FluView Summary ending on March 12, 2022

Updated March 18, 2022

FluView a weekly influenza surveillance report prepared by the Influenza Division

Note: CDC is tracking the COVID-19 pandemic in a weekly publication called COVID Data Tracker Weekly Review.

2019-2020 Influenza Season Week 33, ending August 15, 2020

All data are preliminary and may change as more reports are received.

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 Updates for Week 10, ending March 12, 2022

Influenza activity is increasing in most of the country.

Viruses

Severe Disease

All data are preliminary and may change as more reports are received.

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

  • Influenza activity is increasing in most of the country.
  • The highest levels of influenza percent positivity from clinical labs were seen in states in the central and south-central regions of the country.
  • The majority of influenza viruses detected are A(H3N2). H3N2 viruses identified so far this season are genetically closely related to the vaccine virus. Antigenic data show that the majority of the H3N2 viruses characterized are antigenically different from the vaccine reference viruses. While the number of B/Victoria viruses circulating this season is small, the majority of the B/Victoria viruses characterized are antigenically similar to the vaccine reference virus.
  • The percentage of outpatient visits due to respiratory illness increased slightly this week but is still below baseline. Influenza is contributing to levels of respiratory illness, but other respiratory viruses are also circulating. The relative contribution of influenza varies by location.
  • The number of hospital admissions reported to HHS Protect has increased each week for the past six weeks.
  • The cumulative hospitalization rate in the FluSurv-NET system is higher than the rate for the entire 2020-2021 season, but lower than the rate seen at this time during the four seasons preceding the COVID-19 pandemic.
  • Three influenza-associated pediatric deaths were reported this week. There have been 13 pediatric deaths reported this season.
  • CDC estimates that, so far this season, there have been at least 2.9 million flu illnesses, 28,000 hospitalizations, and 1,700 deaths from flu.
  • An annual flu vaccine is the best way to protect against flu. Vaccination can prevent serious outcomes in people who get vaccinated but still get sick. CDC continues to recommend that everyone ages 6 months and older get a flu vaccine as long as flu activity continues.
  • Flu vaccines are available at many different locations, including pharmacies and health departments. Visit www.vaccines.gov to find a flu vaccine near you.
  • There are also flu antiviral drugs that can be used to treat flu illness.

U.S. Virologic Surveillance

Influenza activity is increasing again in most of the country with the largest increases being reported in the central and south-central parts of the country (Regions 6 and 7). Influenza A(H3N2) viruses have been the most frequently detected influenza viruses this season. Of the 8,408 influenza positives reported this season by the public health labs and also tested for SARS-CoV-2, 473 (5.6%) were also positive for SARS-CoV-2. 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 10 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 39,267 1,819,313
No. of positive specimens (%) 2,685 (6.8%) 53,042 (2.9%)
Positive specimens by type
Influenza A 2,655 (98.9%) 51,897 (97.8%)
Influenza B 30 (1.1%) 1,145 (2.2%)

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 viruses that belong to each influenza subtype/lineage.

results of tests from Public Health Laboratories
Week 10 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 12,542 683,949
No. of positive specimens 347 14,558
Positive specimens by type/subtype    
         Influenza A 347 (100%) 14,456 (99.3%)
            (H1N1)pdm09 0 5 (0.1%)
             H3N2 196 (100%) 9,714 (99.9%)
             H3N2v 0 1 (<0.1%)
             Subtyping not performed 151 4,736
        Influenza B 0 (0%) 102 (0.7%)
            Yamagata lineage 0 1 (2.9%)
            Victoria lineage 0 33 (97.1%)
            Lineage not performed 0 68

 

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

Influenza Virus Characterization

CDC performs genetic and antigenic characterization of U.S. viruses submitted from state and local public health laboratories using 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 817 influenza viruses collected since October 3, 2021. H3N2 viruses identified so far this season are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve.

