Error processing SSI file
Error processing SSI file
Error processing SSI file
Error processing SSI file
Error processing SSI file
Error processing SSI file
  • PDF formatted for print
  • Error processing SSI file

    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 9 ending March 7, 2009

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


    During week 9 (March 1-7, 2009), influenza activity in the United States remained high, but is at approximately the same level as in the previous week.

    • One thousand two hundred fifty-two (23.0%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories and reported to CDC/Influenza Division were positive for influenza.
    • The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
    • Five influenza-associated pediatric deaths were reported.
    • The proportion of outpatient visits for influenza-like illness (ILI) was above the national baseline. ILI increased nationally and in four of the nine regions compared to the previous week. All nine surveillance regions reported ILI above their region-specific baselines.
    • Thirty-five states reported widespread influenza activity, 14 states reported regional activity; the District of Columbia and one state reported local influenza activity; and Puerto Rico reported sporadic influenza activity.

    National and Regional Summary of Select Surveillance Components

    Data for current week Data cumulative for the season
    Out-patient ILI* % positive for flu† Number of jurisdictions reporting regional or widespread activity‡ A (H1) A (H3) A Unsub-typed B Pediatric Deaths
    Nation Elevated 23.0 % 49 of 51 4144 451 7886 4524 26
    New England Elevated 23.6 % 6 of 6 380 62 837 438 1
    Mid-Atlantic Elevated 23.5 % 3 of 3 418 45 801 458 4
    East North Central Elevated 61.2 % 5 of 5 674 54 97 343 1
    West North Central Elevated 23.0 % 7 of 7 611 20 648 287 0
    South Atlantic Elevated 22.3 % 8 of 9 682 49 1227 833 3
    East South Central Elevated 24.9 % 4 of 4 146 6 36 72 1
    West South Central Elevated 21.4 % 4 of 4 330 23 3381 1684 8
    Mountain Elevated 18.1 % 7 of 8 391 113 564 143 6
    Pacific Elevated 16.5 % 5 of 5 512 79 295 266 2

                   * Elevated means the % of visits for ILI is at or above the national or region-specific baseline
                   † National data is for current week; regional data is for the most recent three weeks.
                   ‡ Includes all 50 states and the District of Columbia

    U.S. Virologic Surveillance:

    WHO and NREVSS collaborating laboratories located in all 50 states and Washington D.C. report to CDC the number of respiratory specimens tested for influenza each week. The results of tests performed during the current week and cumulative totals for the season are summarized in the table below.

    Week 9 Cumulative for the Season
    No. of specimens tested 5,446 138,318
    No. of positive specimens (%) 1,252 (23.0%) 17,005 (12.3%)
    Positive specimens by type/subtype
      Influenza A 711 (56.8%) 12,481 (73.4%)
                 A (H1)              193 (27.1%)              4,144 (33.2%)
                 A (H3)              21 (3.0%)              451 (3.6%)
                 A (unsubtyped)              497 (69.9%)              7,886 (63.2%)
      Influenza B 541 (43.2%) 4,524 (26.6%)

    Since week 2 (the week ending January 17, 2009), when influenza activity increased nationally, influenza A (H1) viruses have predominated circulation nationally each week and for the season overall in all regions. However, the relative proportion of influenza B viruses is increasing nationally and regionally. While influenza activity remains at approximately the same level nationally as in the previous week, several surveillance regions reported an increase in influenza virus circulation, and four regions (East North Central, East South Central, Pacific, and West South Central) reported an equal or higher proportion of influenza B viruses compared to influenza A viruses this week.

    INFLUENZA Virus Isolated
    View WHO-NREVSS Regional Bar Charts| View Chart Data | View Full Screen

    Composition of the 2009-10 Influenza Vaccine:

    WHO has recommended vaccine strains for the 2009-10 Northern Hemisphere trivalent influenza vaccine, and FDA has made the same recommendations for the U.S. influenza vaccine. Both agencies recommend that the vaccine contain A/Brisbane/59/2007-like (H1N1), A/Brisbane/10/2007-like (H3N2), and B/Brisbane/60/2008-like (B/Victoria lineage) viruses. Only the influenza B component has been changed from the 2008-09 vaccine formulation. This recommendation was based on surveillance data related to epidemiology and antigenic characteristics, serological responses to 2008-09 vaccines, and the availability of candidate strains and reagents.

    Antigenic Characterization:

    CDC has antigenically characterized 596 influenza viruses [391 influenza A (H1), 53 influenza A (H3) and 152 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

    All 391 influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 53 influenza A (H3N2) viruses are related to the A (H3N2) vaccine component (A/Brisbane/10/2007).

    Influenza B viruses currently circulating can be divided into two distinct lineages represented by the B/Yamagata/16/88 and B/Victoria/02/87 viruses. Thirty-seven influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 115 viruses belong to the B/Victoria lineage and are not related to the vaccine strain.

    Data on antigenic characterization should be interpreted with caution given that antigenic characterization data is based on hemagglutination inhibition (HI) testing using a panel of reference ferret antisera and results may not correlate with clinical protection against circulating viruses provided by influenza vaccination.

