Weekly U.S. Influenza Surveillance Report

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Key Updates for Week 46, ending November 16, 2019

According to this week’s FluView report, seasonal influenza activity in the United States continues to increase but the amount of activity and the predominant influenza virus 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 components 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 Messages from CDC

  • Nationally, ILI activity has been at or above baseline for two weeks; however, the amount of influenza activity across the country varies with the south and parts of the west seeing elevated activity while other parts of the country are still seeing low activity.
  • There is significant cocirculation of influenza A(H3N2), A(H1N1)pdm09 and B/Victoria viruses with the predominant virus varying by region and age group.
  • The flu season is just getting started. It’s not too late to get vaccinated. Flu vaccination is always the best way to prevent flu and its potentially serious complications.

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 46 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 24,405 158,740
No. of positive specimens (%) 1,786 (7.3%)
5,299 (3.3%)
Positive specimens by type    
    Influenza A 470 (26.3%) 1,681 (31.7%)
    Influenza B 1,316 (73.7%) 3,618 (68.3%)
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.

Week 46 Data Cumulative since
September 29, 2019
(week 40)
No. of specimens tested 779 8,105
No. of positive specimens 241 1,676
Positive specimens by type/subtype    
         Influenza A 87 (36.1%) 811 (48.4%)
            (H1N1)pdm09 54 (71.1%) 337 (45.7%)
             H3N2 22 (28.9%) 401 (54.3%)
             Subtyping not performed 11 73
        Influenza B 154 (63.9%) 865 (51.6%)
            Yamagata lineage 3 (2.8%) 26 (4.0%)
            Victoria lineage 106 (97.2%) 630 (96.0%)
            Lineage not performed 45 209

Nationally influenza B/Victoria viruses have been reported more frequently than other influenza viruses this season; followed by A(H1N1)pdm09 and A(H3N2) viruses, which are also circulating in significant numbers. The predominant virus varies by region and the proportion of influenza B/Victoria viruses is increasing in some regions. 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 (48% of reported viruses) and 5-24 years (56% of reported viruses), while A(H3N2) viruses are the most commonly reported influenza viruses among persons 65 years of age and older (70% of reported viruses). Among adults aged 25-64 years, approximately equal proportions of influenza A(H1N1)pdm09, A(H3N2) and B/Victoria viruses (33%, 24% and 31%, 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 174 influenza viruses collected in the U.S. from September 29, 2019 to November 16, 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 35
6B.1A 35 (100%)
A/H3 77
3C.2a 77 (100%) 2a1 77 (100%)
   2a2 0
    2a3 0
2a4 0
3C.3a 0 3a 0
B/Victoria 58
V1A 58 (100%) V1A 0
V1A.1 10 (17.2%)
V1A.3 48 (82.8%)
B/Yamagata 4
Y3 4 (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 13 influenza viruses collected in the U.S. from September 29, 2019 to November 16, 2019.

Influenza A Viruses

  • A (H1N1)pdm09: Two A(H1N1)pdm09 viruses were antigenically characterized by HI with ferret antisera, and all were antigenically similar (reacting at titers that were within 4-fold of the homologous virus titer) to cell-propagated A/Brisbane/02/2018-like reference viruses representing the A(H1N1)pdm09 component for the 2019-20 Northern Hemisphere influenza vaccines.
  • A (H3N2): Antigenic characterization is pending.

Influenza B Viruses

  • B/Victoria: 11 B/Victoria lineage viruses, including viruses from both co-circulating sub-clades, were antigenically characterized by HI with ferret antisera, and seven (63.6%) were antigenically similar to cell-propagated B/Colorado/06/2017-like reference viruses representing the B/Victoria component for the 2019-20 Northern Hemisphere influenza vaccines.
  • B/Yamagata: Antigenic characterization is pending.


