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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 53 ending January 3, 2009

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


    During week 53 (December 28, 2008 – January 3, 2009), influenza activity in the United States remained at approximately the same level as in the previous week.

    • Seventy-nine (3,2 %) 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.
    • The proportion of outpatient visits for influenza-like illness (ILI) was below national and region-specific baseline levels.
    • Four states reported regional activity; 11 states reported local influenza activity; the District of Columbia and 30 states reported sporadic influenza activity; and five 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 3,2 % 4 of 51 368 42 542 221 1
    New England Normal 2 % 1 of 6 6 2 19 5 0
    Mid-Atlantic Normal 2,3 % 1 of 3 20 4 30 15 0
    East North Central Normal 5,6 % 0 of 5 13 2 17 8 0
    West North Central Normal 1,7 % 0 of 7 15 3 26 10 1
    South Atlantic Normal 3,9 % 2 of 9 23 1 126 85 0
    East South Central Normal 1,3 % 0 of 4 1 0 0 4 0
    West South Central Normal 6,4 % 0 of 4 64 1 198 71 0
    Mountain Normal 4,4 % 0 of 8 15 22 51 6 0
    Pacific Normal 3,3 % 0 of 5 211 7 75 17 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 53 Cumulative for the Season
    No. of specimens tested 2 488 52 199
    No. of positive specimens (%) 79 (3,2 %) 1 173 (2,3 %)
    Positive specimens by type/subtype
      Influenza A 70 (88,6 %) 952 (81,2 %)
                 A (H1) 10 (14,3 %) 368 (38,7 %)
                 A (H3) 5 (7,1 %) 42 (4,4 %)
                 A (unsubtyped) 55 (78,6 %) 542 (56,9 %)
      Influenza B 9 (11,4 %) 221 (18,8 %)

    The District of Columbia and 42 states from all nine surveillance regions have reported laboratory-confirmed influenza this season. Three states account for 794 (67,7 %) of the 1 173 reported influenza viruses.

    INFLUENZA Virus Isolated
    View Chart Data | View Full Screen

    Antigenic Characterization:

    CDC has antigenically characterized 110 influenza viruses [68 influenza A (H1), 13 influenza A (H3) and 29 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

    All 68 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. Nine influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 20 viruses belong to the B/Victoria lineage and are not related to the vaccine strain. Seventeen of the 20 viruses belonging to the B/Victoria lineage were from two states.

    Data on antigenic characterization should be interpreted with caution given that:

    1. Few U.S. isolates are available for testing because of limited influenza activity thus far.
    2. The majority of viruses antigenically characterized to date come from only three states and may not be nationally representative.
    3. 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, 88 influenza A (H1N1), 14 influenza A (H3N2), and 40 influenza B viruses from 25 states have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Eighty-eight influenza A (H1N1) and 14 influenza A (H3N2) viruses from 23 states 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) 88 86 (98 %) 0 (0) 88 0 (0 %)
    Influenza A (H3N2) 14 0 (0) 0 (0) 14 14 (100 %)
    Influenza B 40 0 (0) 0 (0) N/A* N/A*
    *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

    With low levels of influenza activity thus far in the 2008-09 season in the United States, overall numbers of virus specimens and the number of states that have submitted specimens for testing is limited. The limited number and geographic diversity of specimens tested for antiviral resistance, as well as the uncertainty regarding which influenza virus types or subtypes will predominate during the season, make it too early to make an accurate determination of the prevalence of influenza viruses resistant to oseltamivir nationally or regionally at this time. CDC has solicited a representative sample of viruses from WHO collaborating laboratories in the United States, and more specimens are expected as influenza activity increases.

    Pneumonia and Influenza (P&I) Mortality Surveillance

    During week 53, 7 % of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below the epidemic threshold of 7,6 % for week 53.

    Pneumonia And Influenza Mortality
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    Influenza-Associated Pediatric Mortality

    No influenza-associated pediatric deaths were reported during week 53. A total of one influenza-associated pediatric death occurring during the 2008-09 season has been reported.

    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 3, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 0.3 per 10 000 and 0.01 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.03 per 10,000, 0.05 per 10,000, and 0.2 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    During week 53, 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 less than the national baseline of 2,4 %. On a regional level, the percentage of visits for ILI ranged from 0,2 % to 4,6 %. All nine surveillance regions reported percentages of visits for ILI below their respective region-specific baselines.

    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 53, the following influenza activity was reported:

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


    A description of surveillance methods is available at:

    • Page last updated January 05, 2008.

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