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Weekly Report: Influenza Summary Update

Week ending December 22, 2001-Week 51

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The following information may be quoted:

Synopsis:

During week 51 (December 16-22, 2001)*, 18 of 756 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 1.5%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 6.4%, which is below the epidemic threshold of 7.8% for week 51. One state and territorial health department reported widespread influenza activity, two reported regional activity, 39 reported sporadic activity, and 5 reported no influenza activity**.

Laboratory Surveillance*:

During week 51, WHO and NREVSS laboratories reported 756 specimens tested for influenza viruses. Six influenza A (H3N2) viruses, 1 influenza A (H1N1) virus, and 11 unsubtyped influenza A viruses were identified.

Since September 30, WHO and NREVSS laboratories have tested a total of 15,389 specimens for influenza viruses and 306 (2.0%) were positive. Of the 306 isolates identified, 296 (97%) were influenza A viruses and 10 (3%) were influenza B viruses. One hundred and ninety (64%) of the 296 influenza A viruses identified have been subtyped; 185 were influenza A (H3N2) viruses and 5 were influenza A (H1N1) viruses. One hundred and ninety-two (63%) of the 306 U.S. isolates were reported from the Pacific region*** and 133 (43%) of the 306 isolates reported were identified in Alaska. Twenty-nine states have reported laboratory-confirmed influenza.

Influenza Virus Isolated
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Antigenic Characterization:

CDC antigenically characterized 10 influenza viruses collected in September and 23 collected in October: 30 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 51, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6.4%. This percentage is below the epidemic threshold of 7.8% for week 51.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 51, 1.5% 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 0.3% to 7.1%.*** 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 widespread in Alaska, and regional in Colorado and Vermont. Thirty-nine states, New York City, and Washington, DC reported sporadic activity, 5 states reported no influenza activity, and 3 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 December 27, 2001

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