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FluView: A Weekly Influenza Surveillance Report Prepared by the Influenza Division

2008-2009 Influenza Season Week 35 ending September 5, 2009

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


During week 35 (August 30-September 5, 2009), influenza activity increased in the U.S.

  • During week 35:
    • 1,085 (20.5%) 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.
    • 97% 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.
    • One influenza-associated pediatric death was reported and was associated with a 2009 influenza A (H1N1) virus infection.
    • The proportion of outpatient visits for influenza-like illness (ILI) was above the national baseline. Regions 2, 4, 6, and 9 reported ILI above region-specific baseline levels.
    • Eleven states and Guam reported geographically widespread influenza activity, 13 states and Puerto Rico reported regional influenza activity, 10 states and the District of Columbia reported local influenza activity, 14 states reported sporadic influenza activity, two states reported no influenza activity, and the U.S. Virgin Islands did not report.
    • The 2009-10 influenza season officially begins October 4, 2009.
National and Regional Summary of Select Surveillance Components
HHS Surveillance Regions*
Data for current week Data cumulative for the season
% 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
Nation Elevated 20.5 % 27 of 54 8,256 4,365 40,490 863 20,570 10,843 112
Region 1 Normal 3.9 % 0 of 6 585 305 2,943 13 1,707 820 4
Region 2 Elevated 3.4 % 1 of 4 297 229 1,836 21 2,396 714 20
Region 3 Normal 13.1 % 2 of 6 1,252 221 4,450 20 1,050 1,363 10
Region 4 Elevated 21.8 % 8 of 8 959 769 6,471 93 3,603 1,315 11
Region 5 Normal 16.5 % 3 of 6 1,661 220 8,482 206 919 1,423 18
Region 6 Elevated 25.8 % 5 of 5 830 311 3,930 7 5,279 2,669 16
Region 7 Normal 18.6 % 1 of 4 537 86 1,203 135 602 537 0
Region 8 Normal 14.8 % 0 of 6 540 219 1,603 80 1,875 505 9
Region 9 Elevated 25.4 % 4 of 5 1,205 1,681 6,827 74 2,607 806 21
Region 10 Normal 21.0 % 2 of 4 390 324 2,745 214 532 691 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, Guam, Puerto Rico, and U.S. Virgin Islands
¥ 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 35
No. of specimens tested 5,296
No. of positive specimens (%) 1,085 (20.5%)
Positive specimens by type/subtype
  Influenza A 1,085 (100.0%)
             A (2009 H1N1)              705 (65.0%)
                          332 (30.6%)
             A (unable to subtype)              28 (2.6%)
             A (H3)              14 (1.3%)
             A (H1)             6 (0.6%)
  Influenza B 0 (0.0%)

During week 35, seasonal influenza A (H1) and A (H3) viruses co-circulated at low levels with 2009 influenza A (H1N1) viruses. Ninety-seven percent of all subtyped influenza A viruses being reported to CDC this week were 2009 influenza A (H1N1) viruses.

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

Pneumonia and Influenza Hospitalization and Death Tracking:

This is the first week that CDC is reporting data from a new system for monitoring the trend of influenza-related hospitalizations and deaths. This new system replaces the weekly report of laboratory confirmed 2009 H1N1-related hospitalizations and deaths. States and territories can now report to CDC either laboratory confirmed or pneumonia and influenza syndromic hospitalizations and deaths resulting from all types or subtypes of influenza, not just those from 2009 H1N1 influenza virus. To allow jurisdictions to implement the new case definition, counts were reset to zero on August 30, 2009. For week 35 (August 30-September 5, 2009) 1,380 hospitalizations and 196 deaths associated with influenza virus infection, or based on syndromic surveillance for influenza and pneumonia, were reported to CDC. This is the first week of data from this new system and reflects reports by 29 states and territories. CDC will continue to use its traditional surveillance systems to track the progress of the remainder of the 2008-09 season, and the 2009-10 influenza season, which officially begins October 4, 2009.

Antigenic Characterization:

CDC has antigenically characterized 2,121 seasonal human influenza viruses [1,189 influenza A (H1), 236 influenza A (H3) and 696 influenza B viruses] collected by U.S. laboratories since October 1, 2008, and 494 2009 influenza A (H1N1) viruses.

