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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week ending February 7, 2009

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


    During week 5 (February 1-7, 2009), influenza activity continued to increase in the United States.

    • One thousand one hundred fifty-four (20.6%) 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.
    • One influenza-associated pediatric death was reported.
    • The proportion of outpatient visits for influenza-like illness (ILI) was above the national baseline. ILI increased in eight of the nine regions compared to the previous week, and the East North Central, East South Central, Mountain, New England, Pacific, South Atlantic, and West South Central regions reported ILI above their region-specific baselines.
    • Sixteen states reported widespread influenza activity, 16 states reported regional activity; the District of Columbia and 14 states reported local influenza activity; and Puerto Rico and four states 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 20.6 % 32 of 51 1402 196 3260 1023 4
    New England Elevated 17.6 % 5 of 6 70 15 241 51 0
    Mid-Atlantic Normal 14.2 % 3 of 3 155 16 239 82 1
    East North Central Elevated 30.3 % 3 of 5 206 36 36 53 0
    West North Central Normal 9.7 % 3 of 7 81 7 197 28 0
    South Atlantic Elevated 14.4 % 7 of 9 210 31 436 212 0
    East South Central Elevated 12.7 % 3 of 4 40 5 6 18 1
    West South Central Elevated 28.5 % 1 of 4 234 14 1667 517 1
    Mountain Elevated 11.6 % 5 of 8 75 49 280 25 1
    Pacific Elevated 5.8 % 2 of 5 331 23 158 37 0

    * 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 5 Cumulative for the Season
    No. of specimens tested 5,596 89,576
    No. of positive specimens (%) 1,154 (20.6%) 5,881 (6.6%)
    Positive specimens by type/subtype
      Influenza A 901 (78.1%) 4,858 (82.6%)
                 A (H1)              146 (16.2%)              1,402 (28.9%)
                 A (H3)              14 (1.6%)              196 (4.0%)
                 A (unsubtyped)              741 (82.2%)              3,260 (67.1%)
      Influenza B 253 (21.9%) 1,023 (17.4%)

    Since week 1 (the week ending January 10, 2009), when influenza activity began to increase nationally, influenza A (H1) viruses have predominated circulation nationally each week and for the season overall in all regions. Since week 1, 88% of subtyped influenza A viruses reported to CDC were influenza A (H1).

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

    Antigenic Characterization:

    CDC has antigenically characterized 309 influenza viruses [194 influenza A (H1), 37 influenza A (H3) and 78 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

    All 194 influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 37 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. Twenty-three influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 55 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, 240 influenza A (H1N1), 49 influenza A (H3N2), and 105 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Two hundred forty influenza A (H1N1) and 49 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) 240 236 (98.3%) 0 (0) 240 2 (0.8%)
    Influenza A (H3N2) 49 0 (0) 0 (0) 49 49 (100%)
    Influenza B 105 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 33 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. Influenza activity in the United States increased this week with influenza A (H1N1) viruses predominating overall. However, the level of activity and the relative proportion of circulating virus types or subtypes have varied by region and may vary over the course of the season. This presents challenges for the selection of antiviral medications for the treatment and chemoprophylaxis of influenza and highlights the importance of testing patients for influenza and consulting 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 5, 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 7.9% for week 5.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    One influenza-associated pediatric death was reported to CDC during week 5 (Tennessee). This death occurred during week 4 (the week ending January 31, 2009). Since September 28, 2008, CDC has received four reports of influenza-associated pediatric deaths that occurred during the current season.

    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).

    No influenza-associated hospitalizations have been reported from the New Vaccine Surveillance Network this season.

    During October 1, 2008 – January 31, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 0.8 per 10,000 and 0.04 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.07 per 10,000, 0.1 per 10,000, and 0.3 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    Nationwide during week 5, 3.2% 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 eight of the nine regions compared to the previous week and ranged from 1.1% to 6.0%. Seven of 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 (%) 3.9 2.6 2.7 1.1 3.0 4.2 6.0 1.7 3.1
    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 5, the following influenza activity was reported:

    • Widespread influenza activity was reported by 16 states (Colorado, Delaware, Georgia, Massachusetts, Nevada, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, and Virginia).
    • Regional influenza activity was reported by 16 states (Alabama, Arizona, California, Connecticut, Florida, Hawaii, Indiana, Iowa, Maine, Maryland, Michigan, Minnesota, Mississippi, Montana, Nebraska, and Wyoming).
    • Local influenza activity was reported by the District of Columbia and 14 states (Arkansas, Illinois, Kansas, Kentucky, Missouri, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Utah, Vermont, Washington, and Wisconsin).
    • Sporadic activity was reported by Puerto Rico and four states (Alaska, Idaho, Louisiana, and West Virginia).


    A description of surveillance methods is available at:

    • Page last updated February 13, 2009.

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