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

2012-2013 Influenza Season Week 13 ending March 30, 2013

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


During week 13 (March 24 – 30, 2013), influenza activity decreased in the United States.

  • Viral Surveillance: Of 4,909 specimens tested and reported by collaborating laboratories, 555 (11.3%) 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 pediatric death was reported.
  • Influenza-Associated Hospitalizations: A cumulative rate for the season of 42.3 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported. Of reported hospitalizations, 50% were among adults 65 years and older.
  • Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 1.8%. This is below the national baseline of 2.2%. Three of 10 regions reported ILI at or above region-specific baseline levels. Seven states and New York City experienced low activity; 43 states experienced minimal activity, and the District of Columbia had insufficient data.
  • Geographic Spread of Influenza: Four states reported widespread influenza activity; 7 states reported regional influenza activity; the District of Columbia, Puerto Rico, and 26 states reported local influenza activity; 12 states reported sporadic influenza activity; Guam and one state reported no influenza activity, and the U.S. Virgin Islands 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 Data cumulative since September 30, 2012 (Week 40)
Out-patient ILI† % positive for flu‡ Number of jurisdictions reporting regional or widespread activity§ 2009 H1N1 A (H3) A(Subtyping not performed) B Pediatric Deaths
Nation Normal 11.3% 11 of 54 1,270 32,798 16,348 18,571 111
Region 1 Elevated 12.3% 4 of 6 62 2,369 604 369 7
Region 2 Elevated 18.0% 2 of 4 166 2,477 2,119 1,318 17
Region 3 Normal 24.7% 2 of 6 235 6,844 483 2,238 4
Region 4 Normal 15.8% 1 of 8 112 2,556 6,275 3,405 17
Region 5 Elevated 21.3% 1 of 6 103 4,855 482 1,391 21
Region 6 Normal 8.3% 0 of 5 74 2,112 3,183 3,864 23
Region 7 Normal 8.8% 0 of 4 37 1,975 198 1,000 4
Region 8 Normal 8.1% 0 of 6 209 2,925 1,931 2,952 10
Region 9 Normal 14.6% 1 of 5 212 3,935 838 1,467 7
Region 10 Normal 9.0% 0 of 4 60 2,750 235 567 1

*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

U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories located in all 50 states and Puerto Rico report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza virus type and influenza A virus subtype.

Region specific data can be found at

Week 13
No. of specimens tested 4,909
No. of positive specimens (%) 555 (11.3%)
Positive specimens by type/subtype
  Influenza A 140 (25.2%)
             2009 H1N1 16 (11.4%) 
             Subtyping not performed 88 (62.9%) 
             H3 36 (25.7%) 
  Influenza B 415 (74.8%)

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

Since the start of the season, influenza A (H3N2) viruses have predominated nationally, however in recent weeks, the proportion of influenza B viruses has been increasing. During week 13, 75% of all influenza positive specimens reported were influenza B viruses and influenza B viruses were reported more frequently than influenza A viruses all 10 regions.

Antigenic Characterization

CDC has antigenically characterized 1,970 influenza viruses [203 2009 H1N1 viruses, 1,158 influenza A (H3N2) viruses, and 609 influenza B viruses] collected by U.S. laboratories since October 1, 2012.

2009 H1N1 [203]:

  • 200 (98.5%) of the 203 2009 H1N1 viruses tested were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2012-2013 influenza vaccine for the Northern Hemisphere.
  • 3 (1.5%) of the 203 2009 H1N1 viruses tested showed reduced titers with antiserum produced against A/California/7/2009.

Influenza A (H3N2) [1,158]:

  • 1,154 (99.7%) of the 1,158 H3N2 influenza viruses tested have been characterized as A/Victoria/361/2011-like, the influenza A (H3N2) component of the 2012-2013 Northern Hemisphere influenza vaccine.
  • 4 (0.3%) of the 1,158 H3N2 viruses tested showed reduced titers with antiserum produced against A/Victoria/361/2011.

Influenza B (B/Yamagata/16/88 and B/Victoria/02/87 lineages) [609]:

  • Yamagata Lineage [426]: 426 (70.0%) of the 609 influenza B viruses tested so far this season have been characterized as B/Wisconsin/1/2010-like, the influenza B component of the 2012-2013 Northern Hemisphere influenza vaccine.
  • Victoria Lineage [183]: 183 (30.0%) of 609 influenza B viruses tested have been from the B/Victoria lineage of viruses.

Composition of the 2013-2014 Influenza Vaccine

The World Health Organization (WHO) has recommended vaccine viruses for the 2013-2014 Northern Hemisphere vaccines, and the Food and Drug Administration’s Vaccines and Related Biological Products Advisory Committee (VRBPAC) has made recommendations for the composition of the 2013-2014 influenza vaccines to be used in the United States. Both agencies recommend that trivalent vaccines contain an A/California/7/2009-like (2009 H1N1) virus, an A(H3N2) virus antigenically like the cell-propagated, or cell-grown, virus A/Victoria/361/2011 (A/Texas/50/2012), and a B/Massachusetts/2/2012-like (B/Yamagata lineage) virus. It is recommended that quadrivalent vaccines containing an additional influenza B virus contain a B/Brisbane/60/2008-like (B/Victoria lineage) virus in addition to the viruses recommended for the trivalent vaccines. These recommendations were based on global influenza virus surveillance data related to epidemiology and antigenic characteristics, serological responses to 2012-2013 seasonal vaccines, and the availability of candidate strains and reagents.

