U.S. Virologic Surveillance

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.

Week 9 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 44 491 1 012 509
No. of positive specimens (%) 10 829 (24,3 %)
207 466 (20,5 %)
Positive specimens by type    
    Influenza A 7 821 (72,2 %) 102 650 (49,5 %)
    Influenza B 3 008 (27,8 %) 104 816 (50,5 %)
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.

Week 9 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 1 594 67 098
No. of positive specimens 953 37 739
Positive specimens by type/subtype    
         Influenza A 692 (72,6 %) 20 452 (54,2 %)
            (H1N1)pdm09 596 (95,2 %) 18 205 (92 %)
             H3N2 30 (4,8 %) 1 583 (8 %)
             Subtyping not performed 66 664
        Influenza B 261 (27,4 %) 17 287 (45,8 %)
            Yamagata lineage 0 (0,0 %) 213 (1,6 %)
            Victoria lineage 207 (100 %) 13 265 (98,4 %)
            Lineage not performed 54 3 809

While influenza B/Victoria viruses predominated earlier in the season, during recent weeks, influenza A(H1N1)pdm09 viruses have been reported more frequently than B/Victoria viruses. For the season, A(H1N1)pdm09 viruses are the predominant virus. The predominant virus varies by region. Regional and state level data about circulating influenza viruses can be found on FluView Interactive.

The predominant virus also varies by age group. Nationally, for the season overall, influenza B viruses are the most commonly reported influenza viruses among children and young adults less than 25 years, while A viruses are the most commonly reported influenza viruses among persons 25 years and older. In the most recent three weeks, influenza A viruses are the most commonly reported influenza viruses in all but the school aged children and young adults (5-24 years old).

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 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 used for developing new influenza vaccines and to monitor evolutionary changes that continually occur in circulating influenza viruses. Antigenic characterization data are based on an animal model (influenza-naive ferrets), and do not reflect pre-existing protection provided by past influenza infections and vaccinations. Additional antigenic characterization studies involving people vaccinated with current influenza vaccines are conducted later in the season; these data account for pre-existing protection in different populations against circulating influenza viruses. Genetic and antigenic characterization data are not used to make calculations about vaccine effectiveness (VE). CDC conducts VE studies each year to measure the benefits of flu vaccines in people. Interim estimates of 2019-2020 flu vaccine effectiveness have been released.

CDC genetically characterized 1 903 influenza viruses collected in the U.S. from September 29, 2019, to February 29, 2020.

Virus Subtype or Lineage Genetic Characterization
Total No. of Subtype/Lineage Tested          Clade Number (% of subtype/lineage tested)          Subclade Number (% of subtype/lineage tested)
A/H1 645
6B.1A 645 (100 %)
A/H3 425
3C.2a 402 (94,6 %) 2a1 402 (94,6 %)
   2a2 0
    2a3 0
2a4 0
3C.3a 23 (5,4 %) 3a 23 (5,4 %)
B/Victoria 754
V1A 754 (100 %) V1A 0
V1A.1 53 (7 %)
V1A.3 701 (93 %)
B/Yamagata 79
Y3 79 (100 %)

CDC antigenically characterizes a subset of influenza viruses by hemagglutination inhibition (HI) or neutralization based Focus Reduction assays (FRA). Antigenic drift is evaluated by comparing antigenic properties of cell-propagated reference viruses representing currently recommended vaccine components with those of cell-propagated circulating viruses. CDC antigenically characterized 363 influenza viruses collected in the United States from September 29, 2019, to February 29, 2020. These data are not used to make calculations about vaccine effectiveness (VE). CDC conducts VE studies each year to measure the benefits of flu vaccines in people.

Influenza A Viruses

Influenza B Viruses


CDC also 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. Viruses collected in the United States since September 29, 2019, were tested for antiviral susceptibility as follows:

Antiviral Medication Total Viruses A/H1 A/H3 B/Victoria B/Yamagata
Neuraminidase Inhibitors
Oseltamivir Viruses Tested 1 884 643 418 744 79
Reduced Inhibition 1 (0,1 %) (0,0 %) (0,0 %) 1 (0,1 %) (0,0 %)
Highly Reduced Inhibition 4 (0,2 %) 4 (0,6 %) (0,0 %) (0,0 %) (0,0 %)
Peramivir Viruses Tested 1 884 643 418 744 79
Reduced Inhibition (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)
Highly Reduced Inhibition 5 (0,3 %) 4 (0,6 %) (0,0 %) 1 (0,1 %) (0,0 %)
Zanamivir Viruses Tested 1 884 643 418 744 79
Reduced Inhibition 2 (0,1 %) (0,0 %) (0,0 %) 2 (0,3 %) (0,0 %)
Highly Reduced Inhibition (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)
PA Endonuclease Inhibitor
Baloxavir Viruses Tested 2 039 663 502 792 82
Reduced Susceptibility (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)

*Six influenza viruses showed reduced or highly reduced inhibition by at least one neuraminidase inhibitor. Four A(H1N1)pdm09 viruses showed highly reduced inhibition to oseltamivir and peramivir while showing normal inhibition to zanamivir. In addition, one B/Victoria virus showed highly reduced inhibition to peramivir and reduced inhibition to oseltamivir and zanamivir, while another influenza B/Victoria virus showed reduced inhibition to zanamivir.


A total of 515 additional viruses (211 A(H1N1)pdm09, 32 A(H3N2), and 272 B) collected in Alabama, Alaska, Florida, Illinois, Iowa, Louisiana, Massachusetts, Michigan, Nevada, New York, North Carolina, Pennsylvania, South Dakota, Virginia and Wisconsin were analyzed for resistance to neuraminidase inhibitors by pyrosequencing assay. Three (1,4 %) of the 211 A(H1N1)pdm09 viruses tested had the H275Y amino acid substitution in the neuraminidase that confers resistance to oseltamivir and potential resistance to peramivir. No markers of resistance to neuraminidase inhibitors were detected in A(H3N2) and type B viruses tested.