CDC assesses susceptibility of influenza viruses to the antiviral medications oseltamivir, zanamivir, peramivir, and baloxavir using next generation sequence analysis supplemented by laboratory assays. One hundred and sixty-nine viruses collected in the U.S. from September 29, 2019 to November 16, 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 169 35 73 57 4
Reduced Inhibition 0 0 0 0 0
Highly Reduced Inhibition 1 (0.6%) 1 (2.9%) 0 0 0
Peramivir Viruses Tested 169 35 73 57 4
Reduced Inhibition 0 0 0 0 0
Highly Reduced Inhibition 1 (0.6%) 1 (2.9%) 0 0 0
Zanamivir Viruses Tested 169 35 73 57 4
Reduced Inhibition 0 0 0 0 0
Highly Reduced Inhibition 0 0 0 0 0
PA Endonuclease Inhibitor
Baloxavir Viruses Tested 169 35 74 56 4
Reduced Susceptibility 0 0 0 0 0


Outpatient Illness Surveillance

ILINet

Nationwide during week 46, 2.5% 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 1.4% to 4.4% during week 46. Region 3 (Delaware, the District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia), Region 4 (Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee), Region 6 (Arkansas, Louisiana, New Mexico, Oklahoma, and Texas), and Region 9 (Arizona, California, Hawaii, and Nevada) reported a percentage of outpatient visits for ILI which is equal to or above their region-specific baselines. Regions 1, 2, 5, 7, 8, and 10 were below their region-specific baselines.

ILI Activity Map

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

During week 46, the following ILI activity levels were experienced:

  • High – Puerto Rico and seven states (Alabama, Arkansas, Georgia, Mississippi, Nevada, South Carolina, and Texas)
  • Low – New York City and 12 states (Alaska, Arizona, California, Colorado, Connecticut, Hawaii, Maryland, New Jersey, New Mexico, Oklahoma, Tennessee, and Virginia)
  • Minimal – the District of Columbia and 30 states (Delaware, Florida, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, New Hampshire, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Rhode Island, South Dakota, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming)
  • Data were insufficient to calculate an ILI activity level from the U.S. Virgin Islands and one state (Louisiana).

*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 46 the following influenza activity was reported:

  • Widespread – six states (Alabama, California, Louisiana, Massachusetts, Nevada and South Carolina)
  • Regional – nine states (Arizona, Connecticut, Florida, Georgia, Hawaii, New Mexico, Oregon, Tennessee and Texas)
  • Local – Puerto Rico and 23 states (Arkansas, Colorado, Delaware, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, Minnesota, Mississippi, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Oklahoma, Virginia, Washington, West Virginia, Wisconsin and Wyoming)
  • Sporadic – the District of Columbia, the U.S. Virgin Islands and 11 states (Alaska, Idaho, Kansas, Maine, Missouri, Montana, Ohio, Pennsylvania, South Dakota, Utah and Vermont)
  • No activity – one state (Rhode Island)
  • Guam did not report.

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 393 laboratory-confirmed influenza-associated hospitalizations were reported by FluSurv-NET sites between October 1, 2019 and November 16, 2019. The overall hospitalization rate was 1.4 per 100,000 population. The highest rate of hospitalization was among adults aged ≥65 (3.6 per 100,000 population), followed by children aged 0-4 (2.2 per 100,000 population) and adults aged 50-64 (1.4 per 100,000 population). Rates are similar to what has been seen at this time during other recent seasons. Among 393 hospitalizations, 246(62.6%) were associated with influenza A virus, 142 (36.1%) with influenza B virus, 3 (0.8%) with influenza A virus and influenza B virus co-infection, and 2 (0.5%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 28 (50.0%) were A(H1N1)pdm09 virus and 28 (50.0%) 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 November 21, 2019, 5.2% of the deaths occurring during the week ending November 9, 2019 (week 45) were due to P&I. This percentage is below the epidemic threshold of 6.2% for week 45.

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

One influenza-associated pediatric death was reported to CDC during week 46. The death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 45 (the week ending November 9, 2019).

A total of four 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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