FluView Summary ending on December 25, 2021

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 51, ending December 25, 2021

Seasonal influenza activity in the United States is increasing, including indicators that track hospitalizations. 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 is increasing, with the eastern and central parts of the country seeing the majority of viruses reported and the western part of the country reporting lower levels of influenza virus circulation.
  • The majority of influenza viruses detected are A(H3N2). Most influenza A(H3N2) infections have occurred among children and young adults ages 5-24 years; however, the proportion of infections occurring among adults age 25 years and older has been increasing.
  • 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.
  • The percentage of outpatient visits due to respiratory illness is trending upwards and is above the national baseline. Influenza is contributing to levels of respiratory illness, but other respiratory viruses are also circulating. The relative contribution of influenza varies by location.
  • Hospitalizations for influenza are starting to increase.
  • The flu season is just getting started. There's still time 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 uptake is down this season compared to last.
  • Flu vaccines are available at many different locations, including pharmacies and health departments. With flu activity just getting started, there is still time to benefit from flu vaccination this season. 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

The week 51 virologic data may have been impacted by delays in reporting due to the holiday therefore, percent positivity and testing numbers should be interpreted with caution. An increasing number of influenza positive tests have been reported by clinical and public health laboratories during recent weeks. Influenza A(H3N2) viruses have been the most frequently detected. Persons aged 5-24 years old account for the largest proportion of influenza A(H3N2) viruses detected, but the proportion of influenza A(H3N2) virus detections occurring among other age groups has increased in recent weeks. Influenza A(H3N2) viruses were reported by public health laboratories in all HHS regions except for Region 10 this week. 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 51 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 70 660 778 091
No. of positive specimens (%) 4 393 (6,2 %) 18 479 (2,4 %)
Positive specimens by type
Influenza A 4 337 (98,7 %) 17 985 (97,3 %)
Influenza B 57 (1,3 %) 494 (2,7 %)

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 51 Data Cumulative since
October 3, 2021
(Week 40)
No. of specimens tested 29 486 282 321
No. of positive specimens 634 5 357
Positive specimens by type/subtype    
         Influenza A 632 (99,7 %) 5 294 (98,8 %)
            (H1N1)pdm09 0 4 (<0,1 %)
             H3N2 375 (100 %) 4 110 (99,9 %)
             H3N2v 0 1 (<0,1 %)
             Subtyping not performed 257 1 179
        Influenza B 2 (0,3 %) 63 (1,2 %)
            Yamagata lineage 0 1 (3,3 %)
            Victoria lineage 0 29 (96,7 %)
            Lineage not performed 2 33

 

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 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 208 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 and antiviral susceptibility data will be reported later this season when a sufficient number of specimens have been tested.

CDC genetically characterized 108 influenza viruses collected October 3, 2021 to present:

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 184
3C.2a1b 189 (100 %)  1a
1b 1 (1 %) 
2a
2a.1
2a.2 183 (99 %)
3C.3a  0 3a
B/Victoria 16
V1A 16 (100 %)  V1A
V1A.1 0
V1A.3 9 (56 %) 
V1A.3a 0
V1A.3a.1 0
V1A.3a.2 7 (44 %)
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
210 3 191 16 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
210 3 191 16 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
210 3 191 16 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
193 3 174 16 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 such as 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 51, 3,8 % 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 percentage is above the national baseline. Nine of the 10 HHS regions are above their region-specific baselines; only Region 6 is below their 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.

The percentage of visits for respiratory illness reported in ILINet increased for all age groups (0–4 years, 5–24 years, 25-49 years, 50–64 years, and 65+).

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 51

(Week ending
Dec. 25, 2021)

Week 50

(Week ending 
Dec. 18, 2021)

Week 51

(Week ending
Dec. 25, 2021)

Week 50

(Week ending
Dec. 18, 2021)

Very High 3 0 7 3
High 16 8 83 49
Moderate 12 14 104 79
Low 13 13 167 163
Minimal 9 20 280 365
Insufficient Data 2 0 288 270

 

*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 51, 122 (0,9 %) of 14 084 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.

A total of 535 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2021, and December 25, 2021, with a cumulative hospitalization rate of 1.8 per 100 000 population, which is more than double the overall cumulative hospitalization rate reported during the 2020-2021 season.

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 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:
Surveillance Methods | FluView Interactive

HHS-Protect Hospitalization Surveillance

Hospitals report to HHS-Protect the number of patients admitted with laboratory-confirmed influenza. During week 51, 1 825 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 December 30, 2021, 19,2 % of the deaths that occurred during the week ending December 25, 2021 (week 51), were due to pneumonia, influenza, and/or COVID-19 (PIC). This percentage is above the epidemic threshold of 6,8 % for this week. Among the 3 381 PIC deaths reported for this week, 2 582 had COVID-19 listed as an underlying or contributing cause of death on the death certificate, and 12 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

No influenza-associated pediatric deaths were reported to CDC during week 51.

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

Click on image to launch interactive tool
 View Full Screen 

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.

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