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27 Dec 2013

2013-2014 Influenza Season, Week 51 ending December 21, 2013 (US CDC/FluView, December 27 2013, edited)

[Source: US Centers for Disease Control and Prevention (CDC), FluView, full page: (LINK). Edited.]

2013-2014 Influenza Season, Week 51 ending December 21, 2013

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



During week 51 (December 15-21, 2013), influenza activity continued to increase in the United States.

  • Viral Surveillance: Of 6,813 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 51, 1,639 (24.1%) were positive for influenza.
  • Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
  • Influenza-Associated Pediatric Deaths: One influenza-associated pediatric death that occurred during the 2012-2013 season was reported.
  • Influenza-associated Hospitalizations: A cumulative rate for the season of 4.3 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.
  • Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 3.0%, above the national baseline of 2.0%. Eight regions reported ILI at or above region-specific baseline levels. Six states experienced high ILI activity; eight states experienced moderate ILI activity; six states experienced low ILI activity; 28 states experienced minimal ILI activity, and the District of Columbia, New York City, and two states had insufficient data.
  • Geographic Spread of Influenza: The geographic spread of influenza in 10 states was reported as widespread; Guam and 23 states reported regional influenza activity; 12 states reported local influenza activity; the District of Columbia, Puerto Rico, and four states reported sporadic influenza activity, and the U.S. Virgin Islands and one state did not report.

A description of surveillance methods is available at:


National and Regional Summary of Select Surveillance Components

[HHS Surveillance Regions* - Data for current week: Out-patient ILI† - % positive for flu‡ - Number of jurisdictions reporting regional or widespread activity§ - Data cumulative since September 29, 2013 (Week 40):  2009 H1N1 - A (H3) - A(Subtyping not performed) – B - Pediatric Deaths]

  • Nation – Elevated - 24.1% - 34 of 54 - 4,396 – 284 - 3,637 – 457 – 4
  • Region 1 – Elevated - 10.6% - 3 of 6 – 87 – 15 – 16 – 8 – 0
  • Region 2 – Normal - 8.2% - 2 of 4 – 169 – 18 – 86 – 28 – 0
  • Region 3 – Elevated - 13.0% - 2 of 6 – 343 – 20 – 4 – 14 – 0
  • Region 4 – Elevated - 22.6% - 7 of 8 – 598 – 6 - 2,083 – 272 – 1
  • Region 5 – Elevated - 24.9% - 4 of 6 – 485 – 27 – 111 – 13 – 1
  • Region 6 – Elevated - 23.1% - 4 of 5 – 663 – 45 – 945 – 57 – 2
  • Region 7 – Elevated - 17.1% - 3 of 4 – 281 – 11 – 24 – 11 – 0
  • Region 8 – Elevated - 25.5% - 3 of 6 - 1,027 – 27 – 290 – 27 – 0
  • Region 9 – Normal - 14.0% - 2 of 5 – 318 – 68 – 54 – 18 – 0
  • Region 10 – Elevated - 26.0% - 4 of 4 – 425 – 47 – 24 – 9 – 0


(*) HHS regions (Region 1 CT, ME, MA, NH, RI, VT; Region 2: NJ, NY, Puerto Rico, US Virgin Islands; Region 3: DE, DC, MD, PA, VA, WV; Region 4: AL, FL, GA, KY, MS, NC, SC, TN; Region 5: IL, IN, MI, MN, OH, WI; Region 6: AR, LA, NM, OK, TX; Region 7: IA, KS, MO, NE; Region 8: CO, MT, ND, SD, UT, WY; Region 9: AZ, CA, Guam, HI, NV; and Region 10: AK, ID, OR, WA).

(†) Elevated means the % of visits for ILI is at or above the national or region-specific baseline

(‡) National data are for current week; regional data are for the most recent three weeks

(§) Includes all 50 states, the District of Columbia, Guam, Puerto Rico, and U.S. Virgin Islands


U.S. Virologic Surveillance

WHO and NREVSS collaborating laboratories located in all 50 states, Puerto Rico, and Washington D.C. report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza virus type and influenza A virus subtype. The results of tests performed during the current week are summarized in the table below.

