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Weekly Report: Influenza Summary Update

2008-2009 Influenza Season Week 11 ending March 21, 2009

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

Synopsis:

During week 11 (March 15-21, 2009), influenza activity continued to decrease in the United States.

  • One thousand one hundred four (21.4%) 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.
  • Three influenza-associated pediatric deaths were reported.
  • The proportion of outpatient visits for influenza-like illness (ILI) was above the national baseline. Six of nine surveillance regions reported ILI at or above their region-specific baselines.
  • Twenty-four states reported widespread influenza activity, 19 states reported regional activity; six states reported local influenza activity; and the District of Columbia and one state reported sporadic influenza activity.

National and Regional Summary of Select Surveillance Components

Region
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) A Unsub-typed B Pediatric Deaths
Nation Elevated 21.4 % 43 of 51 5,533 586 9,333 6,932 35
New England Elevated 17.5 % 6 of 6 429 64 1,003 621 1
Mid-Atlantic Elevated 28.1 % 3 of 3 690 83 890 769 8
East North Central Normal 54.6 % 3 of 5 884 61 243 641 2
West North Central Elevated 23.5 % 6 of 7 827 31 800 545 0
South Atlantic Normal 23.8 % 6 of 9 914 53 1,650 1,411 4
East South Central Elevated 14.2 % 4 of 4 176 6 38 87 2
West South Central Normal 17.8 % 3 of 4 565 39 3,566 2,162 9
Mountain Elevated 15.1 % 7 of 8 479 147 762 275 7
Pacific Elevated 17.3 % 5 of 5 569 102 381 421 2

               * Elevated means the % of visits for ILI is at or above the national or region-specific baseline
               † National data is for current week; regional data is for the most recent three weeks.
               ‡ Includes all 50 states and the District of Columbia

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 each week. The results of tests performed during the current week and cumulative totals for the season are summarized in the table below.

Week 11 Cumulative for the Season
No. of specimens tested 5,161 159,406
No. of positive specimens (%) 1,104 (21.4%) 22,384 (14.0%)
Positive specimens by type/subtype
  Influenza A 524 (47.5%) 15,452 (69.0%)
             A (H1)              158 (30.2%)              5,533 (35.8%)
             A (H3)              27 (5.2%)              586 (3.8%)
             A (unsubtyped)              339 (64.7%)              9,333 (60.4%)
  Influenza B 580 (52.5%) 6,932 (31.0%)

Since week 2 (the week ending January 17, 2009), when influenza activity increased nationally, influenza A (H1) viruses have predominated during the season overall. However, the relative proportion of influenza B viruses is increasing nationally and regionally. While influenza activity continued to decrease nationally, several surveillance regions reported an increase in influenza virus circulation, and seven regions (East North Central, Mid-Atlantic, New England, Pacific, South Atlantic, West North Central, and West South Central) reported a higher proportion of influenza B viruses compared to influenza A viruses this week.

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

Composition of the 2009-10 Influenza Vaccine:

WHO has recommended vaccine strains for the 2009-10 Northern Hemisphere trivalent influenza vaccine, and the Food and Drug Administration (FDA) has made the same recommendations for the U.S. influenza vaccine. Both agencies are recommending that the vaccine contain A/Brisbane/59/2007-like (H1N1), A/Brisbane/10/2007-like (H3N2), and B/Brisbane/60/2008-like (B/Victoria lineage) viruses. Only the influenza B component has been changed from the 2008-09 vaccine formulation. This recommendation was based on surveillance data related to epidemiology and antigenic characteristics, serological responses to 2008-09 vaccines, and the availability of candidate strains and reagents.

Antigenic Characterization:

CDC has antigenically characterized 807 influenza viruses [510 influenza A (H1), 86 influenza A (H3) and 211 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

All 510 influenza A (H1) viruses are related to the influenza A (H1N1) component of the 2008-09 influenza vaccine (A/Brisbane/59/2007). All 86 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. Forty-four (20.9%) influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 167 (79.1%) 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.

Antiviral Resistance:

Since October 1, 2008, 554 influenza A (H1N1), 86 influenza A (H3N2), and 258 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Five hundred fifty-four influenza A (H1N1) and 86 influenza A (H3N2) 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
Influenza A (H1N1) 554 549 (99.1%) 0 (0) 554 3 (0.5%)
Influenza A (H3N2) 86 0 (0) 0 (0) 86 86 (100%)
Influenza B 258 0 (0) 0 (0) N/A* N/A*
       *The adamantanes (amantadine and rimantadine) are not effective against influenza B viruses.

