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

2017-2018 Influenza Season Week 46 ending November 18, 2017

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


During week 46 (November 12-18, 2017), influenza activity increased in the United States.

National and Regional Summary of Select Surveillance Components

HHS Surveillance Regions* Data for current week Data cumulative since October 1, 2017 (week 40)
Out-patient ILI Number of jurisdictions reporting regional or widespread activity§ % respiratory specimens positive for flu in clinical laboratories A(H1N1)pdm09 A (H3) A (Subtyping not Performed)
B Victoria lineage B Yamagata lineage B lineage not performed Pediatric Deaths
Influenza test results from public health laboratories only
Nation Normal 9 of 54 5.3% 136 1,193 28 6 105 85 5
Region 1 Elevated 1 of 6 1.7% 2 21 0 1 1 0 0
Region 2 Elevated 0 of 4 1.8% 2 23 3 0 4 1 0
Region 3 Normal 0 of 6 1.1% 6 40 0 0 3 0 0
Region 4 Elevated 4 of 8 7.3% 49 133 3 0 7 26 2
Region 5 Normal 0 of 6 1.9% 3 167 6 0 19 5 0
Region 6 Elevated 3 of 5 6.0% 34 143 1 1 7 11 1
Region 7 Normal 0 of 4 4.3% 6 68 12 0 13 5 0
Region 8 Normal 0 of 6 3.4% 4 138 0 1 16 1 0
Region 9 Normal 1 of 5 5.8% 21 340 3 2 18 35 2
Region 10 Normal 0 of 4 4.6% 12 120 0 1 17 1 0

† 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, 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 46 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 15,584 114,844
No. of positive specimens (%) 832 (5.3%) 3,764 (3.3%)
Positive specimens by type    
    Influenza A 616 (74.0%) 2,722 (72.3%)
    Influenza B 216 (26.0%) 1,042 (27.7%)
INFLUENZA Virus Isolated
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The results of tests performed by public health laboratories, as well as the age group distribution of influenza positive tests, during the current week are summarized below.

  Week 46 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 937 7,642
No. of positive specimens* 241 1,554
Positive specimens by type/subtype    
    Influenza A 201 (83.4%) 1,358 (87.4%)
    A(H1N1)pmd09 27 (13.4%) 136 (10.0%)
    H3 157 (78.1%) 1,193 (87.8%)
    Subtyping not performed 17 (8.5%) 28 (2.1%)
    Influenza B 40 (16.6%) 196 (12.6%)
     Yamagata lineage 16 (40.0%) 105 (53.6%)
     Victoria lineage 1 (2.5%) 6 (3.1%)
      Lineage not performed 23 (57.5%) 85 (43.4%)

*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
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INFLUENZA Virus Isolated
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Novel Influenza A Virus:

One human infection with a novel influenza A virus was reported from Iowa during week 46. This person was infected with an influenza A(H1N1) variant (H1N1v) virus and reported direct contact to swine during the week preceding illness onset. This patient was an adult < 50 years of age, was not hospitalized, and has fully recovered from their illness. No human-to-human transmission has been identified. This is the first H1N1v virus infection detected in the United States in 2017. This brings the total number of reported H1N1v infections in the United States since 2005 to 21.

A total of 66 variant virus infections have been reported to CDC during 2017. Sixty-one of these were influenza A(H3N2) variant (H3N2v) viruses (Delaware [1], Maryland [39], Michigan [2], Nebraska [1], North Dakota [1], Ohio [15], Pennsylvania [1], and Texas [1]), one was influenza A(H1N1) variant (H1N1v) (Iowa [1]), and four were influenza A(H1N2) variant (H1N2v) viruses (Colorado [1] and Ohio [3]). Six of these 66 infections have resulted in hospitalization; all have recovered.

Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be more fully understood and appropriate public health measures can be taken. Additional information on influenza in swine, variant influenza infection in humans, and strategies to interact safely with swine can be found at http://www.cdc.gov/flu/swineflu/index.htm. .

