Weekly U.S. Influenza Surveillance Report

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 1, ending January 8, 2022

Seasonal influenza activity in the United States declined slightly this week but remains elevated and is expected to continue for several weeks. The amount of activity varies by region.

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 declined slightly this week but remains elevated. While influenza is difficult to predict, influenza activity is expected to continue for several more weeks.
  • The majority of influenza viruses detected are A(H3N2). Most of the H3N2 viruses so far are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve.
  • The percentage of outpatient visits due to respiratory illness decreased nationally but remains above 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 declined slightly this week.
  • 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.
  • It’s not too late to get vaccinated. An annual flu vaccine is the best way to protect against flu and its potentially serious complications. CDC recommends everyone 6 months and older get a flu vaccine.
  • There are early signs that flu vaccination coverage so far is lower this season compared to last.
  • 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 A(H3N2) viruses have been the most frequently detected influenza viruses this season. Earlier in the season most influenza viruses were detected among persons aged 5-24 years; however, during the most recent 2 weeks, persons aged 25-64 years accounted for the largest number of A(H3N2) viruses reported. For regional and state level data about circulating influenza viruses, 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 1 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 102,412 1,095,989
No. of positive specimens (%) 2,203 (2.2%) 30,124 (2.7%)
Positive specimens by type
Influenza A 2,156 (97.9%) 29,434 (97.7%)
Influenza B 47 (2.1%) 690 (2.3%)

INFLUENZA Virus Isolated
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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 1 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 54,888 385,093
No. of positive specimens 617 7,969
Positive specimens by type/subtype    
         Influenza A 613 (99.4%) 7,893 (99.0%)
            (H1N1)pdm09 0 4 (0.1%)
             H3N2 266 (100%) 5,747 (99.9%)
             H3N2v 0 1 (<0.1%)
             Subtyping not performed 347 2,141
        Influenza B 4 (0.6%) 76 (1.0%)
            Yamagata lineage 0 1 (3.1%)
            Victoria lineage 0 31 (96.9%)
            Lineage not performed 4 44

INFLUENZA Virus Isolated

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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 genetically characterized 347 influenza viruses collected since October 3, 2021. While there are little data to date, most of the H3N2 viruses so far are genetically closely related to the vaccine virus, but there are some antigenic differences that have developed as H3N2 viruses have continued to evolve. Virus antigenic data will be reported later this season when a sufficient number of specimens have been tested.

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 (67%)
5a.2 1 (33%)
A/H3 324
3C.2a1b 324 (100%)  1a
1b 1 (0.3%) 
2a
2a.1
2a.2 323 (99.7%)
3C.3a  0 3a
B/Victoria 20
V1A 20 (100%)  V1A
V1A.1 0
V1A.3 11 (55%) 
V1A.3a 0
V1A.3a.1 0
V1A.3a.2 9 (45%)
B/Yamagata 0
Y3

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 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
349 3 326 20 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
349 3 326 20 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
349 3 326 20 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
344 3 321 20 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 1, 4.3% 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 above the national baseline. All 10 HHS regions are above their region-specific 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.”

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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 for four age groups (0–4 years, 5–24 years, 25-49 years, and 50–64 years) and remained stable for one age group (65+ years).

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 1

(Week ending
Jan. 8, 2022)

Week 52

(Week ending 
Jan. 1, 2022)

Week 1

(Week ending
Jan. 8, 2022)

Week 52

(Week ending
Jan. 1, 2022)

Very High 3 9 22 33
High 18 20 116 137
Moderate 14 11 92 107
Low 5 6 157 145
Minimal 13 8 263 232
Insufficient Data 2 1 279 275

 

*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 infections among residents through the National Healthcare Safety Network (NHSN) Long-term Care Facility Component. During week 1, 191 (1.3%) of 14,208 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. Case counts and rates for recent hospital admissions are subject to reporting delays; these delays are likely to be more pronounced around holidays. As hospitalization data are received each week, prior case counts and rates are updated accordingly. As such, end-of-season rates for any given week may vary substantially from in-season reported rates.

A total of 1,005 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2021, and January 8, 2022. The overall hospitalization rate was 3.4 per 100,000 population. This cumulative hospitalization rate is higher than the cumulative in-season hospitalization rate observed for week 1 during the 2020-2021 season (0.4 per 100,000), but lower than the in-season rates observed for week 1 during the 4 seasons preceding the COVID-19 pandemic (these ranged from 7.1 to 22.7 per 100,000 during the 2016-2017 through 2019-2020 seasons). The highest hospitalization rate was among adults aged ≥65 (9.6 per 100,000 population), followed by children aged 0-4 (4.8 per 100,000 population) and adults aged 50-64 (2.9 per 100,000 population). Among 1,005 hospitalizations, 947 (94.2%) were associated with influenza A virus, 50 (5.0%) with influenza B virus, 2 (0.2%) with influenza A virus and influenza B virus co-infection, and 6 (0.6%) with influenza virus for which the type was not determined. Among 227 hospitalizations with influenza A subtype information, 224 (98.7%) were A(H3N2) and 3 (1.3%) were A(H1N1)pdm09.

flusurvnet

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Additional FluSurv-NET hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods | FluView Interactive

HHS-Protect Hospitalization Surveillance

Hospitals report to HHS-Protect the number of patients admitted with laboratory-confirmed influenza. During week 1, 1,804 patients with laboratory-confirmed influenza were admitted to the hospital.

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 January 13, 2022, 23.1% of the deaths that occurred during the week ending January 8, 2022 (week 1), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 7.0% for this week. Among the 4,417 PIC deaths reported for this week, 3,567 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 42 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

One influenza-associated pediatric death occurring during the 2021-2022 season was reported to CDC during week 1. This death was associated with an influenza A virus for which no subtyping was performed and occurred during week 51 (the week ending December 25, 2021).

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

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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.