Weekly U.S. Influenza Surveillance Report

Updated February 3, 2023

FluView a weekly influenza surveillance report prepared by the Influenza Division

Note: CDC is also tracking the impact of other respiratory viruses, including COVID-19.  Data comparing the impact of these viruses can be found in two new dashboards: RESP-NET and NSSP’s Emergency Department Visits for COVID-19, Influenza and Respiratory Syncytial Virus.

Key Updates for Week 4, ending January 28, 2023

Seasonal influenza activity continues to decline across 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 continues to decline across the country.
  • Seven of 10 HHS regions were below their outpatient respiratory illness baselines.
  • The number and weekly rate of flu hospital admissions decreased compared to week 3.
    • Hospitals reported 2,671 influenza hospitalizations to HHS Protect during week 4 compared to 4,028 reported during week 3.
    • The weekly rate of flu hospital admissions in the FluSurv-NET declined again during week 4. However, the season’s cumulative hospitalization rate was 1.1 times higher than the highest cumulative in-season hospitalization rate observed for week 3 during previous seasons going back to 2010-2011. This in-season rate is still lower than end-of-season hospitalization rates for all but 4 pre-COVID-19-pandemic seasons going back to 2010-2011.
  • Of influenza A viruses detected and subtyped during week 4, 62% were influenza A(H3N2) and 38% were influenza A(H1N1).
  • Six influenza-associated pediatric deaths that occurred during the 2022-23 season were reported this week, for a total of 97 pediatric flu deaths reported so far this season.
  • CDC estimates that, so far this season, there have been at least 25 million illnesses, 280,000 hospitalizations, and 17,000 deaths from flu.
  • The majority of influenza viruses tested are in the same genetic subclade as and antigenically similar to the influenza viruses included in this season’s influenza vaccine.
  • All viruses collected and evaluated this season have been susceptible to the influenza antivirals peramivir, zanamivir, and baloxavir, and all viruses except for one (99.9%) have been susceptible to the influenza antiviral oseltamivir.
  • CDC continues to recommend that everyone ages 6 months and older get an annual flu vaccine as long as flu activity continues.

U.S. Virologic Surveillance

The percentage of specimens testing positive for influenza in clinical laboratories declined ≥ 0.5 percentage points compared to the previous week in all regions except regions 3 and 6, which 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) are used to monitor whether influenza activity is increasing or decreasing.

 

results of tests from Clinical Laboratories
Week 4
Data Cumulative since October 2, 2022(Week 40)
No. of specimens tested 69,223 2,119,678
No. of positive specimens (%) 1,483 (2.1%) 323,036 (15.2%)
Positive specimens by type
Influenza A 1,396 (94.1%) 320,603 (99.2%)
Influenza B 87 (5.9%) 2,433 (0.8%)

Public Health Laboratories

The results of tests performed by public health laboratories nationwide are summarized below.  Data from public health laboratories are used to monitor the proportion of circulating viruses that belong to each influenza subtype/lineage.

results of tests from Public Health Laboratories
Week 4 Data Cumulative since
October 2, 2022
(Week 40)
No. of specimens tested 5,495 155,842
No. of positive specimens 166 25,576
Positive specimens by type/subtype    
         Influenza A 165 (99.4%) 25,454 (99.5%)
            (H1N1)pdm09 44 (37.6%) 5,103 (24.3%)
             H3N2 73 (62.4%) 15,858 (75.7%)
             H3N2v 0 1 (<0.1%)
             Subtyping not performed 48 4,492
        Influenza B 1 (0.6%) 122 (0.5%)
            Yamagata lineage 0 0
            Victoria lineage 0 85 (100%)
            Lineage not performed 1 37

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 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 genetically characterized 1,693 influenza viruses collected since October 2, 2022.

