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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 30 ending August 1, 2009

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

    On June 11, the World Health Organization raised the pandemic alert level from Phase 5 to Phase 6 indicating that an influenza pandemic is underway.


    During week 30 (July 26-August 1, 2009), influenza activity decreased in the United States; however, there were still higher levels of influenza-like illness than is normal for this time of year.

    • Eight hundred twenty-six (20.0%) 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.
    • A total of 6,506 hospitalizations and 436 deaths associated with novel influenza A (H1N1) viruses have been reported to CDC.
    • Over 98% of all subtyped influenza A viruses being reported to CDC were novel influenza A (H1N1) viruses.
    • The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
    • One influenza-associated pediatric death was reported and was associated with a novel influenza A (H1N1) virus infection.
    • The proportion of outpatient visits for influenza-like illness (ILI) was below national and region-specific baseline levels.
    • Four states and Puerto Rico reported geographically widespread influenza activity, 11 states reported regional influenza activity, 12 states and the District of Columbia reported local influenza activity, 23 states reported sporadic influenza activity.

    National and Regional Summary of Select Surveillance Components

    HHS Surveillance Regions*
    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) Novel
    A (H1N1)
    A (unable to
    A(Sub-typing not performed) B Pediatric Deaths
    Nation Normal 20.0 % 16 of 52 8,053 2,754 28,210 744 17,866 10,736 98
    Region I Normal 9.5 % 1 of 6 583 305 2,861 13 1,692 820 4
    Region II Normal 10.0 % 3 of 3 296 228 1,803 21 2,372 714 19
    Region III Normal 22.8 % 2 of 6 1,250 219 3,982 18 1,008 1,363 10
    Region IV Normal 26.3 % 3 of 8 855 157 1,806 53 2,908 1,248 7
    Region V Normal 16.4 % 0 of 6 1,651 203 8,084 187 866 1,420 16
    Region VI Normal 29.4 % 1 of 5 828 300 3,042 7 4,476 2,666 15
    Region VII Normal 24.3 % 0 of 4 524 78 888 110 532 536 0
    Region VIII Normal 13.2 % 1 of 6 533 218 1,238 69 1,655 499 8
    Region IX Normal 21.4 % 4 of 4 1,143 726 2,422 56 1,843 779 17
    Region X Normal 32.5 % 1 of 4 390 320 2,084 210 514 691 2

    * HHS regions (Region I: CT, ME, MA, NH, RI, VT; Region II: NJ, NY, Puerto Rico, US Virgin Islands; Region III: DE, DC, MD, PA, VA, WV; Region IV: AL, FL, GA, KY, MS, NC, SC, TN; Region V: IL, IN, MI, MN, OH, WI; Region VI: AR, LA, NM, OK, TX; Region VII: IA, KS, MO, NE; Region VIII: CO, MT, ND, SD, UT, WY; Region IX: AZ, CA, Guam, HI, NV; and Region X: 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, and Puerto Rico
    ¥ The majority of influenza A viruses that cannot be sub-typed as seasonal influenza viruses are novel A (H1N1) influenza viruses upon further testing

    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 and the number positive by influenza type and subtype.

    During the 2008-09 influenza season, influenza A (H1), A (H3), and B viruses have co-circulated in the United States. On April 15 and 17, 2009, CDC confirmed the first two cases of novel influenza A (H1N1) virus infection in the United States, and this virus has been the predominant circulating influenza strain since testing and reporting of novel influenza A (H1N1) viruses by U.S. WHO collaborating laboratories began during week 17 (week ending May 2, 2009).

    Because of the extensive spread of novel influenza A (H1N1) within the United States, it has become extremely resource intensive for local and state health departments to count individual cases. In addition, since only a small proportion of persons with respiratory illness are tested for novel influenza A (H1N1) infection, confirmed and probable case counts represent a significant underestimate of the true number of novel influenza A (H1N1) cases in the U.S. As a result, CDC no longer reports individual case counts; only hospitalizations and deaths associated with confirmed novel influenza A (H1N1) are be reported each week on the CDC H1N1 influenza website

    As of August 6, 2009, 6,506 hospitalizations and 436 deaths (10 deaths in individuals 0-4 years, 90 deaths in individuals 5-24 years, 180 deaths in adults 25-49 years, 111 deaths in adults 50-64 years, 41 deaths in adults age 65 and older, and 4 deaths with unknown age) associated with novel influenza A (H1N1) virus have been identified by CDC and state and local public health departments. The results of tests performed during the current week are summarized in the table below.