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 influenza A (H1N1) clinical samples are tested for a single mutation in the neuraminidase of the virus known to confer oseltamivir resistance (H275Y). 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, 2012

Oseltamivir Zanamivir
Virus Samples tested (n) Resistant Viruses, Number (%) Virus Samples tested (n) Resistant Viruses, Number (%)
Influenza A (H3N2) 1,734* 2 (0.1) 1,734* 0 (0.0)
Influenza B 691 0 (0.0) 691 0 (0.0)
2009 H1N1 446* 2 (0.4) 204 0 (0.0)

*Includes specimens tested in national surveillance and additional specimens tested at public health laboratories in 11 states (AZ, DE, HI, ME, MD, MI, MN, NY, PA, 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 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 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

Novel Influenza A Virus

No new human infections with novel influenza A viruses in the United States were reported to CDC during week 13.

A total of 312 infections with variant influenza viruses (308 H3N2v viruses, 3 H1N2v viruses, and 1 H1N1v virus) have been reported from 11 states since July 2012. More information about H3N2v infections can be found at

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 13, 7.4% of all deaths reported through the 122 Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 7.5% for week 13.

Pneumonia And Influenza Mortality
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Influenza-Associated Pediatric Mortality

One influenza-associated pediatric death was reported to CDC during week 13. This death was associated with an influenza A virus for which the subtype was not determined and occurred during week 12 (week ending March 23, 2013).

A total of 111 influenza-associated pediatric deaths have been reported during the 2012-2013 season from Chicago [1], New York City [4] and 36 states (AL [1], AR [4], AZ [2], CA [3], CO [5], FL [8], HI [1], IA [1], IL [1], IN [4], KS [2], KY [1], LA [1], MA [3], MD [2], ME [1], MI [5], MN [4], MS [1], NE [1], NH [3], NJ [6], NM [2], NV [1], NY [7], OH [4], OK [1], PA [1], SC [4], SD [2], TN [2], TX [15], UT [3], VA [1], WA [1], and WI [2]).

Additional data can be found at

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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 80 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; and IA, MI, OH, RI, and UT during the 2012-2013 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, 2012 and March 30, 2013, 11,798 laboratory-confirmed influenza-associated hospitalizations were reported. This is a rate of 42.3 per 100,000 population. The most affected group is those ≥65 years, accounting for 50% of reported cases. Among all hospitalizations, 9,602 (81.4%) were associated with influenza A and 2,112 (17.9%) with influenza B. There was no virus type information for 46 (0.4%) hospitalizations. Among hospitalizations with influenza A subtype information, 3,361 (96.1%) were attributed to H3 and 125 (3.6%) were attributed to 2009 H1N1. The most commonly reported underlying medical conditions among hospitalized adults were cardiovascular disease, metabolic disorders, obesity, and chronic lung disease (excluding asthma). The most commonly reported underlying medical conditions in hospitalized children were asthma, neurologic disorders, and immune suppression. Approximately 44% of hospitalized children had no identified underlying medical conditions. Among 460 hospitalized women of childbearing age (15-44 years), 141 were pregnant, including 5 pregnancies among the 27 pediatric cases in this category.

Additional FluSurv-NET data can be found at: and

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Outpatient Illness Surveillance

Nationwide during week 13, 1.8% 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 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.)

Region specific data is available at

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 0.9% to 2.3% during week 13. Three of 10 regions (1, 2, and 5) reported a proportion of outpatient visits for ILI at or above their region-specific baseline levels.

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 spring and fall weeks with little or no influenza virus circulation. Activity levels range from minimal, which would correspond to ILI activity from outpatient clinics being below the average, to high, which would correspond to ILI activity from outpatient clinics being much higher than average.

During week 13, the following ILI activity levels were experienced:

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

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*This map uses the proportion of outpatient visits to health care providers for influenza-like illness 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 disproportionately 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 are based on reports from state and territorial epidemiologists. The data presented in this map is preliminary and may change as more data is 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 13, the following influenza activity was reported:

  • Widespread influenza activity was reported by 4 states (Connecticut, Massachusetts, New York, and North Carolina).
  • Seven states reported regional influenza activity (Maine, Nevada, New Hampshire, New Jersey, Ohio, Pennsylvania, and Virginia).
  • The District of Columbia, Puerto Rico, and 26 states reported local influenza activity (Alabama, Alaska, Arizona, Arkansas, California, Georgia, Illinois, Iowa, Kentucky, Louisiana, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, North Dakota, Oklahoma, Rhode Island, South Carolina, South Dakota, Texas, Washington, West Virginia, Wisconsin, and Wyoming).
  • Twelve states reported sporadic influenza activity (Colorado, Delaware, Florida, Hawaii, Idaho, Indiana, Maryland, New Mexico Oregon, Tennessee, Utah, and Vermont).
  • Guam and one state (Kansas) reported no influenza activity.
  • The U.S. Virgin Islands did not report.

U. S. Map for Weekly Influenza Activity

Flu Activity data in XML Format | View Full Screen

View Full Screen

Additional National and International Influenza Surveillance Information

FluView Interactive: This season, 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 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.









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

Virgin Islands

Google Flu Trends: Google Flu Trends uses aggregated Google search data in a model created in collaboration with CDC to estimate influenza activity in the United States. For more information and activity estimates from the U.S. and worldwide, see

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, and the United Kingdom.

Europe: for the most recent influenza surveillance information from Europe, please see WHO/Europe at and visit 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

Health Protection Agency (United Kingdom): 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.


A description of surveillance methods is available at:

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