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 591
6B.1A 591 (100%) 5a.1 1 (0.2%)
5a.2 590 (99.8%)
A/H3 1,073
3C.2a1b 1,073 (100%) 1a 0
1b 0
2a 0
2a.1 0
2a.2 1,073 (100%)
3C.3a 0 3a 0
B/Victoria 29
V1A 29 (100%) V1A 0
V1A.1 0
V1A.3 4 (13.8%)
V1A.3a 0
V1A.3a.1 0
V1A.3a.2 25 (86.2%)
B/Yamagata 0
Y3 0

CDC antigenically characterizes influenza viruses by hemagglutination inhibition (HI) (H1N1pdm09, 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 2022-2023 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 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: Seventy-nine A(H1N1)pdm09 viruses were antigenically characterized by HI, and 78 (98.7%) were well-recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown A/Wisconsin/588/2019-like reference viruses representing the A(H1N1)pdm09 component for the cell- and recombinant- based influenza vaccines.
  • A (H3N2): One hundred and fifty-eight A(H3N2) viruses were antigenically characterized by HINT, and 147 (93.0%) were well-recognized (reacting at titers that were within 8-fold of the homologous virus titer) 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: Twelve 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 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 2, 2022, were tested for antiviral susceptibility as follows:

susceptibility of influenza viruses to the antiviral medications
Antiviral Medication Total
Viruses
 A/H1   A/H3  B/Victoria B/Yamagata
Neuraminidase
Inhibitors
Oseltamivir Viruses
Tested
1,691 591 1,071 29 0
Reduced
Inhibition
1 (0.1%) 1 (0.2%) 0 (0%) 0 (0%) 0 (0%)
Highly
Reduced
Inhibition
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
Peramivir Viruses
Tested
1,691 591 1,071 29 0
Reduced
Inhibition
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
Highly
Reduced
Inhibition
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
Zanamivir Viruses
Tested
1,691 591 1,071 29 0
Reduced
Inhibition
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
Highly
Reduced
Inhibition
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)
PA Cap-Dependent Endonuclease Inhibitor Baloxavir Viruses
Tested
1,647 568 1,050 29 0
Reduced
Susceptibility
0 (0%) 0 (0%) 0 (0%) 0 (0%) 0 (0%)

One A(H1N1)pdm09 virus had NA-S247G amino acid substitution and showed reduced inhibition by oseltamivir.

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, SARS-CoV-2, and RSV. Due to the COVID-19 pandemic, health care-seeking behaviors have changed, and people may be accessing the health care system in alternative settings not captured as a part of ILINet or at a different point in their illness than they might have before the pandemic. Therefore, 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 tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review. 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 4, 2.6% 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 compared to what was reported in week 3 and remains above the national baseline of 2.5%. Seven of the 10 HHS regions are below their respective baselines, and regions 2, 3, and 9 are above their respective baselines. The percent of patient visits for respiratory illness remained stable for seven regions during week 4 compared to week 3, declined in regions 1 and 2, and increased in region 6. 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

More than 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 50-64 years and 65+ years age groups and remained stable (change of ≤ 0.1 percentage point) in the 0-4 years, 5-24 years, and 25-49 years age groups in week 4 compared to week 3.

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 4

(Week ending
Jan. 28, 2023)

Week 3

(Week ending 
Jan. 21, 2022)

Week 4

(Week ending
Jan. 28, 2023)

Week 3

(Week ending
Jan. 21, 2022)

Very High 1 0 3 3
High 3 4 23 26
Moderate 5 6 46 49
Low 15 14 128 126
Minimal 31 30 483 485
Insufficient Data 0 1 246 240

 

*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

Long-term Care Facility (LTCF) Surveillance

LTCFs (e.g., nursing homes/skilled nursing, long-term care for the developmentally disabled, and assisted living facilities) from all 50 states and U.S. territories report data on influenza virus infections among residents through the National Healthcare Safety Network (NHSN) Long-term Care Facility Component. During week 4, 164 (1.1%) of 14,384 reporting facilities reported at least one influenza positive test among their residents. This decreased by > 5% compared to week 3.


