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

2017-2018 Influenza Season Week 42 ending October 21, 2017

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


During week 42 (October 15-21, 2017), influenza activity was low 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 1 of 54 2.5% 27 279 12 1 24 15 1
Region 1 Normal 0 of 6 0.7% 1 6 0 0 0 0 0
Region 2 Normal 0 of 4 1.2% 1 2 0 0 0 0 0
Region 3 Normal 0 of 6 0.8% 0 14 0 0 2 0 0
Region 4 Normal 0 of 8 5.8% 12 45 3 0 1 3 1
Region 5 Normal 0 of 6 0.9% 3 39 3 0 7 0 0
Region 6 Normal 0 of 5 2.8% 2 27 1 0 4 1 0
Region 7 Normal 0 of 4 0.6% 1 10 2 0 2 0 0
Region 8 Normal 0 of 6 0.9% 3 33 0 1 4 1 0
Region 9 Normal 1 of 5 1.4% 2 92 3 0 4 9 0
Region 10 Normal 0 of 4 2.1% 2 11 0 0 0 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 42 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 11,136 39,647
No. of positive specimens (%) 274 (2.5%) 898 (2.3%)
Positive specimens by type    
    Influenza A 188 (68.6%) 616 (68.6%)
    Influenza B 86 (31.4%) 282 (31.4%)
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 42 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 571 1,989
No. of positive specimens* 81 358
Positive specimens by type/subtype    
    Influenza A 78 (96.3%) 318 (88.8%)
    A(H1N1)pmd09 3 (3.8%) 27 (8.5%)
    H3 69 (88.5%) 279 (87.7%)
    Subtyping not performed 6 (7.7%) 12 (3.8%)
    Influenza B 3 (3.7%) 40 (11.2%)
     Yamagata lineage 2 (66.7%) 24 (60.0%)
     Victoria lineage 0 (0%) 1 (2.5%)
      Lineage not performed 1 (33.3%) 15 (37.5%)

*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 Ohio during week 42. This report is an infection with an influenza A (H1N2) variant (H1N2v) virus in a child (younger than 18 years) that occurred in August 2017. Exposure to swine in a fair setting during the week preceding illness onset was reported. This patient became ill with respiratory symptoms, was not hospitalized, and has fully recovered. No human-to-human transmission was identified.

A total of 62 variant virus infections have been reported to CDC during 2017. Fifty-nine of these were influenza A (H3N2) variant (H3N2v) viruses (Delaware [1], Maryland [39], Michigan [1], North Dakota [1], Ohio [15], Pennsylvania [1], and Texas [1]) and three were influenza A (H1N2) variant (H1N2v) viruses (Ohio [3]).

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 the circulating virus populations, 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 – October 21, 2017, 2,068 influenza positive specimens were collected and reported by public health laboratories in the United States (Figure, left). CDC genetically characterized 456 influenza viruses [60 influenza A(H1N1)pdm09, 271 influenza A(H3N2), and 125 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 – October 21, 2017, CDC antigenically characterized 256 influenza viruses [43 influenza A(H1N1)pdm09, 126 influenza A(H3N2), and 87 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 [169]

Influenza B Virus [87]

Antiviral Resistance:

During May 21-October 14, 2017, 421 specimens (57 influenza A(H1N1)pdm09, 243 influenza A(H3N2), and 121 influenza B viruses) collected in the United States were tested for susceptibility to the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir). All tested viruses were sensitive to all three recommended 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 October 26, 2017, 5.4% of the deaths occurring during the week ending October 7, 2017 (week 40) were due to P&I. This percentage is below the epidemic threshold of 6.1% for week 40.

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:

The first influenza-associated pediatric death of the 2017-2018 season occurred and was reported to CDC during week 42 (the week ending October 21, 2017). This death was associated with an influenza A (H1N1)pdm09 virus.

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 select counties in the Emerging Infections Program (EIP) states and Influenza Hospitalization Surveillance Project (IHSP) states. FluSurv-NET estimated hospitalization rates will be updated weekly starting later this season.

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.

Outpatient Illness Surveillance:

Nationwide during week 42, 1.3% 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 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 0.5% to 2.1% during week 42. All 10 regions reported a proportion of outpatient visits for ILI below their region-specific baseline levels.

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 42, 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.

During week 42, 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.