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    Weekly Report: Influenza Summary Update

    2008-2009 Influenza Season Week 33 ending August 22, 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. The novel influenza A (H1N1) virus now will be referred to as “2009 influenza A (H1N1) virus.”


    During week 33 (August 16-22, 2009), influenza activity remained stable or continued to decline in most areas of the U.S. However, activity appears to be increasing in the Southeast.
    A total of 8,843 hospitalizations and 556 deaths associated with 2009 influenza A (H1N1) viruses have been reported to CDC an increase from 7,983 hospitalizations and 522 deaths from the prior week

    • During week 33:
      • 804 (18.0%) 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.
      • 99% of all subtyped influenza A viruses being reported to CDC were 2009 influenza A (H1N1) viruses.
      • The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
      • Five influenza-associated pediatric deaths were reported and all were associated with a 2009 influenza A (H1N1) virus infection.
      • The proportion of outpatient visits for influenza-like illness (ILI) was below the national baseline. Region IV reported ILI above their region-specific baseline.
      • Two states and Puerto Rico reported geographically widespread influenza activity, 13 states reported regional influenza activity, 10 states and the District of Columbia reported local influenza activity, 24 states reported sporadic influenza activity, one state reported no influenza activity, and Guam and the U.S. Virgin Islands did not report.

    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) 2009 A (H1N1) A (unable to sub-type)¥ A(Subtyping not performed) B Pediatric Deaths
    Nation Normal 18.0 % 16 of 52 8,207 4,010 36,872 799 19,438 10,848 110
    Region I Normal 8.8 % 1 of 6 583 305 2,933 13 1,704 820 4
    Region II Normal 3.5 % 2 of 3 297 228 1,823 21 2,388 714 19
    Region III Normal 20.0 % 0 of 6 1,250 220 4,308 20 1,029 1,363 10
    Region IV Elevated 21.7 % 7 of 8 918 441 4,429 69 3,341 1,327 10
    Region V Normal 14.7 % 0 of 6 1,661 209 8,314 198 878 1,422 18
    Region VI Normal 22.8 % 1 of 5 830 307 3,452 7 4,744 2,666 16
    Region VII Normal 15.8 % 1 of 4 535 85 1,078 109 535 537 0
    Region VIII Normal 13.9 % 0 of 6 533 219 1,435 75 1,751 502 9
    Region IX Normal 33.7 % 3 of 4 1,210 1,675 6,528 73 2,539 806 21
    Region X Normal 23.8 % 1 of 4 390 321 2,572 214 529 691 3

    * 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 2009 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. The results of tests performed during the current week are summarized in the table below.

    Week 33
    No. of specimens tested 4,477
    No. of positive specimens (%) 804 (18.0%)
    Positive specimens by type/subtype
      Influenza A 803 (99.9%)
                 A (2009 H1N1)              570 (71.0%)
                              211 (26.3%)
                 A (unable to subtype)              15 (1.9%)
                 A (H3)              6 (0.7%)
                 A (H1)             1 (0.1%)
      Influenza B 1 (0.1%)

    During week 33, seasonal influenza A (H1) and A (H3) and B viruses co-circulated at low levels with 2009 influenza A (H1N1) viruses. 99% of all subtyped influenza A viruses being reported to CDC this week were 2009 influenza A (H1N1) viruses.

    The unusually high percentage of specimens testing positive for influenza by WHO and NREVSS collaborating laboratories remained higher during week 33 than is typically seen this time of year, and may be due to a combination of factors including higher than normal circulation of influenza in the summer with the emergence of the 2009 H1N1 virus, 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.

    As of August 28, 2009, 8,843 hospitalizations and 556 deaths (15 deaths in individuals 0-4 years, 86 deaths in individuals 5-24 years, 235 deaths in adults 25-49 years, 158 deaths in adults 50-64 years, 50 deaths in adults age 65 and older, and 12 deaths for which age was no reported) associated with 2009 influenza A (H1N1) virus have been identified by CDC and state and local public health departments (

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

    Antigenic Characterization:

    CDC has antigenically characterized 2,110 seasonal human influenza viruses [1,189 influenza A (H1), 227 influenza A (H3) and 694 influenza B viruses] collected by U.S. laboratories since October 1, 2008, and 382 2009 influenza A (H1N1) viruses.

    All 1,189 seasonal influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). Two hundred sixteen (95%) of 227 influenza A (H3N2) viruses tested are related to the A (H3N2) vaccine component (A/Brisbane/10/2007) and 11 viruses (5%) tested showed reduced titers with antisera produced against A/Brisbane/10/2007.

    All 382 2009 influenza A (H1N1) viruses are related to the A/California/07/2009 (H1N1)pdm reference virus selected by WHO as a potential candidate for 2009 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-six (11%) of 694 influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 618 (89%) 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 2009 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, 1,148 seasonal influenza A (H1N1), 253 influenza A (H3N2), 651 influenza B, and 426 2009 influenza A (H1N1) virus isolates have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir).  In addition, 596 2009 influenza A (H1N1) original clinical samples were tested for a single known mutation in the virus that confers oseltamivir resistance.   Also, 1,152 seasonal influenza A (H1N1), 252 influenza A (H3N2), and 433 2009 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.