All 1,189 seasonal influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2009-10 Northern Hemisphere influenza vaccine (A/Brisbane/59/2007). Two-hundred twenty (93%) of 236 influenza A (H3N2) viruses tested are related to the influenza A (H3N2) 2009-10 vaccine component (A/Brisbane/10/2007) and 16 viruses (7%) tested showed reduced titers with antisera produced against A/Brisbane/10/2007.

All 494 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. Six hundred twenty (89%) of 696 influenza B viruses tested belong to the B/Victoria lineage and are related to the influenza vaccine component for the 2009-10 Northern Hemisphere influenza vaccine (B/Brisbane/60/2008). The remaining 76 (11%) viruses tested belong to the B/Yamagata lineage.

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), 261 influenza A (H3N2), 654 influenza B, and 574 2009 influenza A (H1N1) virus isolates have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). In addition, 798 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), 258 influenza A (H3N2), and 526 2009 influenza A (H1N1) viruses have been tested for resistance to the adamantanes (amantadine and rimantadine). Three state public health laboratories perform antiviral testing and report their results to CDC. Two additional oseltamivir resistant 2009 influenza A (H1N1) viruses have been identified by these laboratories. The results of antiviral resistance testing performed on these viruses are summarized in the table below.

Viruses tested (n) Resistant Viruses,
Number (%)
Viruses tested (n) Resistant Viruses, Number (%) Viruses 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) 261 0 (0) 261 0 (0) 258 258 (100%)
Influenza B 654 0 (0) 654 0 (0) N/A* N/A*
2009 Influenza A (H1N1) 1,372 8†‡ (0.6) 574 0 (0) 526 526 (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.
‡Three state public health laboratories perform antiviral resistance testing and report their results to CDC. An additional oseltamivir resistant 2009 influenza A (H1N1) viruses has been identified by these laboratories, bringing the total number to 9.

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 nine cases in the United States. All tested viruses retain their sensitivity to the other neuraminidase inhibitor: zanamivir. All of the nine patients (includes the eight viruses detected at CDC and one identified by a state public health laboratory) had documented exposure to oseltamivir through either treatment or chemoprophylaxis, and the remaining patient is currently under investigation to determine exposure to oseltamivir. Occasional 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

Additional information on antiviral recommendations for treatment and chemoprophylaxis of influenza virus infection is available at ( Antiviral treatment with oseltamivir or zanamivir is recommended for all patients with confirmed or suspected influenza virus infection who are hospitalized or who are at higher risk for influenza complications.

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 35, 5.3% 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 35.

Pneumonia And Influenza Mortality
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Influenza-Associated Pediatric Mortality

One influenza-associated pediatric death was reported to CDC during week 35 (Mississippi). This death was associated with 2009 influenza A (H1N1) virus infection. The death reported this week occurred during week 34 (the week ending August 29, 2009). Since September 28, 2008, CDC has received 112 reports of influenza-associated pediatric deaths that occurred during the current influenza season, 44 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 (31.8%) of the 44 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
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Influenza-Associated Hospitalizations

Laboratory-confirmed influenza-associated hospitalizations are monitored through the Emerging Infections Program (EIP), a population-based surveillance network.

During April 15, 2009 – September 5, 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 2.5, 1.0, and 0.8 per 10,000, respectively. Rates for adults aged 18-49 years, 50-64 years, and >= 65 years, the overall flu rates 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 35, 3.6% 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 outpatient visits for ILI ranged from 0.6% to 6.7%. Four regions (Regions 2, 4, 6, and 9) reported a proportion of outpatient visits for ILI above region-specific baseline levels, while the remaining six 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 35, the following influenza activity was reported:

  • Widespread influenza activity was reported by Guam and 11 states (Alaska, Arizona, Florida, Georgia, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, and Tennessee).
  • Regional influenza activity was reported by Puerto Rico and 13 states (Alabama, Arkansas, California, Illinois, Kentucky, Minnesota, Nebraska, Nevada, New Mexico, Ohio, Texas, Virginia, and Washington).
  • Local influenza activity was reported by the District of Columbia and 10 states (Colorado, Hawaii, Iowa, Kansas, Maine, Massachusetts, Montana, New Jersey, Oregon, and Pennsylvania).
  • Sporadic activity was reported by 14 states (Connecticut, Delaware, Idaho, Indiana, Michigan, Missouri, New York, North Dakota, South Dakota, Utah, Vermont, West Virginia, Wisconsin, and Wyoming).
  • No influenza activity was reported by two states (New Hampshire and Rhode Island).
  • The U.S. Virgin Islands did not report.


A description of surveillance methods is available at:

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