Influenza Virus Characterization:

CDC characterizes influenza viruses through one or more tests including genomic sequencing, hemagglutination inhibition (HI) and/or neutralization assays. These data are used to compare how similar currently circulating influenza viruses are to the reference viruses used for developing influenza vaccines, and to monitor for changes in circulating influenza viruses. Antigenic and genetic characterization of circulating influenza viruses can give an indication of the influenza vaccine's ability to produce an immune response against the wide array of influenza viruses co-circulating, but vaccine effectiveness estimates are needed to determine how much protection has been 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. Viruses can be classified into genetic groups/clades based on analysis of their HA gene segments using phylogenetics and key amino acid changes (Klimov Vaccine 2012).A representative subset of influenza-positive surveillance samples are antigenically characterized. However, a proportion of influenza A(H3N2) viruses lack sufficient hemagglutination titers for antigenic characterization using hemagglutination inhibition assays. Therefore, CDC selects a representative subset of influenza A(H3N2) viruses for antigenic characterization using the virus neutralization focus reduction assay to assess the ability of various antisera to neutralize infectivity of the test viruses.

It is important to monitor circulating influenza viruses for evidence of genetic changes. However, genetic changes do not always result in antigenic change. Extensive genetic variation may exist in circulating viruses, with no evidence of substantial antigenic drift. Close monitoring of influenza viruses is required to better assess the potential impact on public health.

Genetic Characterization

During May 21 – November 18, 2017, 3,298 influenza positive specimens were collected and reported by public health laboratories in the United States (Figure, left). CDC genetically characterized 665 influenza viruses [91 influenza A(H1N1)pdm09, 415 influenza A(H3N2), and 159 influenza B viruses] collected by U.S. laboratories.

Influenza A Viruses

Influenza B Viruses

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 issued to the laboratories request that, if available, 2 influenza A (H1N1), 2 A influenza (H3N2), and 2 influenza B viruses be submitted every other week. Because of this, the number of each virus type/subtype characterized should be approximately equal. In the figure below, the results of tests performed by public health labs are presented on the left and sequence results by genetic group of specimens submitted to CDC are presented on the right.

Genetic Characterization
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Antigenic Characterization

During May 21 – November 18, 2017, CDC antigenically characterized 351 influenza viruses [73 influenza A(H1N1)pdm09, 145 influenza A(H3N2), and 133 influenza B viruses] collected by U.S. laboratories. Antigenic similarity is evaluated by comparing cell-propagated circulating viruses with cell-propagated reference viruses representing the recommended vaccine components of the Northern Hemisphere 2017-18 vaccine.

Influenza A Virus [218]

Influenza B Virus [133]

Antiviral Resistance:

A total of 625 specimens collected during May 21-November 18, 2017, were tested for resistance to the influenza neuraminidase inhibitor antiviral medications currently approved for use against seasonal influenza: oseltamivir, zanamivir, and peramivir. A total of 92 influenza A(H1N1)pdm09, 383 influenza A(H3N2), and 150 influenza B viruses were found to be sensitive to all three antiviral medications. An additional one influenza A(H1N1)pmd09 virus was tested for resistance to oseltamivir and peramivir and 14 influenza A(H3N2) viruses were tested for resistance to oseltamivir and zanamivir. All were found to be sensitive to both antiviral medications.

The majority of recently circulating influenza viruses are susceptible to the neuraminidase inhibitor antiviral medications, oseltamivir, zanamivir, and peramivir; however, rare sporadic instances of oseltamivir-resistant and peramivir-resistant influenza A (H1N1)pdm09 viruses and oseltamivir-resistant influenza A (H3N2) viruses have been detected worldwide. 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.

Pneumonia and Influenza (P&I) Mortality Surveillance:

Based on National Center for Health Statistics (NCHS) mortality surveillance data available on November 22, 2017, 5.6% of the deaths occurring during the week ending November 4, 2017 (week 44) were due to P&I. This percentage is below the epidemic threshold of 6.4% for week 44.

