CDC - Centers for Disease Control and Prevention

01/12/2018 | Press release | Distributed by Public on 01/12/2018 10:39

Weekly U.S. Influenza Surveillance Report

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

During week 1 (December 31, 2017-January 6, 2018), influenza activity increased in the United States.

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.

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

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.

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.

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 836 influenza viruses collected during October 1, 2017 - January 6, 2018, and submitted by U.S. laboratories, including 138 influenza A(H1N1)pdm09 viruses, 474 influenza A(H3N2) viruses, and 224 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.

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.




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


2 (1.2)


0 (0.0)


2 (1.2)

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

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

Based on National Center for Health Statistics (NCHS) mortality surveillance data available on January 11, 2018, 7.0% of the deaths occurring during the week ending December 23, 2017 (week 51) were due to P&I. This percentage is at the epidemic threshold of 7.0% for week 51.

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

Seven influenza-associated pediatric deaths were reported to CDC during week 1. One death was associated with an influenza A(H3) virus and occurred during week 1 (the week ending January 6, 2018). One death was associated with an influenza A(H1N1)pdm09 virus and occurred during week 1. Two deaths were associated with an influenza A virus for which no subtyping was performed and occurred during week 1. Three deaths were associated with an influenza B virus and occurred during weeks 50 and 51 (the weeks ending December 16 and December 23, 2017, respectively).

A total of 20 influenza-associated pediatric deaths have been reported for the 2017-2018 season.

Additional data can be found at:

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 6,486 laboratory-confirmed influenza-associated hospitalizations were reported between October 1, 2017 and January 6, 2018. The overall hospitalization rate was 22.7 per 100,000 population. The highest rate of hospitalization was among adults aged ≥65 years (98.0 per 100,000 population), followed by adults aged 50-64 (24.0 per 100,000 population) and children aged 0-4 years (16.0 per 100,000 population). Among 6,486 hospitalizations, 5,836 (90.0%) were associated with influenza A virus, 610 (9.4%) with influenza B virus, 21 (0.3%) with influenza A virus and influenza B virus co-infection, and 19 (0.3%) with influenza virus for which the type was not determined. Among those with influenza A subtype information, 1,346 (85.5%) were A(H3N2) and 228 (14.5%) were A(H1N1)pdm09 virus.

Among 897 hospitalized adults with information on underlying medical conditions, 704 (78.5%) had at least one reported underlying medical condition; the most commonly reported were cardiovascular disease, metabolic disorder, obesity, and chronic lung disease. Among 101 hospitalized children with information on underlying medical conditions, 58 (57.4%) had at least one underlying medical condition; the most commonly reported were asthma, neurologic disorder, and obesity. Among 80 hospitalized women of childbearing age (15-44 years) with information on pregnancy status, 24 (30.0%) were pregnant.

Additional FluSurv-NET data can be found at: and

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.

Nationwide during week 1, 5.8% 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

On a regional level, the percentage of outpatient visits for ILI ranged from 2.7% to 11.2% during week 1. All 10 regions reported percentages of outpatient visits for ILI at or above their region specific baselines.

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 1, the following ILI activity levels were experienced:

  • New York City and 26 states experienced high activity (Alabama, Arizona, Arkansas, California, Colorado, Georgia, Illinois, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Nebraska, Nevada, New Jersey, New Mexico, Ohio, Oklahoma, Oregon, South Carolina, Texas, Virginia, Washington, West Virginia, and Wyoming).
  • Puerto Rico and 10 states experienced moderate ILI activity (Idaho, Massachusetts, Michigan, New York, North Carolina, Pennsylvania, Rhode Island, South Dakota, Tennessee, and Wisconsin).
  • The District of Columbia and six states experienced low ILI activity (Alaska, Hawaii, Iowa, Maryland, Minnesota, and Vermont).
  • Eight states experienced minimal ILI activity (Connecticut, Delaware, Florida, Maine, Montana, New Hampshire, North Dakota, and Utah).

Click on map to launch interactive tool

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

During week 1, the following influenza activity was reported::

  • Widespread influenza activity was reported by 49 states (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, 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, and Wyoming).
  • Regional influenza activity was reported by Guam and one state (Hawaii).
  • Local influenza activity was reported by the District of Columbia.
  • Sporadic activity was reported by the U.S. Virgin Islands.
  • Puerto Rico did not report.

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

U.S. State and local influenza surveillance: Click on a jurisdiction below to access the latest local influenza information.

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

Public Health Agency of Canada: The most up-to-date influenza information from Canada is available at

Public Health England: The most up-to-date influenza information from the United Kingdom is available at

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: