CDC: 2011-2012 Flu Season

2011-2012 Flu Season

This page provides information about the 2011-2012 influenza season, including information about the season’s vaccines, vaccination recommendations and disease activity.

This year’s recommendations are issued in a shortened format because there are relatively few changes from the 2010-2011 recommendations.

Questions & Answers

CDC and Flu

2011-2012 Influenza Season Week 47 ending November 26, 2011

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


During week 47 (November 20-26, 2011), influenza activity remained low in the United States.

National and Regional Summary of Select Surveillance Components

HHS Surveillance Regions* Data for current week Data cumulative since October 2, 2011 (Week 40)
Out-patient ILI† % of respiratory specimens positive for flu‡ Number of jurisdictions reporting regional or widespread activity§ A (H3) 2009 A (H1N1) A(Subtyping not performed) B Pediatric Deaths
Nation Normal 1.9% 0 of 54 79 8 104 75 0
Region 1 Normal 0.0% 0 of 6 1 0 0 3 0
Region 2 Normal 0.4% 0 of 4 2 0 2 1 0
Region 3 Normal 0.3% 0 of 6 2 0 3 0 0
Region 4 Normal 3.4% 0 of 8 8 1 74 48 0
Region 5 Normal 1.5% 0 of 6 10 2 1 6 0
Region 6 Normal 0.3% 0 of 5 4 0 2 8 0
Region 7 Normal 0.9% 0 of 4 6 0 3 1 0
Region 8 Normal 2.3% 0 of 6 20 1 11 0 0
Region 9 Normal 0.5% 0 of 5 18 4 4 6 0
Region 10 Normal 0.7% 0 of 4 8 0 4 2 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 report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza type and subtype. The results of tests performed during the current week are summarized in the table below.
Week 47
No. of specimens tested 2,130
No. of positive specimens (%) 40 (1.9%)
Positive specimens by type/subtype
  Influenza A 33 (82.5%)
             A (2009 H1N1) 1 (3.0%) 
             A (subtyping not performed) 26 (78.8%) 
             A (H3) 6 (18.2%) 
  Influenza B 7 (17.5%)

INFLUENZA Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 21 influenza viruses [one 2009 influenza A (H1N1), 16 influenza A (H3N2) viruses, and four influenza B viruses] collected by U.S. laboratories since October 1, 2011.
Influenza A (H1N1) [1]
  • One virus was characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2011-12 influenza vaccine for the Northern Hemisphere.
Influenza A (H3N2) [16]
  • All 16 were characterized as A/Perth/16/2009-like, the influenza A (H3N2) component of the 2011-12 influenza vaccine for the Northern Hemisphere.
Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages) [4]:
  • Victoria Lineage [3]: Three of four B viruses tested belong to the B/Victoria lineage of viruses and were characterized as B/Brisbane/60/2008-like, the recommended influenza B component for the 2011-12 Northern Hemisphere influenza vaccine.
  • Yamagata Lineage [1]: One of four B viruses tested belongs to the B/Yamagata lineage of viruses.
It is too early in the influenza season to determine how well the seasonal vaccine and circulating strains will match.

Antiviral Resistance:

Testing of 2009 influenza A (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 of the sustained high levels of resistance, data from adamantane resistance testing are not presented in the table below.

Neuraminidase Inhibitor Resistance Testing Results on Samples Collected Since October 1, 2011

Oseltamivir Zanamivir
Virus Samples tested (n) Resistant Viruses, Number (%) Virus Samples tested (n) Resistant Viruses, Number (%)
Influenza A (H3N2) 24 0 (0.0) 24 0 (0.0)
Influenza B 2 0 (0.0) 2 0 (0.0)
Influenza A (2009 H1N1) 5 0 (0.0) 5 0 (0.0)
All viruses tested for the 2011-2012 season since October 1, 2011 have been susceptible to the neuraminidase inhibitor antiviral medications oseltamivir and zanamivir as were the majority of viruses tested last season; however, rare sporadic cases of oseltamivir resistant 2009 influenza A (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 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 47, 6.4% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 7.1% for week 47.

Pneumonia And Influenza Mortality
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Influenza-Associated Pediatric Mortality:

Two influenza-associated pediatric deaths were reported to CDC during week 47. One death was associated with an influenza A (H3) virus and one was associated with a 2009 H1N1 virus. These deaths occurred during the 2010-11 influenza season and bring the total number of reported pediatric deaths occurring during that season to 120. No influenza-associated pediatric deaths occurring during the 2011-12 season have been reported to CDC.

Influenza-Associated Pediatric Mortality
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Influenza-Associated Hospitalizations:

The Influenza Hospitalization Network (FluSurv-NET) conducts population-based surveillance for laboratory-confirmed influenza related hospitalizations in children (persons younger than 18 years) and adults. The network covers more than 80 counties in the 10 Emerging Infections Program (EIP) states (CA, CO, CT, GA, MD, MN, NM, NY, OR, and TN) and four additional states (MI, OH, RI and UT). FluSurv-NET estimated hospitalization rates will be updated weekly starting later this season.

Outpatient Illness Surveillance:

Nationwide during week 47, 1.4% 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 2.4%. (ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and cough and/or sore throat.)

national levels of ILI and ARI
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On a regional level, the percentage of outpatient visits for ILI ranged from 0.6% to 2.1% during week 47. 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 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 intense, which would correspond to ILI activity from outpatient clinics being much higher than average.
During week 47, the following ILI activity levels were experienced:
  • All 50 states and New York City experienced minimal ILI activity (Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, 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).
  • Data were insufficient to calculate an ILI activity level from the District of Columbia.
Click on map to launch interactive tool
Click on map to launch interactive tool

*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 intensity of influenza activity.
During week 47, the following influenza activity was reported:
  • Local influenza activity was reported by one state (Massachusetts).
  • Sporadic influenza activity was reported by the District of Columbia, Guam, and 29 states (Alaska, Arkansas, California, Colorado, Connecticut, Florida, Hawaii, Illinois, Indiana, Iowa, Kentucky, Louisiana, Minnesota, Mississippi, Nebraska, Nevada, New Mexico, New York, North Dakota, Ohio, Pennsylvania, Rhode Island, South Carolina, Texas, Utah, Washington, West Virginia, and Wisconsin).
  • No influenza activity was reported by the U.S. Virgin Islands and 21 states (Alabama, Arizona, Delaware, Georgia, Idaho, Kansas, Maine, Maryland, Michigan, Missouri, Montana, New Hampshire, New Jersey, North Carolina, Oklahoma, Oregon, South Dakota, Tennessee, Vermont, Virginia, and Wyoming).
  • Puerto Rico did not report.

Additional National and International Influenza Surveillance Information

U.S. State and local influenza surveillance: Click on a jurisdiction below to access the latest local influenza information.
Distribute Project: Additional information on the Distribute syndromic surveillance project, developed and piloted by the International Society for Disease Surveillance (ISDS) now working in collaboration with CDC, to enhance and support Emergency Department (ED) surveillance, is available at Web Site Icon
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 Web Site Icon
Europe: for the most recent influenza surveillance information from Europe, please see WHO/Europe at Web Site Icon and visit the European Centre for Disease Prevention and Control at External Web Site Icon
Public Health Agency of Canada: The most up-to-date influenza information from Canada is available at Web Site Icon
World Health Organization FluNet: Additional influenza surveillance information from participating WHO member nations is available at External Web Site Icon