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Weekly U.S. Influenza Surveillance Report

FluView: A Weekly Influenza Surveillance Report Prepared by the Influenza Division

2018-2019 Influenza Season Week 50 ending December 15, 2018


All data are preliminary and may change as more reports are received.

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.,

Synopsis:

Influenza activity in the United States is increasing. Influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B viruses continue to co-circulate. Below is a summary of the key influenza indicators for the week ending December 15, 2018:

  • Viral Surveillance: Influenza A viruses have predominated in the United States since the beginning of October. Influenza A(H1N1)pdm09 viruses are predominating in most areas of the country. However, in the most recent three weeks, influenza A(H3) viruses were most commonly reported in the southeastern United States (HHS Region 4). The percentage of respiratory specimens testing positive for influenza in clinical laboratories is increasing.
    • Virus Characterization: The majority of influenza viruses characterized antigenically and genetically are similar to the cell-grown reference viruses representing the 2018–2019 Northern Hemisphere influenza vaccine viruses.
    • Antiviral Resistance: All viruses tested show susceptibility to the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir).
  • Influenza-like Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) increased to 2.7%, which is above the national baseline of 2.2%. Eight of 10 regions reported ILI at or above their region-specific baseline level.
    • ILI State Activity Indictor Map: Two states experienced high ILI activity; New York City and nine states experienced moderate ILI activity; Puerto Rico and 11 states experienced low ILI activity; the District of Columbia and 28 states experienced minimal ILI activity.
  • Geographic Spread of Influenza: The geographic spread of influenza in Guam and six states was reported as widespread; 18 states reported regional activity; 19 states reported local activity; and the District of Columbia, Puerto Rico, the U.S. Virgin Islands and seven states reported sporadic activity.
  • Influenza-associated Hospitalizations A cumulative rate of 2.9 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. The highest hospitalization rate is among children younger than 5 years (7.7 hospitalizations per 100,000 population).
  • Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I;) was below the system-specific epidemic threshold in the National Center for Health Statistics (NCHS) Mortality Surveillance System.
  • Influenza-associated Pediatric Deaths: One influenza-associated pediatric death was reported to CDC during week 50.

National and Regional Summary of Select Surveillance Components

HHS Surveillance Regions* Data for current weekPredominant flu virus reported by public health laboratories for the most recent three weeks
Out-patient ILI Number of jurisdictions reporting regional or widespread activity§ % respiratory specimens positive for flu in clinical laboratories
Nation Elevated 25 of 54 11.0% Influenza A(H1N1)pdm09
Region 1 Elevated 5 of 6 4.5% Influenza A(H1N1)pdm09
Region 2 Elevated 2 of 4 3.7% Influenza A(H1N1)pdm09
Region 3 Elevated 3 of 6 2.7% Influenza A(H1N1)pdm09
Region 4 Elevated 5 of 8 14.7% Influenza A(H3)
Region 5 Normal 1 of 6 4.1% Influenza A(H1N1)pdm09
Region 6 Normal 2 of 5 6.1% Influenza A(H1N1)pdm09
Region 7 Elevated 1 of 4 4.8% Influenza A(H1N1)pdm09
Region 8 Elevated 0 of 6 6.6% Influenza A(H1N1)pdm09
Region 9 Elevated 4 of 5 9.3% Influenza A(H1N1)pdm09
Region 10 Elevated 2 of 4 2.9% Influenza A(H1N1)pdm09

*https://www.hhs.gov/about/agencies/iea/regional-offices/index.html
† Elevated means the % of visits for ILI is at or above the national or region-specific baseline
§ Includes all 50 states, the District of Columbia, Guam, Puerto Rico, and U.S. Virgin Islands
‡ National data are for current week; regional data are for the most recent three weeks


    U.S. Virologic Surveillance:

    WHO and NREVSS collaborating laboratories, which include both public health and clinical laboratories located in all 50 states, Puerto Rico, Guam, and the District of Columbia, report to CDC the total number of respiratory specimens tested for influenza and the number positive for influenza by virus type. In addition, public health laboratories also report the influenza A subtype (H1 or H3) and influenza B lineage information of the viruses they test and the age or age group of the persons from whom the specimens were collected.

