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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 20 ending May 23, 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 20 (May 17 - 23, 2009), influenza activity decreased in the United States, however there are still higher levels of influenza-like illness than is normal for this time of year.

    • One thousand four hundred fifty (22.4%) 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 below the national baseline. Two of the 10 surveillance regions reported ILI above their region-specific baseline.
    • Four states reported geographically widespread influenza activity, 11 states reported regional activity, the District of Columbia and 13 states reported local influenza activity, 21 states reported sporadic influenza activity, and one state reported no 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 Normal 22.4 % 15 of 51 7,525 1,919 4,010 519 12,143 10,047 62
    Region I Normal 14.1 % 2 of 6 525 143 422 15 1,088 799 1
    Region II Elevated 15.1 % 2 of 2 270 121 174 13 1,121 710 8
    Region III Normal 18.2 % 3 of 6 1,219 178 286 144 622 1,349 9
    Region IV Normal 10.5 % 3 of 8 805 89 104 32 1,699 1,179 6
    Region V Normal 22.7 % 1 of 6 1,621 170 2,095 101 489 1,282 10
    Region VI Normal 6.0 % 1 of 5 716 95 184 4 3,971 2,434 12
    Region VII Normal 12.7 % 0 of 4 493 54 101 139 453 526 0
    Region VIII Normal 8.8 % 0 of 6 468 201 153 53 1,405 489 5
    Region IX Normal 42.1 % 3 of 4 1,045 600 356 13 954 622 10
    Region X Elevated 30.3 % 0 of 4 363 268 135 5 341 657 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
    ¥ 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.

    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 29, 2009, 8,975 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 20
    No. of specimens tested 6,469
    No. of positive specimens (%) 1,450 (22.4%)
    Positive specimens by type/subtype
      Influenza A 1,423 (98.1%)
                 A (H1)              24 (1.7%)
                 A (H3)              70 (4.9%)
                 A (unsubtyped)              116 (8.2%)
                 A (could not be subtyped)              117 (8.2%)
                 A (novel influenza H1N1)              1,096 (77.0%)
      Influenza B 27 (1.9%)

    During week 20, seasonal influenza A (H1), A (H3), and B viruses continue to co-circulate with novel influenza A (H1N1). Approximately 84% of all influenza viruses being reported to CDC are novel influenza A (H1N1) viruses.

    The increase in the percentage of specimens testing positive for influenza by WHO and NREVSS collaborating laboratories 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 WHO-NREVSS Regional Bar Charts| View Chart Data | View Full Screen

    Antigenic Characterization:

    CDC has antigenically characterized 1,440 seasonal human influenza viruses [877 influenza A (H1), 162 influenza A (H3) and 401 influenza B viruses] collected by U.S. laboratories since October 1, 2008, and 84 novel A (H1N1) viruses.

    All 877 influenza seasonal A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 162 influenza A (H3N2) viruses are related to the A (H3N2) vaccine component (A/Brisbane/10/2007).

    All 84 novel 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. Fifty-eight influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 343 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, 904 seasonal influenza A (H1N1), 158 influenza A (H3N2), and 444 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Nine hundred fourteen seasonal influenza A (H1N1) and 158 influenza A (H3N2) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). One hundred thirty-nine novel influenza A (H1N1) viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). One hundred nineteen 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) 904 899 (99.4%) 0 (0) 914 5 (0.5%)
    Influenza A (H3N2) 158 0 (0) 0 (0) 158 145 (100%)
    Influenza B 444 0 (0) 0 (0) N/A* N/A*
    Novel Influenza A (H1N1) 139 0 (0) 0 (0) 119 119 (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 20, 6.7% 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 7.2% for week 20.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    One influenza-associated pediatric death was reported to CDC during week 20 (Ohio); and was due to an influenza A virus. The death reported this week occurred during week 5 (the week ending February 7, 2009). Since September 28, 2008, CDC has received 62 reports of influenza-associated pediatric deaths that occurred during the current influenza season.

    Of the 31 children who had specimens collected for bacterial culture from normally sterile sites, 13 (41.9%) 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).

    During October 12, 2008 to May 16, 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 16, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 3.7 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 1.5 per 10,000, respectively.

    Influenza-Associated Pediatric Mortality
    View Full Screen

    Outpatient Illness Surveillance:

    Nationwide during week 20, 2.0% 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 0.4% to 5.3%. Two of the 10 surveillance regions reported an ILI percentage above their region specific baseline (Regions II and X). ILI increased during week 20 in six of 10 regions compared to week 19.

    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 20, the following influenza activity was reported:

    • Widespread influenza activity was reported by four states (Arizona, California, New Jersey, and Virginia).
    • Regional influenza activity was reported by 11 states (Alabama, Connecticut, Hawaii, Illinois, Maryland, Massachusetts, New York, Pennsylvania, South Carolina, Tennessee, and Texas).
    • Local influenza activity was reported by the District of Columbia and 13 states (Alaska, Georgia, Kansas, Louisiana, Maine, Michigan, Nevada, New Mexico, North Carolina, Oklahoma, Oregon, Rhode Island, and Washington).
    • Sporadic activity was reported by 21 states (Arkansas, Colorado, Delaware, Florida, Idaho, Indiana, Iowa, Kentucky, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Hampshire, North Dakota, Ohio, South Dakota, Utah, Vermont, Wisconsin, and Wyoming).
    • No influenza activity was reported by one state (West Virginia).


    A description of surveillance methods is available at:

    • Page last updated May 29, 2009.

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