    Annual influenza vaccination is expected to provide the best protection against those virus strains that are related to the vaccine strains, but limited to no protection may be expected when the vaccine and circulating virus strains are so different as to be from different lineages, as is seen with the two lineages of influenza B viruses.

    Antiviral Resistance:

    Since October 1, 2008, 422 influenza A (H1N1), 64 influenza A (H3N2), and 200 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Four hundred twenty-two influenza A (H1N1) and 61 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). The results of antiviral resistance testing performed on these viruses are summarized in the table below.

    Isolates tested (n) Resistant Viruses,
    Number (%)
    Isolates tested (n) Resistant Viruses, Number (%)
    Oseltamivir Zanamivir Adamantanes
    Influenza A (H1N1) 422 417 (98.8%) 0 (0) 422 3 (0.7%)
    Influenza A (H3N2) 64 0 (0) 0 (0) 61 61 (100%)
    Influenza B 200 0 (0) 0 (0) N/A* N/A*
           *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

    Influenza A (H1N1) viruses from 37 states have been tested for antiviral resistance to oseltamivir so far this season. To date, all influenza A (H3N2) viruses tested are resistant to the adamantanes and all oseltamivir-resistant influenza A (H1N1) viruses tested are sensitive to the adamantanes. Nationally, influenza A (H1N1) viruses have predominated during the season overall. While influenza activity remains at similar levels as last week nationally, several regions are reporting increasing levels of influenza virus circulation, and may have a higher relative proportion of influenza B viruses than at the national level or in other regions. This presents challenges for the selection of antiviral medications for the treatment and chemoprophylaxis of influenza. Health care providers should be aware of the possibility of increased influenza B circulation in their area, and continue test patients for influenza and consult local surveillance data when evaluating patients with acute respiratory infections during the influenza season. CDC issued interim recommendations for the use of influenza antiviral medications in the setting of oseltamivir resistance among circulating influenza A (H1N1) viruses on December 19, 2008. These interim recommendations are available at

    Pneumonia and Influenza (P&I) Mortality Surveillance

    During week 9, 7.2% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below the epidemic threshold of 8.0% for week 9.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    Five influenza-associated pediatric deaths were reported to CDC during week 9 (Illinois, New York City, Texas [2], and Utah). The deaths reported this week occurred between February 13 and February 28, 2009. The pediatric death that was reported from Florida during week 6 was later reclassified by the state as not due to influenza. Since September 28, 2008, CDC has received 26 reports of influenza-associated pediatric deaths that occurred during the current season.

    Of the 24 children who were tested for bacterial coinfections, 18 (75.0%) were positive; Staphylococcus aureus was identified in 12 (66.7%) of the 18 children. Five of the S. aureus isolates were sensitive to methicillin, six were methicillin resistant, and one had no sensitivity results reported. Fourteen (77.8%) children with bacterial coinfections were five years of age or older and twelve (66.7%) of the eighteen children were 12 years of age or older. An increase in the number of influenza-associated pediatric deaths with bacterial coinfections was first recognized during the 2006-07 influenza season. In January 2008, interim testing and reporting recommendations were released regarding influenza and bacterial coinfections in children and are available at (

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Influenza-Associated Hospitalizations

    Laboratory-confirmed influenza-associated hospitalizations are monitored in two population-based surveillance networks: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). These two systems provide updates of surveillance data every two weeks.

    During October 12, 2008 to February 21, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 0.22 per 10,000. Due to case identification methods utilized in this study, a delay exists from the date of hospitalization to the date of report.

    Influenza-Associated Hospitalizations
    View Full Screen

    During October 1, 2008 – February 28, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 1.5 per 10,000 and 0.2 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.1 per 10,000, 0.2 per 10,000, and 0.5 per 10,000, respectively.”

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    Nationwide during week 9, 3.5% 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 2.4%.

    national levels of ILI and ARI
    View Sentinel Providers Regional Charts | View Chart Data |View Full Screen

    On a regional level, the percentage of visits for ILI increased in four of the nine regions (East North Central, East South Central, Pacific, and West North Central) compared to the previous week and ranged from 2.1% to 4.9%. All nine surveillance regions reported ILI percentages above their region specific baselines.

    Region New England Mid- Atlantic East North Central West North Central South Atlantic East South Central West South Central Mountain Pacific
    Reported ILI (%) 2.1 3.8 3.1 2.8 3.5 4.0 4.9 2.1 3.6
    Region-Specific Baseline 1.5 2.9 1.9 1.7 2.2 2.5 4.8 1.5 3.0

    Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists:

    During week 9, the following influenza activity was reported:

    • Widespread influenza activity was reported by 35 states (Alabama, Alaska, Arizona, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Indiana, Iowa, Kansas, Mississippi, Montana, 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, Vermont, Virginia, Washington, and West Virginia).
    • Regional influenza activity was reported by 14 states (Arkansas, California, Illinois, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, Wisconsin, and Wyoming).
    • Local influenza activity was reported by the District of Columbia and one state (Utah).
    • Sporadic activity was reported by Puerto Rico.


    A description of surveillance methods is available at:

    • Page last updated March 13, 2009.

    Error processing SSI file
    Error processing SSI file
    Error processing SSI file
    Error processing SSI file