    Week 30
    No. of specimens tested 4,138
    No. of positive specimens (%) 826 (20.0%)
    Positive specimens by type/subtype
      Influenza A 826 (100.0%)
                 A (novel H1N1)              556 (67.3%)
                              234 (28.3%)
                 A (unable to subtype)             30 (3.6%)
                 A (H3)              2 (0.2%)
                 A (H1)             4 (0.5%)
      Influenza B 0 (0.0%)

    During week 30, seasonal influenza A (H1) and A (H3) viruses co-circulated at low levels with novel influenza A (H1N1) viruses. Over 98% of all subtyped influenza A viruses being reported to CDC this week were novel influenza A (H1N1) viruses.

    The unusually high percentage of specimens testing positive for influenza by WHO and NREVSS collaborating laboratories remained higher during week 30 than is typically seen this time of year, and may be due in part to changes in testing practices by health care providers, triaging of specimens by public health laboratories, an increase in the number of specimens collected from outbreaks, and other factors.

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

    Antigenic Characterization:

    CDC has antigenically characterized 2,012 seasonal human influenza viruses [1,186 influenza A (H1), 201 influenza A (H3) and 625 influenza B viruses] collected by U.S. laboratories since October 1, 2008, and 248 novel influenza A (H1N1) viruses.

    All 1,186 seasonal influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). One hundred ninety-three (96%) of 201 influenza A (H3N2) viruses tested are related to the A (H3N2) vaccine component (A/Brisbane/10/2007) and eight viruses (4%) tested showed reduced titers with antisera produced against A/Brisbane/10/2007.

    All 248 novel influenza A (H1N1) viruses are related to the A/California/07/2009 (H1N1) reference virus selected by WHO as a potential candidate for novel influenza A (H1N1) vaccine.

    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. Seventy-two (12%) of 625 influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 553 (88%) 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 are 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. Antigenic characterization of novel influenza A (H1N1) viruses indicates that these viruses are antigenically and genetically unrelated to seasonal influenza A (H1N1) viruses, suggesting that little to no protection would be expected from vaccination with seasonal influenza vaccine.

    Antiviral Resistance:

    Since October 1, 2008, 1,140 seasonal influenza A (H1N1), 229 influenza A (H3N2), and 640 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Also, 1,141 seasonal influenza A (H1N1) and 232 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). Two hundred eighty novel influenza A (H1N1) viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Three hundred thirty-one novel influenza A (H1N1) 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
    Seasonal Influenza A (H1N1) 1,140 1,135 (99.6%) 0 (0) 1,141 6 (0.5%)
    Influenza A (H3N2) 229 0 (0) 0 (0) 232 232 (100%)
    Influenza B 640 0 (0) 0 (0) N/A* N/A*
    Novel Influenza A (H1N1) 280 0 (0) 0 (0) 331 331 (100%)
            *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

    The novel influenza A (H1N1) virus is susceptible to both neuraminidase inhibitor antiviral medications zanamivir and oseltamivir. It is resistant to the adamantane antiviral medications, amantadine and rimantadine. Antiviral treatment with either oseltamivir or zanamivir is recommended for all patients with confirmed, probable or suspected cases of novel influenza A (H1N1) virus infection who are hospitalized or who are at higher risk for seasonal influenza complications.