Influenza A (H1N1) viruses from 42 states have been tested for antiviral resistance to oseltamivir so far this season. To date, all influenza A (H3N2) viruses tested are resistant to the adamantanes and all oseltamivir-resistant influenza A (H1N1) viruses tested are sensitive to the adamantanes. Nationally, influenza A (H1N1) viruses have predominated during the season overall. While influenza activity continued to decrease nationally, the relative proportion of influenza B viruses is increasing. During week 11, influenza B viruses accounted for 53% of the influenza viruses identified nationally, and more than 50% of the influenza viruses identified in seven of the nine surveillance regions. This presents challenges for the selection of antiviral medications for the treatment and chemoprophylaxis of influenza. Health care providers should be aware of the possibility of increased influenza B circulation in their area, and continue to test patients for influenza and consult local surveillance data when evaluating patients with acute respiratory infections during influenza season. CDC issued interim recommendations for the use of influenza antiviral medications in the setting of oseltamivir resistance among circulating influenza A (H1N1) viruses on December 19, 2008. These interim recommendations are available at http://www2a.cdc.gov/HAN/ArchiveSys/ViewMsgV.asp?AlertNum=00279.

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 11, 7.2% of all deaths reported through the 122-Cities Mortality Reporting System were due to P&I. This percentage is below the epidemic threshold of 7.9% for week 11.

Pneumonia And Influenza Mortality
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Influenza-Associated Pediatric Mortality

Three influenza-associated pediatric deaths were reported to CDC during week 11 (Kentucky, New York, and West Virginia). The deaths reported this week occurred between March 5 and March 10, 2009. Since September 28, 2008, CDC has received 35 reports of influenza-associated pediatric deaths that occurred during the current season.

Of the 21 children who had specimens collected for bacterial culture from normally sterile sites, eight (38.1%) were positive; Staphylococcus aureus was identified in five (62.5%) of the eight children. Two of the S. aureus isolates were sensitive to methicillin and three were methicillin resistant. All eight children with bacterial coinfections were five years of age or older and seven (87.5%) of the eight 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 (http://www2a.cdc.gov/HAN/ArchiveSys/ViewMsgV.asp?AlertNum=00268).

Influenza-Associated Pediatric Mortality
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Influenza-Associated Hospitalizations

Laboratory-confirmed influenza-associated hospitalizations are monitored in two population-based surveillance networks: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). These two systems provide updates of surveillance data every two weeks.

During October 12, 2008 to March 7, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 0.55 per 10,000. Due to case identification methods utilized in this study, a delay exists from the date of hospitalization to the date of report.

Influenza-Associated Hospitalizations
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During October 1, 2008 – March 14, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 2.1 per 10,000 and 0.4 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.2 per 10,000, 0.3 per 10,000, and 0.8 per 10,000, respectively.

Influenza-Associated Pediatric Mortality
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Outpatient Illness Surveillance:

Nationwide during week 11, 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 Sentinel Providers Regional Charts | View Chart Data |View Full Screen

On a regional level, the percentage of visits for ILI ranged from 1.6% to 4.0%. Six of the nine surveillance regions reported ILI percentages at or above their region specific baselines.

Region New England Mid- Atlantic East North Central West North Central South Atlantic East South Central West South Central Mountain Pacific
Reported ILI (%) 1.6 2.9 1.8 2.1 2.0 3.2 4.0 2.5 3.0
Region-Specific Baseline 1.5 2.9 1.9 1.7 2.2 2.5 4.8 1.5 3.0

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

During week 11, the following influenza activity was reported:

  • Widespread influenza activity was reported by 24 states (Alabama, Alaska, Arizona, California, Colorado, Connecticut, Delaware, Idaho, Indiana, Kansas, Montana, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Oregon, South Dakota, Tennessee, Vermont, Virginia, Washington, and Wisconsin).
  • Regional influenza activity was reported by 19 states (Arkansas, Florida, Georgia, Hawaii, Iowa, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Mississippi, Missouri, Nevada, New Mexico, Ohio, Pennsylvania, Rhode Island, Texas, and Wyoming).
  • Local influenza activity was reported by six states (Illinois, Michigan, Minnesota, Oklahoma, South Carolina, and Utah).
  • Sporadic activity was reported by the District of Columbia and one state (West Virginia).

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A description of surveillance methods is available at: http://www.cdc.gov/flu/weekly/fluactivity.htm

  • Page last updated March 20, 2009.

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