Additional information about long-term care facility surveillance:
Surveillance Methods | Additional Data

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 13 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 17,149 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2022, and January 28, 2023. The weekly hospitalization rate observed in week 4 was 0.3 per 100,000 population. The weekly rate observed during week 48 (week ending December 3, 2022) is the third highest peak weekly rate observed during all seasons going back to 2010-2011; this follows the 2017-18 season which peaked during week 1 (week ending January 6, 2018) and the 2014-15 season which peaked during week 52 (week ending December 27, 2014).

The overall cumulative hospitalization rate was 58.6 per 100,000 population. This cumulative hospitalization rate is 1.1 times higher than the highest cumulative in-season hospitalization rate observed in week 4 during previous seasons going back to 2010-2011 (prior season rates ranged from 0.5 per 100,000 to 51.4 per 100,000). However, this in-season cumulative hospitalization rate is still lower than end-of-season hospitalization rates for all but 4 pre-COVID-19-pandemic seasons (2015-16, 2013-14, 2011-12, 2010-11 seasons).

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged 65 and older (173.2). Among adults aged 65 and older, rates were highest among adults aged 85 and older (316.3). Among persons aged <65 years, hospitalization rates per 100,000 population were highest among children aged 0-4 years (78), followed by adults aged 50-64 years (62.5). When examining rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (83.5), followed by non-Hispanic American Indian or Alaska Native persons (71.8), non-Hispanic White persons (49.0), Hispanic/Latino persons (44.3), and non-Hispanic Asian/Pacific Islander persons (24.6).

Among 17,149 hospitalizations, 16,667 (97.2%) were associated with influenza A virus, 312 (1.8%) with influenza B virus, 24 (0.1%) with influenza A virus and influenza B virus co-infection, and 146 (0.9%) with influenza virus for which the type was not determined. Among 3,626 hospitalizations with influenza A subtype information, 2,769 (76.4%) were A(H3N2), and 857 (23.6%) were A(H1N1)pdm09. Based on preliminary data, of the 3,181 laboratory-confirmed influenza-associated hospitalizations with more complete data, 3.4% (95% CI: 2.8%-4.1%) also tested positive for SARS-CoV-2.

Among 2,256 hospitalized adults with information on underlying medical conditions, 96.7% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 754 hospitalized women of childbearing age (15-49 years) with information on pregnancy status, 40.8% were pregnant. Among 764 hospitalized children with information on underlying medical conditions, 66.8% had at least one reported underlying medical condition; the most commonly reported was asthma, followed by obesity, and neurologic disease.


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

HHS Protect Hospitalization Surveillance

Hospitals report to HHS Protect the number of patients admitted with laboratory-confirmed influenza. During week 4, 2,671 patients with laboratory-confirmed influenza were admitted to a hospital. This was a decrease of > 5% compared to week 3.


Additional HHS Protect hospitalization surveillance information:
Surveillance MethodsAdditional Data

Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on February 2, 2023, 11.1% of the deaths that occurred during the week ending January 28, 2023 (week 4), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage decreased (> 0.3 percentage point change) compared to week 3 and is above the epidemic threshold of 7.2% for this week. Among the 2,690 PIC deaths reported for this week, 1,246 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 129 listed influenza. While current PIC mortality is due primarily to COVID-19, the proportion due to influenza increased from October through mid-December and has been decreasing for the past six weeks. 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

Six influenza-associated pediatric deaths occurring during the 2022-2023 season were reported to CDC during week 4. The deaths occurred during week 47 of 2022 (the week ending November 26, 2022) and during weeks 1, 2, and 3 of 2023 (the weeks ending January 7, 2023, January 14, 2023, and January 21, 2023, respectively). All six deaths were associated with influenza A viruses. Four of the influenza A viruses had subtyping performed; two were A(H1N1) viruses and two were A(H3) viruses.

A total of 97 influenza-associated pediatric deaths occurring 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: 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.
Long-term Care Facilities: Up or down arrows indicate change of greater than or equal to 5% of the percent of facilities reporting at least one influenza positive test among their residents compared to the previous week.
HHS Protect 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.

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.