Region specific data can be found at


Week 51

  • No. of specimens tested - 6,813
    • No. of positive specimens (%) - 1,639 (24.1%)
      • Positive specimens by type/subtype:
        • Influenza A - 1,610 (98.2%)
          • 2009 H1N1 - 915 (56.8%)
          • H3 - 22 (1.4%)
          • Subtyping not performed - 673 (41.8%)
        • Influenza B - 28 (1.8%)


View National and Regional Level Graphs and Data


Antigenic Characterization*

CDC has antigenically characterized 408 influenza viruses [354 2009 H1N1 viruses, 46 influenza A (H3N2) viruses, and 8 influenza B viruses] collected by U.S. laboratories since October 1, 2013 by hemagglutination inhibition (HI).

  • 2009 H1N1 [354]:
    • All 354 2009 H1N1 viruses tested were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2013-2014 Northern Hemisphere influenza vaccine.
  • Influenza A (H3N2) [46]:
    • All 46 influenza A (H3N2) viruses tested have been characterized as A/Texas/50/2012-like, the influenza A (H3N2) component of the 2013-2014 Northern Hemisphere influenza vaccine.
  • Influenza B [8]: Three (38%) of the eight influenza B viruses tested belong to B/Yamagata/16/88-lineage and the remaining five (62%) influenza B viruses tested belong to B/Victoria/02/87 lineage.
    • Yamagata Lineage [3]:
      • Three influenza B/Yamagata-lineage viruses were characterized as B/ Massachusetts/2/1012-like, which is included as an influenza B component of the 2013-2014 Northern Hemisphere trivalent and quadrivalent influenza vaccines.
    • Victoria Lineage [5]:
      • Five influenza B/Victoria-lineage viruses were characterized as B/Brisbane/60/2008-like, which is included as an influenza B component of the 2013-2014 Northern Hemisphere quadrivalent influenza vaccine.


(*) For more information see the section on antigenic characterization in the MMWR “Update: Influenza Activity — United States and Worldwide, May 19–September 28, 2013”.


Antiviral Resistance

Testing of 2009 H1N1, influenza A (H3N2), and influenza B virus isolates for resistance to neuraminidase inhibitors (oseltamivir and zanamivir) is performed at CDC using a functional assay. Additional 2009 H1N1 and influenza A (H3N2) clinical samples are tested for mutations of the virus 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 2009 influenza A (H1N1) and A (H3N2) viruses (the adamantanes are not effective against influenza B viruses). As a result, data from adamantane resistance testing are not presented below.


Neuraminidase Inhibitor Resistance Testing Results on Samples Collected Since October 1, 2013

[Oseltamivir: Virus Samples tested (n) - Resistant Viruses, Number (%) – Zanamivir: Virus Samples tested (n) - Resistant Viruses, Number (%)]

  • Influenza A (H3N2) – 56 - 0 (0.0) – 56 - 0 (0.0)
  • Influenza B – 12 - 0 (0.0) – 12 - 0 (0.0)
  • 2009 H1N1 - 768* - 10 (1.3) – 476 - 0 (0.0)


(*) Includes specimens tested in national surveillance and additional specimens tested at public health laboratories in 10 states (AZ, CO, FL, HI, MD, MI, NY, TX, WA, and WI) who share testing results with CDC.

The majority of currently circulating influenza viruses are susceptible to the neuraminidase inhibitor antiviral medications, oseltamivir and zanamivir; however, rare sporadic cases of oseltamivir-resistant 2009 H1N1 and A (H3N2) viruses have been detected worldwide. Antiviral treatment with oseltamivir or zanamivir is recommended as early as possible for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at greater risk for serious influenza-related complications. Additional information on recommendations for treatment and chemoprophylaxis of influenza virus infection with antiviral agents is available at


Pneumonia and Influenza (P&I) Mortality Surveillance

During week 51, 6.7% of all deaths reported through the 122 Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 6.9% for week 51.