    Rare sporadic cases of oseltamivir resistant novel influenza A (H1N1) viruses have been detected worldwide. Additional information on antiviral recommendations for treatment and chemoprophylaxis of novel influenza A (H1N1) infection is available at

    Three seasonal influenza A (H1N1) viruses collected between February 8 and May 11, 2009, were found to be resistant to both oseltamivir and the adamantanes (amantadine and rimantadine). All seasonal influenza A (H1N1) viruses tested retain their sensitivity to zanamivir. The three dually-resistant viruses represent less than 1% of all seasonal influenza A (H1N1) viruses tested during the 2008-09 influenza season, and as a result, no changes to the influenza antiviral treatment or prophylaxis recommendations will be made at this time. CDC will continue to monitor trends in antiviral resistance over the summer and throughout the upcoming 2009-10 influenza season.

    Pneumonia and Influenza (P&I) Mortality Surveillance

    During week 30, 5.9% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 6.4% for week 30.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    One influenza-associated pediatric death was reported to CDC during week 30 (New York City). This death was associated with novel influenza A (H1N1) virus infection. The death reported this week occurred during week 27 (the week ending July 11, 2009). Since September 28, 2008, CDC has received 98 reports of influenza-associated pediatric deaths that occurred during the current influenza season, 30 of which were due to novel influenza A (H1N1) virus infections.

    Of the 41 children who had specimens collected for bacterial culture from normally sterile sites, 16 (39.0%) were positive; Staphylococcus aureus was identified in 10 (62.5%) of the 16 children. Four of the S. aureus isolates were sensitive to methicillin and six were methicillin resistant. Fourteen (87.5%) of the 16 children with bacterial coinfections were five years of age or older and 10 (62.5%) of the 16 children were 12 years of age or older. Ten of the 30 children with confirmed novel influenza A (H1N1) infection had a specimen collected from a normally sterile site; two of the 10 children had a positive bacterial culture (methicillin resistant S. aureus and Streptococcus constellatus). 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 New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP).

    During October 12, 2008 to July 11, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 4.42 per 10,000. Because of case identification methods utilized in this study, there is a delay from the date of hospitalization to the date of report.

    Data collection for influenza-associated hospitalizations through the NVSN has been completed for the 2008-09 influenza season. There will be no updates to this system after week 28 (the week ending July 18, 2009).

    Influenza-Associated Hospitalizations
    View Full Screen

    During April 15, 2009 to August 1, 2009, the following preliminary laboratory-confirmed overall influenza associated hospitalization rates were reported by the EIP (rates include type A, type B, and novel H1N1):

    Rates for children aged 0-23 months, 2-4 years, and 5-17 years were 2.0, 0.7, and 0.6 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years were 0.4, 0.4, and 0.4 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    *This value represents an age group-specific average influenza rate from October 1 to April 30 from the 2005-06, 2006-07, and 2007-08 influenza seasons.
    **Note: The scales for the 0-23 month and the >= 65 years age groups differ from other age groups.

    View Full Screen

    Outpatient Illness Surveillance:

    Nationwide during week 30, 1.1% 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%.

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

    On a regional level, the percentage of outpatient visits for ILI ranged from 0.3% to 2.0%. All 10 regions reported percentages of visits for ILI below their respective region-specific baselines. ILI decreased during week 30 in six of 10 regions compared to week 29.

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

    The influenza activity reported by state and territorial epidemiologists indicates geographic spread of both seasonal influenza and novel influenza A (H1N1) viruses and does not measure the severity of influenza activity.

    During week 30, the following influenza activity was reported:

    • Widespread influenza activity was reported by Puerto Rico and four states (Alaska, California, Hawaii, and Maine).
    • Regional influenza activity was reported by 11 states (Arizona, Arkansas, Florida, Georgia, Montana, Nevada, New Jersey, New York, North Carolina, Pennsylvania, and West Virginia).
    • Local influenza activity was reported by the District of Columbia and 12 states (Connecticut, Idaho, Iowa, Maryland, Michigan, Minnesota, Oregon, Rhode Island, Tennessee, Texas, Virginia, and Washington).
    • Sporadic activity was reported by 23 states (Alabama, Colorado, Delaware, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Mississippi, Missouri, Nebraska, New Hampshire, New Mexico, North Dakota, Ohio, Oklahoma, South Carolina, South Dakota, Utah, Vermont, Wisconsin, and Wyoming).


    A description of surveillance methods is available at:

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