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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 2 ending January 17, 2009

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


    During week 2 (January 11-17, 2009), influenza activity continued to slowly increase in the United States.

    • Four hundred nine (11,5 %) 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.
    • Two influenza-associated pediatric deaths were reported.
    • The proportion of outpatient visits for influenza-like illness (ILI) was below national and region-specific baseline levels.
    • One state reported widespread influenza activity, six states reported regional activity; 11 states reported local influenza activity; the District of Columbia, Puerto Rico and 30 states reported sporadic influenza activity; and two states reported no 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 Normal 11,5 % 7 of 51 641 104 1292 413 2
    New England Normal 5,2 % 1 of 6 24 5 45 8 0
    Mid-Atlantic Normal 5,3 % 2 of 3 69 11 102 23 0
    East North Central Normal 16,8 % 0 of 5 59 7 24 18 0
    West North Central Normal 4,1 % 0 of 7 31 7 49 20 0
    South Atlantic Normal 5,3 % 2 of 9 54 7 192 113 0
    East South Central Normal 1,3 % 0 of 4 2 0 0 7 0
    West South Central Normal 13,4 % 1 of 4 93 1 629 189 1
    Mountain Normal 12,1 % 1 of 8 49 56 149 11 1
    Pacific Normal 5,7 % 0 of 5 260 10 102 24 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 2 Cumulative for the Season
    No. of specimens tested 3544 66499
    No. of positive specimens (%) 409 (11,5 %) 2 450 (3,7 %)
    Positive specimens by type/subtype
      Influenza A 346 (84,6 %) 2,037(83,1 %)
                 A (H1) 44 (12,7 %) 641 (31,5 %)
                 A (H3) 4 (1,2 %) 104 (5,1 %)
                 A (unsubtyped) 298 (86,1 %) 1 292 (63,4 %)
      Influenza B 63 (15,4 %) 413 (16,9 %)

    The District of Columbia and 45 states from all nine surveillance regions have reported laboratory-confirmed influenza this season.

    INFLUENZA Virus Isolated
    View Chart Data | View Full Screen

    Antigenic Characterization:

    CDC has antigenically characterized 207 influenza viruses [142 influenza A (H1), 13 influenza A (H3) and 52 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

    All 142 influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 13 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. Seventeen influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 35 viruses belong to the B/Victoria lineage and are not related to the vaccine strain. Thirty of the 35 viruses belonging to the B/Victoria lineage were from two states.

    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, 160 influenza A (H1N1), 30 influenza A (H3N2), and 66 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). One hundred thirty-three influenza A (H1N1) and 26 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) 103 101 (98 %) 0 (0) 103 1 (1 %)
    Influenza A (H3N2) 23 0 (0) 0 (0) 23 23 (100 %)
    Influenza B 61 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 25 states have been tested for antiviral resistance to oseltamivir so far this season. In all 25 states, at least one oseltamivir-resistant influenza A (H1N1) virus has been identified. To date, all influenza A (H3N2) viruses tested are resistant to the adamantanes. Influenza activity in the United States, although increasing, remains relatively low with influenza A(H1N1) viruses predominating overall. However, the level of activity and the predominating virus has 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 2, 7,5 % of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below the epidemic threshold of 7,8 % for week 2.

    Pneumonia And Influenza Mortality
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    Influenza-Associated Pediatric Mortality

    Two influenza-associated pediatric deaths were reported to CDC during week 2 (Colorado and Texas). These deaths occurred during week 1 (week ending January 10, 2009). Since September 28, 2008, CDC has received a total of two reports of influenza-associated pediatric deaths that occurred during the current season.

    Influenza-Associated Pediatric Mortality
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    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 17, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 0.6 per 10 000 and 0.02 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.05 per 10,000, 0.08 per 10,000, and 0.2 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    During week 2, 1,5 % 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 less than the national baseline of 2,4 %. On a regional level, the percentage of visits for ILI ranged from 0,3 % to 3,7 %. The percentage of visits for ILI reported from all nine surveillance regions was below their respective region-specific baselines.

    The increase in the percentage of patient visits for ILI over the previous weeks may have been influenced by a reduction in routine health care visits during the holiday season, as has occurred in previous seasons.

    national levels of ILI and ARI
    View Chart Data |View Full Screen

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

    During week 2, the following influenza activity was reported:

    • Widespread influenza activity was reported by one state (Virginia).
    • Regional influenza activity was reported by six states (Colorado, New Hampshire, New Jersey, New York, North Carolina, and Texas).
    • Local influenza activity was reported by 11 states (Arizona, Connecticut, Florida, Hawaii, Illinois, Maryland, Massachusetts, Michigan, Montana, Pennsylvania, and Tennessee).
    • Sporadic activity was reported in the District of Columbia, Puerto Rico, and 30 states (Alabama, Alaska, Arkansas, California, Delaware, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Minnesota, Missouri, Nebraska, Nevada, New Mexico, North Dakota, Ohio, Oregon, Rhode Island, South Carolina, South Dakota, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming).
    • No influenza activity was reported by two states (Mississippi and Oklahoma).


    A description of surveillance methods is available at:

    • Page last updated January 23, 2009.

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