    Additional virologic data, including national, regional and select state-level data, can be found at: http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html. Age group proportions and totals by influenza subtype reported by public health laboratories can be found at: http://gis.cdc.gov/grasp/fluview/flu_by_age_virus.html.

    The results of tests performed by clinical laboratories are summarized below.

      Week 50 Data Cumulative since
    September 30, 2018 (Week 40)
    No. of specimens tested 24,176 242,938
    No. of positive specimens (%) 2,666 (11.0%) 9,582 (3.9%)
    Positive specimens by type    
        Influenza A 2,522 (94.6%) 8,540 (89.1%)
        Influenza B 144 (5.4%) 1,042 (10.9%)
         

    INFLUENZA Virus Isolated
    View National and Regional Level Graphs and Data | View Chart Data | View Full Screen | View PowerPoint Presentation


    The results of tests performed by public health laboratories are summarized below.

      Week 50 Data Cumulative since
    September 30, 2018 (Week 40)
    No. of specimens tested 1,154 11,587
    No. of positive specimens* 408 2,086
    Positive specimens by type/subtype    
             Influenza A 399 (97.8%) 1,957 (93.8%)
                (H1N1)pdm09 308 (81.9%) 1,455 (79.0%)
                 H3N2 68 (18.1%) 386 (21.0%)
                 Subtyping not performed 23 116
           Influenza B 9 (2.2%) 129 (6.2%)
                Yamagata lineage 3 (42.9%) 78 (72.9%)
                Victoria lineage 4 (57.1%) 29 (27.1%)
                Lineage not performed 2 22

    *The percent of specimens testing positive for influenza is not reported because public health laboratories often receive samples that have already tested positive for influenza at a clinical laboratory and therefore percent positive would not be a valid indicator of influenza activity. Additional information is available at http://www.cdc.gov/flu/weekly/overview.htm.

    INFLUENZA Virus Isolated
    View National and Regional Level Graphs and Data | View Chart Data | View Full Screen | View PowerPoint Presentation


    Influenza Virus Characterization:

    Close monitoring of influenza viruses is required to better assess the potential impact on public health. CDC characterizes influenza viruses through one or more tests including genomic sequencing, hemagglutination inhibition (HI) and/or neutralization assays based Focus Reduction assays (FRA). These data are used to compare how similar currently circulating influenza viruses are to the reference viruses used for developing new influenza vaccines and to monitor evolutionary changes that continually occur in influenza viruses circulating in humans. Antigenic and genetic characterization of circulating influenza viruses gives an indication of the influenza vaccine's ability to induce an immune response against the wide array of influenza viruses that are co-circulating every season. However, annual vaccine effectiveness estimates are needed to determine how much protection was provided to the population by vaccination.

    For nearly all influenza-positive surveillance samples received at CDC, next-generation sequencing is performed to determine the genetic identity of circulating influenza viruses and to monitor the evolutionary trajectory of viruses circulating in our population. Virus gene segments are classified into genetic clades/subclades based on phylogenetic analysis. However, genetic changes that classify the clades/subclades do not always result in antigenic changes. “Antigenic drift” is a term used to describe gradual antigenic change that occurs as viruses evolve changes to escape host immune pressure. 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 has antigenically or genetically characterized 264 influenza viruses collected September 30, 2018 – December 15, 2018, and submitted by U.S. laboratories, including 163 influenza A(H1N1)pdm09 viruses, 70 influenza A(H3N2) viruses, and 31 influenza B viruses.