    Samples tested (n) Resistant Viruses,
    Number (%)
    Samples tested (n) Resistant Viruses, Number (%) Samples tested (n) Resistant Viruses, Number (%)
    Oseltamivir Zanamivir Adamantanes
    Seasonal Influenza A (H1N1) 1,148 1,143 (99.6%) 1,148 0 (0) 1,152 6 (0.5%)
    Influenza A (H3N2) 253 0 (0) 253 0 (0) 252 245 (100%)
    Influenza B 651 0 (0) 651 0 (0) N/A* N/A*
    2009 Influenza A (H1N1) 1,022 6** (0.6) 426 0 (0) 433 433 (100%)
            *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.
             **Two screening tools were used to determine oseltamivir resistance: sequence analysis of viral genes or a neuraminidase inhibition assay.

    2009 influenza A (H1N1) viruses were tested for oseltamivir resistance by a neuraminidase inhibition assay and/or detection of genetic sequence mutation, depending on the type of specimen tested: original clinical samples were examined for a single known mutation in the virus that confers oseltamivir resistance in currently circulating seasonal influenza A (H1N1) viruses, while influenza virus isolates were tested using a neuraminidase inhibition assay that determines the presence or absence of neuraminidase inhibitor resistance, followed by the neuraminidase gene sequence analysis of resistant viruses.

    The majority of 2009 influenza A (H1N1) viruses are susceptible to the neuraminidase inhibitor antiviral medication oseltamivir, however rare sporadic cases of oseltamivir resistant 2009 influenza A (H1N1) viruses have been detected worldwide, including seven viruses in the United States.  All tested viruses retain their sensitivity to the other neuraminidase inhibitor zanamivir.  Additional information on antiviral recommendations for treatment and chemoprophylaxis of 2009 influenza A (H1N1) infection is available at All 2009 influenza A (H1N1) viruses tested to date are 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 2009 influenza A (H1N1) virus infection who are hospitalized or who are at higher risk for seasonal influenza complications.

    Six of the seven patients had documented exposure to oseltamivir through either treatment or chemoprophylaxis, and the remaining patient is currently under investigation to determine exposure to oseltamivir. Occasssional development of oseltamivir resistance during treatment or prophylaxis is not unexpected. Enhanced surveillance is expected to detect additional cases of oseltamivir resistant 2009 influenza A (H1N1) viruses and such cases will be investigated to assess the spread of resistant strains in the community. To prevent the spread of antiviral resistant virus strains, CDC reminds clinicians and the public of the need to continue hand and cough hygiene measures for the duration of any symptoms of influenza

    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 33, 6.1% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 6.3% for week 33.

    Pneumonia And Influenza Mortality
    View Full Screen

    Influenza-Associated Pediatric Mortality

    Five influenza-associated pediatric deaths were reported to CDC during week 33 (California [2], Georgia, Illinois, and Texas).  These deaths were associated with 2009 influenza A (H1N1) virus infection.  The deaths reported this week occurred between June 7 and August 15, 2009.  Since September 28, 2008, CDC has received 110 reports of influenza-associated pediatric deaths that occurred during the current influenza season, 42 of which were due to 2009 influenza A (H1N1) virus infections.

    Of the 45 children who had specimens collected for bacterial culture from normally sterile sites, 17 (37.8%) were positive; Staphylococcus aureus was identified in 11 (64.7%) of the 17 children.    Five of the S. aureus isolates were sensitive to methicillin and six were methicillin resistant.   Fifteen (88.2%) of the 17 children with bacterial coinfections were five years of age or older and 11 (64.7%) of the 17 children were 12 years of age or older.  Fourteen (33.3%) of the 42 children with confirmed 2009 influenza A (H1N1) infection had a specimen collected from a normally sterile site; three (21.4%) of the 14 children had a positive bacterial culture (methicillin sensitive S. aureus, methicillin resistant S. aureus and Streptococcus constellatus). .

    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. 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

    Influenza-Associated Hospitalizations
    View Full Screen

    During April 15, 2009 to August 22, 2009, the following preliminary laboratory-confirmed overall influenza associated hospitalization rates were reported by the EIP (rates include type A, type B, and 2009 H1N1):

    Rates for children aged 0-23 months, 2-4 years, and 5-17 years were 2.3, 0.9, and 0.7 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years were 0.5, 0.6, and 0.5 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 33, 1.4% 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 2.6%. One region (Region IV) reported 2.2% of outpatient visits for ILI, which is above their region-specific baseline of 2.0%, while the remaining nine regions reported percentages of visits for ILI below region-specific baseline levels.

    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 2009 influenza A (H1N1) viruses and does not measure the severity of influenza activity.

    During week 33, the following influenza activity was reported:

    • Widespread influenza activity was reported by Puerto Rico and two states (Alaska and Georgia).
    • Regional influenza activity was reported by 13 states (Alabama, California, Florida, Hawaii, Kentucky, Maine, Mississippi, Nebraska, Nevada, New Jersey, North Carolina, Oklahoma, and South Carolina).
    • Local influenza activity was reported by the District of Columbia and 10 states (Arizona, Arkansas, Louisiana, Montana, New Mexico, Pennsylvania, Tennessee, Texas, Virginia, and Washington).
    • Sporadic activity was reported by 24 states (Colorado, Connecticut, Delaware, Idaho, Illinois, Indiana, Iowa, Kansas, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New York, North Dakota, Ohio, Oregon, Rhode Island, South Dakota, Utah, Vermont, West Virginia, Wisconsin, and Wyoming).
    • No influenza activity was reported by one state (New Hampshire).
    • Guam and the U.S. Virgin Islands did not report.


    A description of surveillance methods is available at:

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