A/H3 375
2a.3a 5 (1.3%) G.1.3.1 5 (1.3%)
2a.3a.1 370 (98.7%) J.1 1 (0.3%)
J.1.1 3 (0.8%)
J.2 334 (89.1%)
J.2.1 7 (1.9%)
J.2.2 25 (6.7%)
B/Victoria 53
3a.2 53 (100%) C.3 1 (1.9%)
C.5 6 (11.3%)
C.5.1 32 (60.4%)
C.5.6 8 (15.1%)
C.5.7 6 (11.3%)
B/Yamagata 0
Y3 0 Y3 0

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) assay (H1N1pdm09, H3N2, and B/Victoria viruses) or neutralization-based HINT (H3N2 viruses) using antisera that ferrets make after being infected with reference viruses representing the 2024-2025 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 of the recent genetically characterized viruses and are chosen based on the genetic changes in their surface proteins and may not be proportional to the number of such viruses circulating in the United States.

Influenza A Viruses

  • A (H1N1)pdm09: 40 A(H1N1)pdm09 viruses were antigenically characterized by HI, and 40 (100%) 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): 34 A(H3N2) viruses were antigenically characterized by HI or HINT, and 19 (55.9%) 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/Massachusetts/18/2022-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines.

Influenza B Viruses

  • B/Victoria: 7 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 September 29, 2024, 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 638 249 345 44
Reduced Inhibition 1 (0.2%) 1 (0.4%) 0 0
Highly Reduced Inhibition 0 0 0 0
Peramivir Viruses Tested 638 249 345 44
Reduced Inhibition 0 0 0 0
Highly Reduced Inhibition 0 0 0 0
Zanamivir Viruses Tested 638 249 345 44
Reduced Inhibition 0 0 0 0
Highly Reduced Inhibition 0 0 0 0
PA Cap-Dependent Endonuclease Inhibitor Baloxavir Viruses Tested 635 229 360 46
Decreased Susceptibility 0 0 0 0

One A(H1N1)pdm09 virus had NA-I223V and NA-S247N amino acid substitutions and showed reduced inhibition by oseltamivir.

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 and Emergency Department Illness Surveillance

Outpatient respiratory illness visits

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.

Nationally, during Week 52, 6.8% 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 week's percentage increased (change of > 0.1 percentage points) compared to Week 51 and is above the national baseline of 3.0% for the fifth consecutive week. The percentage of visits for ILI increased (change of > 0.1 percentage points) in all HHS regions compared to Week 51. All HHS regions are above their respective baselines. Multiple respiratory viruses are co-circulating, and the relative contribution of influenza virus infections 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. Based on these data, the percentage of visits for respiratory illness increased (change of > 0.1 percentage point) in all age groups in Week 52 compared to Week 51.

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 52
(Week ending
Dec. 28, 2024)
Week 51
(Week ending
Dec. 21, 2024)
Week 52
(Week ending
Dec. 28, 2024)
Week 51
(Week ending
Dec. 21, 2024)
Very High 25 9 76 22
High 17 20 192 143
Moderate 6 7 133 118
Low 3 9 157 158
Minimal 3 9 131 256
Insufficient Data 1 1 240 232

*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:

National Syndromic Surveillance System (NSSP)

The overall percentage of emergency department (ED) visits with a discharge diagnosis of influenza reported in NSSP was 5.2% during Week 52, an increase (change of > 0.1 percentage points) from 3.2% the previous week. In all 10 HHS regions and among all age groups, the percentage of ED visits for influenza increased (change of > 0.1 percentage points) in Week 52 compared to the previous week.

Additional information about emergency department visits for flu 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 4,049 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2024, and December 28, 2024. The weekly hospitalization rate observed in Week 52 was 4.3 per 100,000 population. The cumulative hospitalization rate observed in Week 52 was 13.2 per 100,000 population.

When examining rates by age, the highest cumulative hospitalization rate per 100,000 population was among adults aged 65 years and older (38.6), followed by adults aged 50-64 years (15.1), children aged 0-4 years (12.2), adults aged 18-49 (6.2), and children aged 5-17 (4.8).

When examining age-adjusted rates by race and ethnicity, the highest cumulative hospitalization rate per 100,000 population was among non-Hispanic Black persons (20.5), followed by American Indian/Alaska Native persons (19.7), Hispanic persons (13.7), non-Hispanic White persons (10.6), and Asian/Pacific Islander persons (9.3).

**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) Hospital Respiratory Data

Hospitals report to NHSN the weekly number of patients with laboratory-confirmed influenza who were admitted to the hospital. Nationally, during Week 52, 26,088 laboratory confirmed influenza-associated hospitalizations were reported. This is an increase (change of >5%) compared to Week 51, when 14,755 hospitalizations were reported.

The weekly hospital admission rate observed in Week 52 was 7.8 per 100,000. The weekly rate of hospital admissions in the 10 HHS regions ranged from 3.6 (Region 1) to 11.1 (Region 9) and increased in all regions.

When examining rates by age for Week 52, all age groups increased this week (change of >5%) compared to the previous week. The highest hospital admission rate per 100,000 population was among those 75+ years (34.2), followed by 65-to-74-year age group (15.4), and 50-to-64-year age group (8.3).

NHSN Week 52
NHSN Week 52

Additional NHSN Hospital Respiratory Data information:

Mortality surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on January 2, 2025, 0.5% of the deaths that occurred during the week ending December 28, 2024 (Week 52), were due to influenza. This percentage remained stable (≤ 0.1 percentage point change) compared to Week 51. 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

Two influenza-associated pediatric deaths occurring during the 2024-2025 season were reported to CDC during week 52. One death was associated with an influenza B virus with no lineage determined and occurred during week 51 (the week ending December 21, 2024). The other death was associated with an influenza A virus with no subtyping performed and occurred during week 52 (the week ending December 28, 2024).

A total of 11 influenza-associated pediatric deaths occurring during the 2024-2025 season have been reported to CDC.

Additional pediatric mortality surveillance information for current and past seasons:

Additional National and International Influenza Surveillance Information

Indicators Status by System

IncreasingIncreasing
DecreasingDecreasing
StableStable

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.

Additional 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.