    Influenza A Viruses

    • A (H1N1)pdm09: Phylogenetic analysis of the HA genes from 163 A(H1N1)pdm09 viruses showed that all belonged to clade 6B.1. Seventy-nine A(H1N1)pdm09 viruses were antigenically characterized, and 78 (98.7%) were antigenically similar (analyzed using HI with ferret antisera) to A/Michigan/45/2015 (6B.1), a cell-propagated A/Michigan/45/2015-like reference virus representing the A(H1N1)pdm09 component for the 2018-19 Northern Hemisphere influenza vaccines.
    • A (H3N2): Phylogenetic analysis of the HA genes from 70 A(H3N2) viruses revealed extensive genetic diversity with multiple clades/subclades co-circulating. The HA genes of circulating viruses belonged to clade 3C.2a (n=29), subclade 3C.2a1 (n=37) or clade 3C.3a (n=4). Six A(H3N2) viruses were antigenically characterized by FRA with ferret antisera, and all 6 (100%) A(H3N2) viruses tested were well-inhibited (reacting at titers that were within 4-fold of the homologous virus titer) by ferret antisera raised against A/Singapore/INFIMH-16-0019/2016 (3C.2a1), a cell-propagated A/Singapore/INFIMH-16-0019/2016-like reference virus representing the A(H3N2) component of 2018-19 Northern Hemisphere influenza vaccines.

    Influenza B Viruses

    • B/Victoria: Phylogenetic analysis of 8 B/Victoria-lineage viruses indicate that all HA genes belonged to genetic clade V1A, however genetic subclades which are antigenically distinct have emerged. Genetic subclades which are antigenically distinct include viruses with a two amino acid deletion (162-163) in the HA protein (V1A.1, previously abbreviated as V1A-2Del) and viruses with a three amino acid deletion (162-164) in the HA protein (abbreviated as V1A-3Del). Three B/Victoria lineage viruses were antigenically characterized and all 3 (100%) reacted poorly (at titers that were 8-fold or greater reduced compared with the homologous virus titer) with ferret antisera raised against cell-propagated B/Colorado/06/2017-like V1A.1 reference virus, and belonged to clade V1A.
    • B/Yamagata: Phylogenetic analysis of 23 influenza B/Yamagata-lineage viruses indicate that the HA genes belonged to clade Y3. A total of 16 influenza B/Yamagata-lineage viruses were antigenically characterized, and all were antigenically similar to cell-propagated B/Phuket/3073/2013 (Y3), the reference vaccine virus representing the influenza B/Yamagata-lineage component of the 2018-19 Northern Hemisphere quadrivalent vaccines.

    The majority of U.S. viruses submitted for characterization come from state and local public health laboratories. Due to Right Size Roadmap considerations, specimen submission guidance to laboratories is that, if available, 2 influenza A(H1N1)pdm09, 2 influenza A(H3N2), and 2 influenza B viruses be submitted every other week. Therefore, the numbers of each virus type/subtype characterized should be more balanced across subtypes/lineages but will not reflect the actual proportion of circulating viruses. In the figure below, the results of tests performed by public health labs are shown on the left and CDC sequence results (by genetic clade/subclade) are shown on the right.


    Genetic Characterization
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      Antiviral Resistance:

      Testing of influenza A(H1N1)pdm09, influenza A(H3N2), and influenza B viruses for resistance to the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) is performed at CDC using next-generation sequencing analysis and/or a functional assay. Neuraminidase sequences of viruses are inspected to detect the presence of amino acid substitutions,previously associated with reduced or highly reduced inhibition by any of three neuraminidase inhibitors. In addition, a subset of viruses are tested using the neuraminidase inhibition assay with three neuraminidase inhibitors. The level of neuraminidase activity inhibition is reported using the thresholds recommended by the World Health Organization Expert Working Group of the Global Influenza Surveillance and Response System (GISRS)These samples are routinely obtained for surveillance purposes rather than for diagnostic testing of patients suspected to be infected with an antiviral-resistant virus.

      Reporting of baloxavir susceptibility testing for the 2018-2019 influenza season will begin later this season. More information regarding influenza antiviral drug resistance can be found here.

