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Weekly Report: Influenza Summary Update

Week ending December 29, 2001-Week 52

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The following information may be quoted:

Synopsis:

During week 52 (December 23-29, 2001)*, 20 of 608 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 2.5%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.0%, which is below the epidemic threshold of 7.9% for week 52. One state and territorial health department reported widespread influenza activity, 3 reported regional activity, 42 reported sporadic activity, and 3 reported no influenza activity**.

Laboratory Surveillance*:

During week 52, WHO and NREVSS laboratories reported 608 specimens tested for influenza viruses. Five influenza A (H3N2) viruses and 15 unsubtyped influenza A viruses were identified.

Since September 30, WHO and NREVSS laboratories have tested a total of 17,212 specimens for influenza viruses and 356 (2.2%) were positive. Of the 356 isolates identified, 343 (96%) were influenza A viruses and 13 (4%) were influenza B viruses. Two hundred and nine (61%) of the 343 influenza A viruses identified have been subtyped; 203 were influenza A (H3N2) viruses and 6 were influenza A (H1N1) viruses. One hundred and ninety-eight (56%) of the 356 U.S. isolates were reported from the Pacific region*** and 136 (38%) of the 356 isolates reported were identified in Alaska. Thirty-six 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 since 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 52, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.0%. This percentage is below the epidemic threshold of 7.9% for week 52.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 52, 2.5% of patient visits to U.S. sentinel physicians were due to ILI. This is above the national baseline of 1.9%. On a regional level, the percentage of visits for ILI ranged from 0.9% to 6.0%.*** 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 Colorado, and regional in Connecticut, Pennsylvania, and Utah. Forty-two states and New York City reported sporadic activity, 3 states reported no influenza activity, and 1 state 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 January 4, 2002

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