Background: Weekly mortality surveillance data include a combination of machine coded and manually coded causes of death collected from death certificates. There is a backlog of data requiring manual coding within NCHS mortality surveillance data. The percentages of deaths due to P&I are higher among manually coded records than more rapidly available machine coded records and may result in initially reported P&I percentages that are lower than percentages calculated from final data. Efforts continue to reduce and monitor the number of records awaiting manual coding.

Region and state-specific data are available at http://gis.cdc.gov/grasp/fluview/mortality.html.

INFLUENZA Virus Isolated
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Influenza-Associated Pediatric Mortality:

Five influenza-associated pediatric deaths were reported to CDC during week 46.

Two deaths were associated with an influenza A (H3) virus and occurred during weeks 45 and 46 (the weeks ending November 11 and November 18, 2017, respectively). One death was associated with an influenza A (H1N1)pdm09 virus and occurred during week 44 (the week ending November 4, 2017). One death was associated with an influenza A virus for which no subtyping was performed and occurred during week 44.

A total of five influenza-associated pediatric deaths have been reported for the 2017-2018 season.

One death that occurred during the 2016-2017 season was associated with an influenza A (H3) virus and occurred during week 15 (the week ending April 15, 2017). This death brings the total number of reported influenza-associated pediatric deaths occurring during that season to 110.

Additional data can be found at: http://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.

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Influenza-Associated Hospitalizations:

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza-related hospitalizations in children younger than 18 years of age (since the 2003-2004 influenza season) and adults (since the 2005-2006 influenza season).

The FluSurv-NET covers more than 70 counties in the 10 Emerging Infections Program (EIP) states (CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN) and additional Influenza Hospitalization Surveillance Project (IHSP) states. The IHSP began during the 2009-2010 season to enhance surveillance during the 2009 H1N1 pandemic. IHSP sites included IA, ID, MI, OK and SD during the 2009-2010 season; ID, MI, OH, OK, RI, and UT during the 2010-2011 season; MI, OH, RI, and UT during the 2011-2012 season; IA, MI, OH, RI, and UT during the 2012-2013 season; and MI, OH, and UT during the 2013-2014, 2014-15, 2015-16, 2016-17, and 2017-18 seasons.

Data gathered are used to estimate age-specific hospitalization rates on a weekly basis, and describe characteristics of persons hospitalized with influenza illness. The rates provided are likely to be an underestimate as influenza-related hospitalizations can be missed, either because testing is not performed, or because cases may be attributed to other causes of pneumonia or other common influenza-related complications.

A total of 400 laboratory-confirmed influenza-associated hospitalizations were reported between October 1, 2017 and November 18, 2017. The overall hospitalization rate was 1.4 per 100,000 population. The highest rate of hospitalization was among adults aged ≥65 years (5.1 per 100,000 population), followed by adults aged 50-64 (1.7 per 100,000 population) and children aged 0-4 years (1.0 per 100,000 population). Among 400 hospitalizations, 330 (82.5%) were associated with influenza A virus, 65 (16.3%) with influenza B virus, 2 (0.5%) with influenza A virus and influenza B virus co-infection, and 3 (0.8%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 87 (91.6%) were A(H3N2) and 8 (8.4%) were A(H1N1)pdm09 virus.

Additional FluSurv-NET data can be found at: http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.

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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.

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Outpatient Illness Surveillance:

Nationwide during week 46, 2.0% 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 below 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.)

Additional ILINet data, including national, regional and select state-level data, are available at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

national levels of ILI and ARI
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On a regional level, the percentage of outpatient visits for ILI ranged from 0.7% to 4.5% during week 46. Regions 1, 2, 4 and 6 reported percentages of outpatient visits for ILI at or above their region specific baselines.

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.

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

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*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 can be found at https://gis.cdc.gov/grasp/fluview/FluView8.html.

During week 46, the following influenza activity was reported::

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.









District of Columbia





















New Hampshire

New Jersey

New Mexico

New York

North Carolina

North Dakota





Rhode Island

South Carolina

South Dakota







West Virginia



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.