      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, data from adamantane resistance testing are not presented below.

      Assessment of Virus Susceptibility to Neuraminidase Inhibitors Using Next-Generation Sequencing Analysis and/or Neuraminidase Inhibition Assay

      Type/Subtype or Lineage Inhibition of Neuraminidase Activity by Antiviral Drug
      Oseltamivir Peramivir Zanamivir
      Virus Tested (n) Reduced, Number (%) Highly Reduced, Number (%) Virus Tested (n) Reduced, Number (%) Highly Reduced, Number (%) Virus Tested (n) Reduced, Number (%) Highly Reduced, Number (%)
      Total Viruses 259 0 (0%) 0 (0%) 259 0 (0%) 0 (0%) 259 0 (0%) 0 (0%)
      A(H1N1)pdm09 161 0 (0%) 0 (0%) 161 0 (0%) 0 (0%) 161 0 (0%) 0 (0%)
      A(H3N2) 68 0 (0%) 0 (0%) 68 0 (0%) 0 (0%) 68 0 (0%) 0 (0%)
      B/Victoria 8 0 (0%) 0 (0%) 8 0 (0%) 0 (0%) 8 0 (0%) 0 (0%)
      B/Yamagata 22 0 (0%) 0 (0%) 22 0 (0%) 0 (0%) 22 0 (0%) 0 (0%)

      Antiviral treatment as early as possible is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for serious influenza-related complications. Additional information on recommendations for treatment and chemoprophylaxis of influenza virus infection with antiviral agents is available at:http://www.cdc.gov/flu/antivirals/index.htm.




      Outpatient Illness Surveillance:

      Nationwide during week 50, 2.7% 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 2.2%.(ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and cough and/or sore throat.)

      On a regional level, the percentage of outpatient visits for ILI ranged from 1.1% to 3.9% during week 50.  Eight of 10 regions (Regions 1, 2, 3, 4, 7, 8, 9 and 10) reported a percentage of outpatient visits for ILI at or above their region-specific baseline.

      Additional data on medically attended visits for ILI for current and past seasons and by geography (national, HHS region, or select states) are available on FluView Interactive http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

      national levels of ILI and ARI
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      ILINet State Activity Indicator Map:

      Data collected in ILINet are used to produce a measure of ILI activity* by state. Activity levels are based on the percent of outpatient visits in a state due to ILI and are compared to the average percent of ILI visits that occur during weeks with little or no influenza virus circulation. Activity levels range from minimal, which would correspond to ILI activity from outpatient clinics being below, or only slightly above, the average, to high, which would correspond to ILI activity from outpatient clinics being much higher than average.

      The ILI Activity Indicator Map displays state-specific activity levels for multiple seasons and allows a visual representation of relative activity from state to state. More information is available on FluView Interactive at https://gis.cdc.gov/grasp/fluview/main.html.

      During week 50, the following ILI activity levels were experienced:

      • Two states (Colorado and Georgia) experienced high ILI activity.
      • New York City and nine states (Alabama, Arizona, Arkansas, Kentucky, Louisiana, Missouri, New Jersey, South Carolina, and Virginia) experienced moderate ILI activity.
      • Puerto Rico and 11 states (California, Massachusetts, Minnesota, Mississippi, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, Texas, and Utah) experienced low ILI activity.
      • The District of Columbia and 28 states (Alaska, Connecticut, Delaware, Florida, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Maine, Maryland, Michigan, Montana, Nebraska, Nevada, New Hampshire, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Tennessee, Vermont, Washington, West Virginia, Wisconsin, and Wyoming) experienced minimal ILI activity.

      *This map uses the proportion of outpatient visits to health care providers for ILI to measure the ILI activity level within a state. It does not, however, measure the extent of geographic spread of flu within a state. Therefore, outbreaks occurring in a single city could cause the state to display high activity levels.
      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.
      Data displayed in this map are based on data collected in ILINet, whereas the State and Territorial flu activity map is based on reports from state and territorial epidemiologists. The data presented in this map are preliminary and may change as more data are received.
      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.



