Welcome to the CDC Seasonal Flu website.
Skip directly to the search box, site navigation, or .

Plan and Prepare
  • Contact Us
  • 800–CDC–INFO
  • (800-232-4636)
  • 888–232–6348 (TTY)
  • cdcinfo@cdc.gov

Weekly Report: Influenza Summary Update

2008-2009 Influenza Season Week 12 ending March 28, 2009

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


During week 12 (March 22-28, 2009), influenza activity continued to decrease in the United States.

  • Seven hundred twenty-two (16.8%) 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.
  • Eight influenza-associated pediatric deaths were reported.
  • The proportion of outpatient visits for influenza-like illness (ILI) was below the national baseline. Two of nine surveillance regions reported ILI above their region-specific baselines (Mid-Atlantic and Pacific).
  • Thirteen states reported widespread influenza activity, 19 states reported regional activity; 14 states reported local influenza activity; the District of Columbia, Puerto Rico, and three states reported sporadic influenza activity; and one state did not report.

National and Regional Summary of Select Surveillance Components

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 Normal 16.8 % 32 of 51 5913 646 9633 7657 43
New England Normal 13.6 % 4 of 6 433 67 1036 667 1
Mid-Atlantic Elevated 25.2 % 3 of 3 703 85 895 808 8
East North Central Normal 42.6 % 1 of 5 947 61 266 762 4
West North Central Normal 19.6 % 5 of 7 885 33 834 651 0
South Atlantic Normal 21.4 % 5 of 9 962 56 1757 1566 6
East South Central Normal 14.2 % 3 of 4 181 6 41 90 3
West South Central Normal 13.9 % 2 of 4 568 39 3595 2194 9
Mountain Normal 21.5 % 5 of 8 643 184 798 390 8
Pacific Elevated 16.0 % 4 of 5 591 115 411 529 4

               * 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 12 Cumulative for the Season
No. of specimens tested 4,300 167,639
No. of positive specimens (%) 722 (16.8%) 23,849 (14.2%)
Positive specimens by type/subtype
  Influenza A 328 (45.4%) 16,192 (67.9%)
             A (H1)              130 (39.6%)              5,913 (36.5%)
             A (H3)              18 (5.5%)              646 (4.0%)
             A (unsubtyped)              180 (54.9%)              9,633 (59.5%)
  Influenza B 394 (54.6%) 7,657 (32.1%)

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, while influenza activity continued to decrease nationally, the relative proportion of influenza B viruses is increasing nationally and regionally. Six surveillance regions (East North Central, Mid-Atlantic, New England, Pacific, South Atlantic, and West North 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 846 influenza viruses [549 influenza A (H1), 86 influenza A (H3) and 211 influenza B viruses] collected by U.S. laboratories since October 1, 2008.

All 549 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 (H3) 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 influenza B viruses tested belong to the B/Yamagata lineage and are related to the vaccine strain (B/Florida/04/2006). The remaining 167 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, 654 influenza A (H1N1), 94 influenza A (H3N2), and 274 influenza B viruses have been tested for resistance to the neuraminidase inhibitors (oseltamivir and zanamivir). Six hundred five influenza A (H1N1) and 94 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) 654 649 (99.2%) 0 (0) 605 3 (0.5%)
Influenza A (H3N2) 94 0 (0) 0 (0) 94 94 (100%)
Influenza B 274 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 43 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 12, influenza B viruses accounted for 55% of the influenza viruses identified nationally, and more than 50% of the influenza viruses identified in six 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 the 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 12, 7.3% 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 12.

Pneumonia And Influenza Mortality
View Full Screen

Influenza-Associated Pediatric Mortality

Eight influenza-associated pediatric deaths were reported to CDC during week 12 (California, Kentucky, Maryland, North Carolina, Nevada, Ohio [2], and Washington). The deaths reported this week occurred between February 15 and March 28, 2009. Since September 28, 2008, CDC has received 43 reports of influenza-associated pediatric deaths that occurred during the current season.

Of the 26 children who had specimens collected for bacterial culture from normally sterile sites, 11 (42.3%) were positive; Staphylococcus aureus was identified in eight (72.7%) of the 11 children. Three of the S. aureus isolates were sensitive to methicillin and five were methicillin resistant. All 11 children with bacterial coinfections were five years of age or older and nine (81.8%) of the 11 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
View Full Screen

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 21, 2009, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 1.46 per 10,000. Because of case identification methods utilized in this study, a delay exists from the date of hospitalization to the date of report.

Influenza-Associated Hospitalizations
View Full Screen

During October 1, 2008 – March 28, 2009, preliminary laboratory-confirmed influenza-associated hospitalization rates reported by the EIP for children aged 0-4 years and 5-17 years were 2.8 per 10,000 and 0.5 per 10,000, respectively. For adults aged 18-49 years, 50-64 years, and = 65 years, the rates were 0.3 per 10,000, 0.4 per 10,000, and 1.0 per 10,000, respectively.

Influenza-Associated Pediatric Mortality
View Full Screen

Outpatient Illness Surveillance:

Nationwide during week 12, 2.1% 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 Sentinel Providers Regional Charts | View Chart Data |View Full Screen

On a regional level, the percentage of visits for ILI ranged from 1.1% to 3.3%. Two of the nine surveillance regions reported ILI percentages 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 (%) 1.1 3.1 1.7 1.4 1.6 2.1 1.9 1.4 3.3
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 12, the following influenza activity was reported:

  • Widespread influenza activity was reported by 13 states (Alabama, Arizona, California, Colorado, Delaware, Idaho, Nebraska, New Jersey, New York, North Carolina, Oregon, Virginia, and Washington).
  • Regional influenza activity was reported by 19 states (Connecticut, Florida, Hawaii, Kansas, Kentucky, Louisiana, Massachusetts, Missouri, Montana, Nevada, New Hampshire, North Dakota, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, and Wisconsin).
  • Local influenza activity was reported by 14 states (Arkansas, Illinois, Iowa, Maine, Maryland, Michigan, Minnesota, Mississippi, New Mexico, Ohio, Utah, Vermont, West Virginia, and Wyoming).
  • Sporadic activity was reported by the District of Columbia, Puerto Rico, and three states (Alaska, Georgia, and Oklahoma).
  • One state (Indiana) did not report.


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

  • Page last updated April 3, 2009.

CDC logo
USA.gov: The U.S. Government's Official Web Portal Department of Health and Human Services