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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 18 ending May 9, 2009

    All data are preliminary and may change as more reports are received.
    (Due to the response to the novel influenza A (H1N1) investigation, surveillance regions were changed from Census Divisions to Department of Health and Human Services (HHS) Regions.)


    During week 18 (May 3 - 9, 2009), influenza activity remained at approximately the same level as last week in the United States, indicating that there are higher levels of influenza-like illness than is normal for this time of year.

    • One thousand four hundred fifty-four (11.9%) 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.
    • Three influenza-associated pediatric deaths were reported.
    • The proportion of outpatient visits for influenza-like illness (ILI) was above the national baseline. Three of the ten surveillance regions reported ILI above their region-specific baselines.
    • Eight states reported geographically widespread influenza activity, 14 states reported regional activity, the District of Columbia and 15 states reported local influenza activity; and 13 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 (could not be subyped)¥ A(Unsub-typed) B Pediatric Deaths
    Nation Elevated 11.9 % 22 of 51 7114 1351 1036 425 11046 9679 59
    Region I Normal 9.8 % 4 of 6 524 130 132 13 1079 794 1
    Region II Normal 14.5 % 2 of 2 265 100 101 4 1035 704 8
    Region III Normal 11.9 % 3 of 6 1144 132 71 40 610 1330 9
    Region IV Elevated 9.0 % 4 of 8 749 61 34 24 1615 1120 6
    Region V Normal 14.1 % 0 of 6 1561 128 487 170 467 1249 9
    Region VI Normal 4.7 % 2 of 5 713 83 34 4 3884 2414 12
    Region VII Normal 23.7 % 0 of 4 483 44 74 107 443 518 0
    Region VIII Normal 10.5 % 2 of 6 461 191 36 47 1358 470 5
    Region IX Elevated 11.6 % 4 of 4 890 263 12 12 257 436 8
    Region X Elevated 25.0 % 1 of 4 324 219 55 4 298 644 1

    * 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 and the District of Columbia
    ¥ >99% of influenza A viruses that cannot be sub-typed as seasonal influenza viruses are novel A (H1N1) influenza viruses upon further

    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.

    During the 2008-09 season, influenza A (H1), A (H3), and B viruses have co-circulated in the United States. On April 15 and 17, 2009, CDC confirmed that two cases of febrile respiratory illness occurring in children who reside in adjacent counties in southern California were caused by infection with a novel influenza A (H1N1) virus. As of May 15, 2009, 4,714 confirmed infections with novel influenza A (H1N1) virus have been identified by CDC and state and local public health departments. Reporting of novel influenza A (H1N1) viruses by U.S. WHO collaborating laboratories began during week 17. The results of tests performed during the current week are summarized in the table below.

    Week 18
    No. of specimens tested 12,202
    No. of positive specimens (%) 1,454 (11.9%)
    Positive specimens by type/subtype
      Influenza A 1,286 (88.4%)
                 A (H1)              178 (13.8%)
                 A (H3)              150 (11.7%)
                 A (unsubtyped)              314 (24.4%)
                 A (could not be subtyped)              203 (15.8%)
                 A (novel influenza H1N1)              441 (34.3%)
      Influenza B 168 (11.6%)

    During week 18, seasonal influenza A (H1), A (H3), and B viruses continue to co-circulate with novel influenza A (H1N1). About half of all influenza viruses being detected are novel H1N1 viruses.

    The number of specimens tested by WHO and NREVSS collaborating laboratories during week 18 increased in response to the ongoing novel influenza A (H1N1) investigations. The increase in the percentage of specimens testing positive for influenza also may be due in part to changes in testing practices by healthcare 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 Chart Data | View Full Screen

    Antigenic Characterization:

    CDC has antigenically characterized 1,317 seasonal human influenza viruses [827 influenza A (H1), 140 influenza A (H3) and 350 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

    All 827 influenza seasonal A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 140 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. Fifty-six influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 294 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. 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, 865 seasonal influenza A (H1N1), 134 influenza A (H3N2), and 424 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Eight hundred seventy-six seasonal influenza A (H1N1) and 145 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). One hundred one novel influenza A (H1N1) viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Ninety-six 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) 865 860 (99.4%) 0 (0) 876 4 (0.5%)
    Influenza A (H3N2) 134 0 (0) 0 (0) 145 145 (100%)
    Influenza B 424 0 (0) 0 (0) N/A* N/A*
    Novel Influenza A (H1N1) 101 0 (0) 0 (0) 96 96 (100%)
            *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

    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. 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. Additional information on antiviral recommendations for treatment and chemoprophylaxis of novel influenza A (H1N1) infection is available at

    In areas that continue to have seasonal influenza activity, especially those with circulation of oseltamivir-resistant seasonal human influenza A (H1N1) viruses, clinicians might prefer to use either zanamivir or a combination of oseltamivir and either rimantadine or amantadine to provide adequate empiric treatment or chemoprophylaxis for patients who might have seasonal human influenza A (H1N1) virus infection.

    Pneumonia and Influenza (P&I) Mortality Surveillance

    During week 18, 7.2% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below to the epidemic threshold of 7.4% for week 18.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    Three influenza-associated pediatric deaths were reported to CDC during week 18 (Arizona, California, and Texas); one was due to influenza A virus (rapid test positive) and two were due to influenza B virus infections. The deaths reported this week occurred between April 5 and May 2, 2009. Since September 28, 2008, CDC has received 59 reports of influenza-associated pediatric deaths that occurred during the current season.

    Of the 30 children who had specimens collected for bacterial culture from normally sterile sites, 13 (43.3%) were positive; Staphylococcus aureus was identified in eight (61.5%) of the 13 children. Three of the S. aureus isolates were sensitive to methicillin and five were methicillin resistant. Twelve of the 13 children with bacterial coinfections were five years of age or older and 10 (76.9%) of the 13 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 New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP). These two systems provide updates of surveillance data every two weeks.

    During October 12, 2008 to May 2, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 3.85 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.

    Influenza-Associated Hospitalizations
    View Full Screen

    During October 1, 2008 – May 9, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 3.6 per 10,000 and 0.7 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.4 per 10,000, 0.5 per 10,000, and 1.4 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    Nationwide during week 18, 2.6% 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 ILINet Regional Charts | View Chart Data |View Full Screen

    On a regional level, the percentage of visits for ILI ranged from 1.1% to 6.2%. Three of the ten surveillance regions reported an ILI percentage above their region specific baselines.

    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 18, the following influenza activity was reported:

    • Widespread influenza activity was reported by eight states (Arizona, California, Delaware, Georgia, New Jersey, New Mexico, Texas, and Virginia).
    • Regional influenza activity was reported by 14 states (Alabama, Alaska, Colorado, Connecticut, Florida, Hawaii, Maine, Maryland, Massachusetts, Nevada, New Hampshire, New York, Tennessee, and Utah).
    • Local influenza activity was reported by the District of Columbia and 15 states (Idaho, Illinois, Iowa, Kansas, Michigan, Montana, North Carolina, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Washington, Wisconsin, and Wyoming).
    • Sporadic activity was reported by 13 states (Arkansas, Indiana, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, Nebraska, North Dakota, Ohio, South Dakota, Vermont, and West Virginia).


    A description of surveillance methods is available at:

    • Page last updated May 15, 2009.

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