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Weekly Report: Influenza Summary Update

Week ending November 24, 2001-Week 47

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The following information may be quoted:

Synopsis:

During week 47 (November 18-24, 2001)*, 3 of 717 respiratory specimens tested by WHO and NREVSS laboratories were positive for influenza. The overall proportion of patient visits to sentinel physicians for influenza-like illness (ILI) was 1.4%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 6.1%, which is below the epidemic threshold of 7.4% for week 47. One state and territorial health department reported regional influenza activity, 20 reported sporadic influenza activity, and 18 reported no influenza activity**.

Laboratory Surveillance*:

During week 47, WHO and NREVSS laboratories reported 717 specimens tested for influenza viruses, of which 3 (0.4%) were positive. Two were influenza A (H3N2) viruses and one was an influenza B virus.

Since September 30, WHO and NREVSS laboratories have tested a total of 8,140 specimens for influenza viruses and 74 (0.9%) were positive. Of the 74 isolates identified, 71 (96%) were influenza A viruses and 3 (4%) were influenza B viruses. Forty-five (63%) of the 71 influenza A viruses identified have been subtyped; 44 were influenza A (H3N2) viruses and one was an influenza A (H1N1) virus. Influenza A (H3N2) isolates were identified in Alaska, Arizona, Colorado, Florida, New York, North Carolina, North Dakota, Texas, Utah, and Wisconsin; the influenza A (H1N1) isolate was identified in Washington; and unsubtyped influenza A isolates were identified in Alabama, Alaska, Hawaii, Kentucky, Louisiana, Minnesota, New York, Washington, and Wisconsin. Influenza B isolates were identified in Louisiana, Michigan, and Texas. Thirty-nine (53%) of the 74 influenza viruses isolated were identified in Alaska.

Influenza Virus Isolated
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Antigenic Characterization:

CDC antigenically characterized 10 influenza viruses collected in September and 13 collected in October: 20 influenza A (H3N2) viruses, 2 influenza A (H1N1) viruses, and 1 influenza B virus. All viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2), A/New Caledonia/ 20/99 (H1N1), and B/Sichuan/379/99, respectively.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 47, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6.1%. This percentage is below the epidemic threshold of 7.4% for week 47.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 47, 1.4% of patient visits to U.S. sentinel physicians were due to ILI. This is below the national baseline of 1.9%. On a regional level, the percentage of visits for ILI ranged from 0.6% to 3.9%.*** Due to wide variability in regional level data, it is not possible to calculate region-specific baselines and it is not appropriate to apply the national baseline to regional level data.

Bar Chart for Influenza-like Illness
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Influenza Activity as Assessed by State and Territorial Epidemiologists**:

Influenza activity was reported as regional in Alaska and sporadic in 20 states (Arizona, California, Colorado, Georgia, Indiana, Iowa, Kansas, Kentucky, Maine, Michigan, Missouri, New Mexico, Nevada, New York, North Carolina, Texas, Vermont, West Virginia, Wisconsin, and Wyoming), New York City, and Washington, DC. Eighteen states reported no influenza activity, and 11 states did not report.

U. S. map for Weekly Influenza Activity


* Reporting is incomplete for this week. Numbers may change as more reports are received.

** Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

*** Surveillance Regions: New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, Rhode Island); Mid-Atlantic (New Jersey, New York City, Pennsylvania, Upstate New York); East North Central (Illinois, Indiana, Michigan, Ohio, Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota); South Atlantic (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., West Virginia); East South Central (Alabama, Kentucky, Mississippi, Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming); Pacific (Alaska, California, Hawaii, Oregon, Washington)

Report prepared November 29, 2001

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