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 3
6B.1A 3 (100%)  5a.1 2 (66.7%)
5a.2 1 (33.3%)
A/H3 791
3C.2a1b 791 (100%) 1a 1 (0.1%)
1b 1 (0.1%)
2a
2a.1
2a.2 789 (99.8%)
3C.3a  0 3a
B/Victoria 23
V1A 23 (100%) V1A
V1A.1 0
V1A.3 9 (39.1%)
V1A.3a 0
V1A.3a.1 0
V1A.3a.2 14 (60.9%)
B/Yamagata 0
Y3

 

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 2021-2022 Northern Hemisphere recommended egg-based and 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: Two A(H1N1)pdm09 viruses were antigenically characterized by HI, and 1 (50%) was 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 and 1 (50%) was well recognized by ferret antisera to egg-grown A/Victoria/2570/2019-like reference viruses representing the A(H1N1)pdm09 component for the egg-based influenza vaccines.
  • A (H3N2): A subset of fifty-eight A(H3N2) viruses were antigenically characterized by HINT, and 2 (3%) were well recognized (reacting at titers that were within 8-fold of the homologous virus titer) by ferret antisera to cell-grown A/Cambodia/E0826360/2020-like reference viruses representing the A(H3N2) component for the cell- and recombinant-based influenza vaccines and 18 (31%) were well recognized by ferret antisera to egg-grown A/Cambodia/E0826360/2020-like reference viruses representing the A(H3N2) component for egg-based influenza vaccines.

Influenza B Viruses

  • B/Victoria: Fourteen B/Victoria lineage viruses were antigenically characterized by HI, and 11 (79%) were well recognized (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera to cell-grown B/Washington/02/2019-like reference viruses representing the B/Victoria component for the cell- and recombinant-based influenza vaccines and 11 (79%) were well recognized by ferret antisera to egg-grown B/Washington/02/2019-like reference viruses representing the B/Victoria component for egg-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 United States since October 3, 2021, 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
833 3 807 23 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%)
Peramivir Viruses
Tested
833 3 807 23 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
833 3 807 23 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
819 3 793 23 0
Reduced
Susceptibility
(0.0%) (0.0%) (0.0%) (0.0%) (0.0%)

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 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 10, 1.7% 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 percentage is below the national baseline. Nine of the 10 HHS regions are below their region-specific baselines; Region 7 is above its baseline. 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.

In week 10, the percentage of visits for respiratory illness reported in ILINet increased among two age groups (0–4 years and 5–24 years), while remaining stable for all other age groups (25–49 years, 50–64 years, and 65+ years) compared to week 9.

national levels of ILI and ARI by age group

* 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 | View Full Screen

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 10

(Week ending
Mar. 12, 2022)

Week 9

(Week ending 
Mar. 5, 2022)

Week 10

(Week ending
Mar. 12, 2022)

Week 9

(Week ending
Mar. 5, 2022)

Very High 0 0 3 3
High 1 1 5 4
Moderate 2 1 23 11
Low 5 6 54 51
Minimal 46 44 573 597
Insufficient Data 1 3 271 263

 

*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 10, 94 (0.7%) of 14,266 reporting LTCFs reported at least one influenza positive test among their residents.

national levels of ltcf influenza
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Additional information about long-term care facility surveillance:
Surveillance Methods | Additional Dataexternal icon

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 1,734 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2021, and March 12, 2022. The overall cumulative hospitalization rate was 5.9 per 100,000 population. This cumulative hospitalization rate is higher than the cumulative in-season hospitalization rate observed in week 10 during the 2020-2021 season (0.7 per 100,000), but lower than the in-season rates observed in week 10 during the 4 seasons preceding the COVID-19 pandemic (these ranged from 41.3 to 89.9 per 100,000 during the 2016-17 through 2019-20 seasons).

