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Weekly Report: Influenza Summary Update

Week ending January 26, 2002-Week 4

The following information may be quoted:

Synopsis:

During week 4 (January 20-26, 2002)*, 392 (17.6%) of 2,224 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 3.2%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.8%, which is below the epidemic threshold of 8.2% for week 4. Eight state and territorial health departments reported widespread influenza activity, 15 reported regional activity, and 25 reported sporadic activity **.

Laboratory Surveillance*:

During week 4, WHO and NREVSS laboratories reported 2,224 specimens tested for influenza viruses, of which 392 (17.6%) were positive. Ninety-one influenza A (H3N2) viruses, 297 unsubtyped influenza A viruses, and 4 influenza B virus were identified. During the past 3 weeks (weeks 2-4), 28% of the specimens tested for influenza in the West South Central region and 27% of the specimens tested for influenza in the Mountain region were positive. Between 10% and 20% of the specimens tested for influenza in the New England, Mid-Atlantic, South Atlantic, and East North Central regions were positive. Six percent or less of the specimens tested for influenza in the Pacific, East South Central, and West North Central regions were positive.

Since September 30, WHO and NREVSS laboratories have tested a total of 32,266 specimens for influenza viruses and 2,180 (6.8%) were positive. Of the 2,180 isolates identified, 2,148 (99%) were influenza A viruses and 32 (1%) were influenza B viruses. Six hundred and ninety-three (32%) of the 2,148 influenza A viruses identified have been subtyped; 684 (99%) were influenza A (H3N2) viruses and 9 (1%) were influenza A (H1N1) viruses. Forty-eight states have reported laboratory-confirmed influenza.

Influenza Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 134 influenza viruses collected from U.S. laboratories since October 1: 132 influenza A (H3N2) viruses and 2 influenza A (H1N1) viruses. All viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2) and A/New Caledonia/ 20/99 (H1N1), respectively.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 4, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.8%. This percentage is below the epidemic threshold of 8.2% for week 4.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 4, 3.2% 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 1.5% to 5.8%.*** 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 8 states (Colorado, Maryland, New Mexico, New York, Tennessee, Texas, Utah, and Virginia) and New York City, and regional in 15 states (Arizona, Connecticut, Georgia, Kansas, Louisiana, Michigan, Minnesota, Mississippi, Nebraska, North Carolina, Oregon, Pennsylvania, South Dakota, Washington, and Wyoming). Twenty-five states and Washington, DC reported sporadic activity, and 2 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 January 31, 2002

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