      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. Additional data displaying the influenza activity reported by state and territorial epidemiologists for the current and past seasons are available on FluView Interactive at https://gis.cdc.gov/grasp/fluview/FluView8.html

      During week 50, the following influenza activity was reported:

      • Widespread influenza activity was reported by Guam and six states (Alabama, California, Delaware, Georgia, Massachusetts, and New York).
      • Regional influenza activity was reported by 18 states (Arizona, Connecticut, Florida, Idaho, Kentucky, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, North Carolina, Ohio, Oregon, Pennsylvania, Rhode Island, Texas, Vermont, and Virginia).
      • Local influenza activity was reported by 19 states (Arkansas, Colorado, Illinois, Indiana, Iowa, Kansas, Louisiana, Maryland, Michigan, Minnesota, Mississippi, Missouri, Montana, North Dakota, Oklahoma, South Carolina, Tennessee, Utah, and Wyoming).
      • Sporadic influenza activity was reported by the District of Columbia, Puerto Rico, the U.S. Virgin Islands and seven states (Alaska, Hawaii, Maine, South Dakota, Washington, West Virginia, and Wisconsin).

      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 835 laboratory-confirmed influenza-associated hospitalizations were reported between October 1, 2018 and December 15, 2018. The overall hospitalization rate was 2.9 per 100,000 population. The highest rate of hospitalization was among children aged 0-4 (7.7 per 100,000 population), followed by adults aged ≥65 (6.6 per 100,000 population) and adults aged 50-64 (3.4 per 100,000 population). Among 835 hospitalizations, 701 (84.0%) were associated with influenza A virus, 115 (13.8%) with influenza B virus, 11 (1.3%) with influenza A virus and influenza B virus co-infection, and 8 (1.0%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 123 (73.7%) were A(H1N1)pdm09 virus and 44 (26.3%) were A(H3N2).

       Additional FluSurv-NET data displaying hospitalization rates for the current and past seasons and different age groups, as well as data on patient characteristics (such as influenza virus type, demographic, and clinical information), are available on FluView Interactive at: http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.


      Click on graph to launch interactive tool

      Data from the Influenza Hospitalization Surveillance Network (FluSurv-NET), a population-based surveillance for influenza related hospitalizations in children and adults in 13 U.S. states. Cumulative incidence rates are calculated using the National Center for Health Statistics’ (NCHS) population estimates for the counties included in the surveillance catchment area.

      View Interactive Application | View Full Screen | View PowerPoint Presentation


      Pneumonia and Influenza (P&I) Mortality Surveillance:

      Based on National Center for Health Statistics (NCHS) mortality surveillance data available on December 20, 2018, 6.0% of the deaths occurring during the week ending December 8, 2018 (week 49) were due to P&I. This percentage is below the epidemic threshold of 6.7% for week 49.

      Additional pneumonia and influenza mortality data for current and past seasons and by geography (national, HHS region, or state) are available at on FluView Interactive http://gis.cdc.gov/grasp/fluview/mortality.html. Data displayed on the regional and state-level are aggregated by the state of residence of the decedent.

      INFLUENZA Virus Isolated
      View Regional and State Level Data | View Chart Data | View Full Screen | View PowerPoint Presentation


      Influenza-Associated Pediatric Mortality:

      One influenza-associated pediatric death was reported to CDC during week 50. This death was associated with an influenza A virus for which no subtyping was performed and occurred during week 49 (the week ending December 8, 2018).

      A total of seven influenza-associated pediatric deaths have been reported for the 2018-2019 season.

      Additional information on influenza-associated pediatric deaths including basic demographics, underlying conditions, bacterial co-infections, and place of death for the current and past seasons, is available on FluView Interactive http://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.

      Click on image to launch interactive tool

      View Interactive Application | View Full Screen | View PowerPoint Presentation


      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.

      U.S. State and local influenza surveillance: Click on 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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