When examining rates by age, the highest rate of hospitalization per 100,000 population was among adults aged 65 and older (15.9). Among adults aged 65 and older, rates were highest among adults aged 85 and older (32.4). Among persons aged less than 65 years, hospitalization rates per 100,000 population were highest among children aged 0-4 years (8.5) followed by adults aged 50-64 years (5.4). When examining rates by race and ethnicity, the highest rate of hospitalization per 100,000 population was among non-Hispanic Black persons (7.3), followed by non-Hispanic American Indian or Alaska Native persons (6.9).

Among 1,734 hospitalizations, 1,641 (94.6%) were associated with influenza A virus, 86 (5%) with influenza B virus, 4 (0.2%) with influenza A virus and influenza B virus co-infection, and 3 (0.2%) with influenza virus for which the type was not determined. Among 433 hospitalizations with influenza A subtype information, 428 (98.8%) were A(H3N2), and 5 (1.2%) were A(H1N1)pdm09. Based on preliminary data, of the 1,734 laboratory-confirmed influenza-associated hospitalizations, 3.9% also tested positive for SARS-CoV-2.

Among 875 hospitalized adults with information on underlying medical conditions, 92.8% had at least one reported underlying medical condition, the most commonly reported were hypertension, cardiovascular disease, metabolic disorder, and obesity. Among 97 hospitalized children with information on underlying medical conditions, 71.1% had at least one reported underlying medical condition; the most commonly reported condition was asthma.

flusurvnet

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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

FluSurv-Net data are used to generate national estimates of the total numbers of influenza cases, medical visits, hospitalizations, and deaths. This season, CDC is reporting preliminary cumulative in-season estimates, which are available at https://www.cdc.gov/flu/about/burden/preliminary-in-season-estimates.htm

HHS Protect Hospitalization Surveillance

Hospitals report to HHS Protect the number of patients admitted with laboratory-confirmed influenza. During week 10, 2,082 patients with laboratory-confirmed influenza were admitted to the hospital.

Effective February 2, 2022, hospitals are required to report laboratory-confirmed influenza hospitalizations to HHS Protect daily. Prior to this update, reporting influenza hospitalizations was optional. See COVID-19 Guidance for Hospital Reporting and FAQspdf iconexternal icon for additional details on this guidance.

national levels of influenza hospitalizations
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Additional HHS Protect hospitalization surveillance information:
Surveillance MethodsAdditional Dataexternal icon

Mortality Surveillance

National Center for Health Statistics (NCHS) Mortality Surveillance

Based on NCHS mortality surveillance data available on March 17, 2022, 10.8% of the deaths that occurred during the week ending March 12, 2022 (week 10), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 7.2% for this week. Among the 1,890 PIC deaths reported for this week, 1,215 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 15 listed influenza, indicating that current PIC mortality is due primarily to COVID-19 and not influenza. The data presented are preliminary and may change as more data are received and processed.

INFLUENZA Virus Isolated
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Additional pneumonia, influenza and COVID-19 mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

Influenza-Associated Pediatric Mortality

Three influenza-associated pediatric deaths were reported to CDC during week 10. The deaths occurred in weeks 3, 8, and 9 of the 2021-2022 season (weeks ending January 22, February 26, and March 5, 2022, respectively). All three deaths were associated with influenza A viruses for which no subtyping was performed.

A total of 13 influenza-associated pediatric deaths occurring during the 2021-2022 season have been reported to CDC.

Click on image to launch interactive tool
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Additional pediatric mortality surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive

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
FluNetexternal icon and the Global Epidemiology Reports.external icon

WHO Collaborating Centers for Influenza:
Australiaexternal icon, Chinaexternal icon, Japanexternal icon, the United Kingdomexternal icon, 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 Controlexternal icon.

Public Health Agency of Canada:
The most up-to-date influenza information from Canada is available in Canada’s weekly FluWatch reportexternal icon.

Public Health England:
The most up-to-date influenza information from the United Kingdom is available from Public Health Englandexternal icon.

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.