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Influenza-Associated Pediatric Mortality

One influenza-associated pediatric death that occurred during the 2012-2013 season was reported to CDC during week 51 and was associated with an influenza B virus. This death brings the total number of reported pediatric deaths for that season to 171. A total of four influenza-associated pediatric deaths for the 2013-2014 season have been reported.

Additional data can be found at


View Interactive Application


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, 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 season.

Data gathered are used to estimate age-specific hospitalization rates on a weekly basis, and describe characteristics of persons hospitalized with severe 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.

Between October 1, 2013 and December 21, 2013, 1,156 laboratory-confirmed influenza-associated hospitalizations were reported. This is a rate of 4.3 per 100,000 population.

Among cases, 1,071 (92.6%) were influenza A, 72 (6.2%) were influenza B, 6 (0.5%) were influenza A and B co-infection, and 7 (0.6%) had no virus type information.

Among those with influenza A subtype information, 7 (1.6%) were H3 and 420 (97.7%) were 2009 H1N1.

The most commonly reported underlying medical conditions among adults were obesity, metabolic disorders, cardiovascular disease, and chronic lung disease (excluding asthma).

The most commonly reported underlying medical conditions in children were asthma, obesity, neurologic disorders, and cardiovascular disease.

Approximately 45.5% of hospitalized children had no identified underlying medical conditions. Among 50 hospitalized women of childbearing age (15-44 years), eleven were pregnant.

Additional FluSurv-NET data can be found at: and


View Interactive Application


View Interactive Application


Outpatient Illness Surveillance

Nationwide during week 51, 3.0% 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 above the national baseline of 2.0%. (ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and cough and/or sore throat.)


View National and Regional Level Graphs and Data

On a regional level, the percentage of outpatient visits for ILI ranged from 0.9% to 7.1% during week 51. Eight regions (Regions 1, 3, 4, 5, 6, 7, 8, and 10) reported a proportion of outpatient visits for ILI at or above their region-specific baseline level.

Region specific data is available at


ILINet 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 51, the following ILI activity levels were experienced:

  • Six states experienced high ILI activity (Alabama, Louisiana, Mississippi, Missouri, Oklahoma, and Texas).
  • Eight states experienced moderate ILI activity (Colorado, Indiana, Kansas, North Carolina, Pennsylvania, South Carolina, Tennessee, and Utah).
  • Six states experienced low ILI activity (Alaska, Arizona, Delaware, Florida, Georgia, and Virginia).
  • Twenty-eight states experienced minimal ILI activity (California, Connecticut, Hawaii, Illinois, Idaho, Iowa, Maine, Maryland, Massachusetts, Michigan, Minnesota, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Vermont, Washington, West Virginia, Wisconsin, and Wyoming).
  • Data were insufficient to calculate an ILI activity level from the District of Columbia, New York City, and two states (Arkansas and Kentucky).


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 51, the following influenza activity was reported:

  • Widespread influenza activity was reported by 10 states (Alabama, Alaska, Kansas, Louisiana, Massachusetts, New York, Pennsylvania, Texas, Virginia, and Wyoming).
  • Regional influenza activity was reported by Guam and 23 states (Arkansas, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kentucky, Maine, Michigan, Minnesota, Mississippi, Nebraska, Nevada, New Jersey, Oklahoma, Oregon, South Carolina, Tennessee, Utah, and Washington).
  • Local influenza activity was reported by 12 states (Arizona, California, Delaware, Maryland, Missouri, Montana, New Hampshire, New Mexico, North Dakota, Ohio, South Dakota, and Wisconsin).
  • Sporadic influenza activity was reported by the District of Columbia, Puerto Rico, and four states (Hawaii, Rhode Island, Vermont, and West Virginia).
  • The U.S. Virgin Islands and one state (North Carolina) did not report.

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