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

2017-2018 Influenza Season Week 10 ending March 10, 2018

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


During week 10 (March 4-10, 2018), influenza activity decreased 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 Elevated 46 of 54 15.0% 4,415 28,950 413 722 6,562 2,748 128
Region 1 Elevated 5 of 6 25.5% 249 1,723 4 43 428 38 4
Region 2 Elevated 3 of 4 22.0% 263 1,628 10 15 254 279 8
Region 3 Elevated 4 of 6 18.8% 858 3,153 10 159 867 93 12
Region 4 Elevated 8 of 8 14.0% 779 2,705 96 32 562 531 33
Region 5 Elevated 6 of 6 24.2% 692 6,349 83 48 1,149 169 24
Region 6 Elevated 4 of 5 16.3% 345 1,285 8 2 355 373 22
Region 7 Elevated 4 of 4 20.2% 76 1,134 12 3 434 34 2
Region 8 Elevated 6 of 6 15.6% 253 2,524 17 96 714 61 2
Region 9 Elevated 3 of 5 13.4% 583 7,068 166 313 1,344 769 17
Region 10 Elevated 3 of 4 17.6% 317 1,381 7 11 455 401 4

† 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 10 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 28,157 971,395
No. of positive specimens (%) 4,223 (15.0%) 195,266 (20.1%)
Positive specimens by type    
    Influenza A 1,963 (46.5%) 140,352 (71.9%)
    Influenza B 2,260 (53.5%) 54,914 (28.1%)
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, as well as the age group distribution of influenza positive tests, during the current week are summarized below.

  Week 10 Data Cumulative since
October 1, 2017 (Week 40)
No. of specimens tested 1,307 78,351
No. of positive specimens* 561 43,810
Positive specimens by type/subtype    
    Influenza A 242 (43.1%) 33,778 (77.1%)
    A(H1N1)pdm09 55 (22.7%) 4,415 (13.1%)
    H3N2 173 (71.5%) 28,950 (85.7%)
    Subtyping not performed 14 (5.8%) 413 (1.2%)
    Influenza B 319 (56.9%) 10,032 (22.9%)
     Yamagata lineage 173 (54.2%) 6,562 (65.4%)
     Victoria lineage 25 (7.8%) 722 (7.2%)
      Lineage not performed 121 (37.9%) 2,748 (27.4%)

*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
View National and Regional Level Graphs and Data | View Chart Data | View Full Screen | View PowerPoint Presentation

INFLUENZA Virus Isolated
View Interactive Application | View Full Screen

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 and hemagglutination inhibition (HI) ((i.e., hemagglutination inhibition (HI) and/or neutralization assays). These data are used to monitor for changes in circulating influenza viruses and to compare how similar currently circulating influenza viruses are to the reference viruses used for developing influenza vaccines. 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 annual vaccine effectiveness estimates are needed to determine how much protection has been provided to the population by vaccination. On February 15, 2018, interim influenza vaccine effectiveness estimates for the 2017-2018 season were released and are available here.

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 viruses for evidence of genetic changes. Viruses are classified into genetic clades/subclades based on analysis of the genetic sequences of the HA gene segments. 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. Antigenic drift is evaluated by comparing cell-propagated circulating viruses with cell-propagated reference viruses representing currently recommended vaccine components.

CDC has antigenically or genetically characterized 2,075 influenza viruses collected during October 1, 2017 – March 10, 2018, and submitted by U.S. laboratories, including 499 influenza A(H1N1)pdm09 viruses, 941 influenza A(H3N2) viruses, and 635 influenza B viruses.

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 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
View Chart Data | View Full Screen | View PowerPoint Presentation

Antiviral Resistance:

Testing of influenza A (H1N1)pdm09, influenza A (H3N2), and influenza B virus isolates for resistance to neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir) is performed at CDC using a functional assay. Additional influenza A (H1N1)pdm09 and influenza A (H3N2) viruses from clinical samples are tested for mutations known to confer oseltamivir resistance. The data summarized below combine the results of both testing methods. These samples are routinely obtained for surveillance purposes rather than for diagnostic testing of patients suspected to be infected with antiviral-resistant virus.

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.

Neuraminidase Inhibitor Resistance Testing Results on Samples Collected Since October 1, 2017






Virus Samples tested (n)

Resistant Viruses, Number (%)

Virus Samples tested (n)

Resistant Viruses, Number (%)

Virus Samples tested (n)

Resistant Viruses, Number (%)

Influenza A (H1N1)pdm09


9 (1.4)


0 (0.0)


9 (1.4)

Influenza A (H3N2)


0 (0.0)


0 (0.0)


0 (0.0)

Influenza B


0 (0.0)


0 (0.0)


0 (0.0)

On December 27, 2017, a Health Advisory was released by CDC providing: 1) a notice about increased influenza A(H3N2) activity and its clinical implications; 2) a summary of influenza antiviral drug treatment recommendations; 3) an update about approved treatment drugs and supply this season; and 4) background information for patients about influenza treatment. More information is available at https://emergency.cdc.gov/han/han00409.asp.

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 March 15, 2018, 8.5% of the deaths occurring during the week ending February 24, 2018 (week 8) were due to P&I. This percentage is above the epidemic threshold of 7.4% for week 8.

Background: Weekly mortality surveillance data include a combination of machine coded and manually coded causes of death collected from death certificates.  Percentages of deaths due to P&I are higher among manually coded records than more rapidly available machine coded records. There is currently a delay in manual coding for deaths occurring in 2018.  Because of this delay initially reported P&I percentages will be lower than those calculated from the final data.

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

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

Influenza-Associated Pediatric Mortality:

Nine influenza-associated pediatric deaths were reported to CDC during week 10. Two deaths were associated with an influenza A(H1N1)pdm09 virus and occurred during weeks 1 and 9 (the weeks ending January 6, 2018 and March 3, 2018, respectively). Two deaths were associated with an influenza A(H3) virus and occurred during weeks 6 and 9 (the weeks ending February 10, 2018 and March 3, 2018, respectively). Five deaths were associated with an influenza B virus and occurred during weeks 6 and 9 (the weeks ending February 10, 2018, March 3, 2018, respectively).

A total of 128 influenza-associated pediatric deaths have been reported for the 2017-2018 season.

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

Click on image to launch interactive tool

View Interactive Application | View Full Screen | View PowerPoint Presentation

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 25,676 laboratory-confirmed influenza-associated hospitalizations were reported between October 1, 2017 and March 10, 2018. The overall hospitalization rate was 89.9 per 100,000 population. The highest rate of hospitalization was among adults aged ≥65 years (386.2 per 100,000 population), followed by adults aged 50-64 (97.3 per 100,000 population) and children aged 0-4 years (64.9 per 100,000 population). Among 25,676 hospitalizations, 20,293 (79.0%) were associated with influenza A virus, 5,228 (20.4%) with influenza B virus, 87 (0.3%) with influenza A virus and influenza B virus co-infection, and 68 (0.3%) with influenza virus for which the type was not determined. Among those with influenza A subtype information 4,967 (85.2%) were A(H3N2) and 860 (14.8) were A(H1N1)pdm09 virus.

Among 2,565 hospitalized adults with information on underlying medical conditions, 2,384 (92.9%) had at least one reported underlying medical condition; the most commonly reported were cardiovascular disease, metabolic disorder, obesity, and chronic lung disease. Among 237 hospitalized children with information on underlying medical conditions, 130 (54.9%) had at least one underlying medical condition; the most commonly reported were asthma, neurologic disorder, and obesity. Among 200 hospitalized women of childbearing age (15-44 years) with information on pregnancy status, 60 (30.0%) were pregnant.

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.

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

Click on graph to launch interactive tool2

FluSurv-NET data are preliminary and displayed as they become available. Therefore, figures are based on varying denominators as some variables represent information that may require more time to be collected. Data are refreshed and updated weekly. Asthma includes a medical diagnosis of asthma or reactive airway disease; Cardiovascular diseases include conditions such as coronary heart disease, cardiac valve disorders, congestive heart failure, and pulmonary hypertension; does not include isolated hypertension; Chronic lung diseases include conditions such as chronic obstructive pulmonary disease, bronchiolitis obliterans, chronic aspiration pneumonia, and interstitial lung disease; Immune suppression includes conditions such as immunoglobulin deficiency, leukemia, lymphoma, HIV/AIDS, and individuals taking immunosuppressive medications; Metabolic disorders include conditions such as diabetes mellitus; Neurologic diseases include conditions such as seizure disorders, cerebral palsy, and cognitive dysfunction; Neuromuscular diseases include conditions such as multiple sclerosis and muscular dystrophy; Obesity was assigned if indicated in patient's medical chart or if body mass index (BMI) >30 kg/m2; Pregnancy percentage calculated using number of female cases aged between 15 and 44 years of age as the denominator; Renal diseases include conditions such as acute or chronic renal failure, nephrotic syndrome, glomerulonephritis, and impaired creatinine clearance; No known condition indicates that the case did not have any known high risk medical condition indicated in medical chart at the time of hospitalization.

View Interactive Application | View Full Screen | View PowerPoint Presentation

Outpatient Illness Surveillance:

Nationwide during week 10, 3.3% 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 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.)

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

On a regional level, the percentage of outpatient visits for ILI ranged from 1.9% to 4.9% during week 10. All 10 regions 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 10, the following ILI activity levels were experienced:

Click on map to launch interactive tool

*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 10, 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.