Outpatient Illness Surveillance

ILINet

Nationwide during week 9, 5,3 % of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is above the national baseline of 2,4 %.

national levels of ILI and ARI
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On a regional level, the percentage of outpatient visits for ILI ranged from 3,9 % to 8,7 % during week 9. All regions reported a percentage of outpatient visits for ILI above their region-specific baselines.

ILI Activity Map

Data collected in ILINet are used to produce a measure of ILI activity* by state.

During week 9, the following ILI activity levels were experienced:

*Data collected in ILINet may disproportionally represent certain populations within a state, and therefore, may not accurately depict the full picture of influenza activity for the whole state. Differences in the data presented here by CDC and independently by some state health departments likely represent differing levels of data completeness with data presented by the state 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



Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists

The influenza activity reported by state and territorial epidemiologists indicates geographic spread of influenza viruses but does not measure the severity of influenza activity.

During week 9, the following influenza activity was reported:

Additional geographic spread surveillance information for current and past seasons:
Surveillance Methods | FluView Interactive



Influenza-Associated Hospitalizations

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in the Emerging Infections Program (EIP) states and Influenza Hospitalization Surveillance Project (IHSP) states.

A total of 16 819 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2019 and February 29, 2020; 11 707 (69,6 %) were associated with influenza A virus, 5 020 (29,8 %) with influenza B virus, 51 (0,3 %) with influenza A virus and influenza B virus co-infection, and 41 (0,2 %) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 3 183 (94,2 %) were A(H1N1)pdm09 virus and 196 (5,8 %) were A(H3N2).

The overall cumulative hospitalization rate was 57.9 per 100 000 population which is similar to what has been seen during recent previous influenza seasons at this time of year. Rates in school aged children and young adults are higher than at this time in recent seasons, and rates among children 0-4 years old are now the highest CDC has on record at this point in the season, surpassing rates reported during the second wave of the 2009 H1N1 pandemic.

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The highest rate of hospitalization is among adults aged ≥ 65, followed by children aged 0-4 years and adults aged 50-64 years.

Age Group 2019-2020 Season
Cumulative Rate per 100 000 Population
Overall 57.9
0-4 years 84.9
5-17 years 21.6
18-49 years 31.2
50-64 years 76.1
65+ years 147.5

Among 2 528 hospitalized adults with information on underlying medical conditions, 91,9 % had at least one reported underlying medical condition, the most commonly reported were cardiovascular disease, metabolic disorder, obesity, and chronic lung disease. Among 443 hospitalized children with information on underlying medical conditions, 48,5 % had at least one underlying medical condition; the most commonly reported was asthma. Among 427 hospitalized women of childbearing age (15-44 years) with information on pregnancy status, 25,8 % were pregnant.

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Additional hospitalization surveillance information for current and past seasons and additional age groups:
Surveillance Methods | FluView Interactive: Rates by Age or Patient Characteristics



Pneumonia and Influenza (P&I) Mortality Surveillance

Based on National Center for Health Statistics (NCHS) mortality surveillance data available on March 5, 2020, 6,9 % of the deaths occurring during the week ending February 22, 2020 (week 8) were due to P&I. This percentage is below the epidemic threshold of 7,3 % for week 8.

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



Influenza-Associated Pediatric Mortality

11 influenza-associated pediatric deaths occurring during the 2019-2020 season between weeks 51 and 9 (the weeks ending December 21, 2019 and February 29, 2020) were reported to CDC during week 9. Six were associated with influenza B viruses, and none had a lineage determined. Five were associated with influenza A viruses, and one was subtyped and was an A(H1N1)pdm09 virus.

Of the 136 influenza-associated pediatric deaths occurring during the 2019-2020 season and 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. To access these tools, visit http://www.cdc.gov/flu/weekly/fluviewinteractive.htm

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 at https://www.cdc.gov/niosh/topics/absences/default.html

U.S. State and local influenza surveillance:Select a jurisdiction below to access the latest local influenza information

Alabama

Alaska

Arizona

Arkansas

California

Colorado

Connecticut

Delaware

District of Columbia

Florida

Georgia

Hawaii

Idaho

Illinois

Indiana

Iowa

Kansas

Kentucky

Louisiana

Maine

Maryland

Massachusetts

Michigan

Minnesota

Mississippi

Missouri

Montana

Nebraska

Nevada

New Hampshire

New Jersey

New Mexico

New York

North Carolina

North Dakota

Ohio

Oklahoma

Oregon

Pennsylvania

Rhode Island

South Carolina

South Dakota

Tennessee

Texas

Utah

Vermont

Virginia

Washington

West Virginia

Wisconsin

Wyoming

New York City

Puerto Rico

Virgin Islands



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 located in Australia, China, Japan, the United Kingdom, and the United States (CDC in Atlanta, Georgia).

Europe: For the most recent influenza surveillance information from Europe, please see WHO/Europe and the European Centre for Disease Prevention and Control at http://www.flunewseurope.org/.

Public Health Agency of Canada: The most up-to-date influenza information from Canada is available at http://www.phac-aspc.gc.ca/fluwatch/

Public Health England: The most up-to-date influenza information from the United Kingdom is available at https://www.gov.uk/government/statistics/weekly-national-flu-reports



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.

An overview of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available at: http://www.cdc.gov/flu/weekly/overview.htm.

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