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 51 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 30 054 358 869
No. of positive specimens (%) 6 629 (22,1 %)
31 350 (8,7 %)
Positive specimens by type    
    Influenza A 2 035 (30,7 %) 9 182 (29,3 %)
    Influenza B 4 594 (69,3 %) 22 168 (70,7 %)
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 51 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 1 848 21 741
No. of positive specimens 1 074 8 151
Positive specimens by type/subtype    
         Influenza A 443 (41,2 %) 3 325 (40,8 %)
            (H1N1)pdm09 386 (94,8 %) 2 358 (74,9 %)
             H3N2 21 (5,2 %) 789 (25,1 %)
             Subtyping not performed 36 178
        Influenza B 631 (58,8 %) 4 826 (59,2 %)
            Yamagata lineage 6 (1,3 %) 87 (2,5 %)
            Victoria lineage 456 (98,7 %) 3 394 (97,5 %)
            Lineage not performed 169 1 345

Nationally influenza B/Victoria viruses have been reported more frequently than other influenza viruses this season followed by A(H1N1)pdm09. 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, influenza B/Victoria viruses are the most commonly reported influenza viruses among children age 0-4 years (46 % of reported viruses) and 5-24 years (57 % of reported viruses), while A(H1N1)pdm09 viruses are the most commonly reported influenza viruses among persons 25-64 years (41 % of reported viruses). Among adults aged 65 years of age and older, approximately equal proportions of influenza A(H1N1)pdm09 and A(H3N2) viruses (38 % and 37 %, respectively) have been reported. Additional age data can be found on FluView Interactive.

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. CDC also tests susceptibility of influenza viruses to antiviral medications including the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) and the PA endonuclease inhibitor baloxavir.

CDC genetically characterized 554 influenza viruses collected in the U.S. from September 29, 2019 to December 21, 2019.

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 143
6B.1A 143 (100 %)
A/H3 168
3C.2a 168 (100 %) 2a1 168 (100 %)
   2a2 0
    2a3 0
2a4 0
3C.3a 0 3a 0
B/Victoria 221
V1A 221 (100 %) V1A 0
V1A.1 24 (10,9 %)
V1A.3 197 (89,1 %)
B/Yamagata 22
Y3 22 (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 148 influenza viruses collected in the United States from September 29, 2019, to December 21, 2019.

Influenza A Viruses

Influenza B Viruses


CDC assesses susceptibility of influenza viruses to the antiviral medications oseltamivir, zanamivir, peramivir, and 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 546 142 166 216 22
Reduced Inhibition (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)
Highly Reduced Inhibition 1 (0,2 %) 1 (0,7 %) (0,0 %) (0,0 %) (0,0 %)
Peramivir Viruses Tested 546 142 166 216 22
Reduced Inhibition (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)
Highly Reduced Inhibition 1 (0,2 %) 1 (0,7 %) (0,0 %) (0,0 %) (0,0 %)
Zanamivir Viruses Tested 546 142 166 216 22
Reduced Inhibition 1 (0,2 %) (0,0 %) (0,0 %) 1 (0,5 %) (0,0 %)
Highly Reduced Inhibition (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)
PA Endonuclease Inhibitor
Baloxavir Viruses Tested 546 144 164 216 22
Reduced Susceptibility (0,0 %) (0,0 %) (0,0 %) (0,0 %) (0,0 %)


Outpatient Illness Surveillance

ILINet

Nationwide during week 51, 5,1 % 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 2,5 % to 10 % during week 51. All regions reported a percentage of outpatient visits for ILI which is equal to or 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 51, 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 51 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 1 931 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2019 and December 21, 2019. The overall hospitalization rate was 6.6 per 100 000 population. The highest rate of hospitalization was among adults aged ≥65 (14.4 per 100 000 population), followed by children aged 0-4 (12.5 per 100 000 population) and adults aged 50-64 (7 per 100 000 population). Among 1 931 hospitalizations, 1 021 (52,9 %) were associated with influenza A virus, 896 (46,4 %) with influenza B virus, 8 (0,4 %) with influenza A virus and influenza B virus co-infection, and 6 (0,3 %) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 164 (74,5 %) were A(H1N1)pdm09 virus and 56 (25,5 %) were A(H3N2).

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



Pneumonia and Influenza (P&I) Mortality Surveillance

Based on National Center for Health Statistics (NCHS) mortality surveillance data available on December 26, 2019, 5,7 % of the deaths occurring during the week ending December 14, 2019 (week 50) were due to P&I. This percentage is below the epidemic threshold of 6,7 % for week 50.

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

Three influenza-associated pediatric deaths occurring in weeks 49 (the week ending December 7, 2019) and 51 (the week ending December 21, 2019) were reported to CDC during week 51. All three were associated with influenza B viruses.

A total of 22 influenza-associated pediatric deaths occurring during the 2019-2020 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. 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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