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Weekly Report: Influenza Summary Update

Week ending October 13, 2001-Week 41

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The following information may be quoted:

Synopsis:

During week 41 (October 7-13, 2001)*, five of the specimens tested by WHO and NREVSS laboratories were positive for influenza. The proportion of patient visits to sentinel physicians for influenza-like illness (ILI) overall was 1.3%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 6.0%. Thirteen state and territorial health departments reported sporadic influenza activity and 34 reported no influenza activity**.

Laboratory Surveillance*:

During week 41, WHO and NREVSS laboratories reported 374 specimens tested for influenza viruses, of which 5 (1%) were positive. Two were influenza A (H3N2) viruses and 3 were unsubtyped influenza A viruses.

Since September 30, WHO and NREVSS laboratories have tested a total of 1,013 specimens for influenza viruses and 8 (1%) were positive. Of the positive results, 2 were influenza A (H3N2) viruses and 6 were unsubtyped influenza A viruses. Influenza isolates have been identified in Alaska, Arizona, and New York.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 2 influenza A (H3N2) viruses collected in the United States since October 1. Both viruses were antigenically similar to the vaccine strain A/Panama/2007/99 (H3N2).

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 41, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6.0%. This percentage is below the epidemic threshold of 6.8% for week 41.

Prior to the 1999-2000 season, a new case definition for a pneumonia and influenza death was introduced in the 122 Cities Mortality Reporting System. During the summer of 2000, the baseline and epidemic threshold were manually adjusted to take into account these changes in the case definition. At this point, sufficient data have been collected using the new case definition to allow us to readjust the P&I mortality baseline and epidemic threshold for the 2001-02 season.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 41, 1.3% of patient visits to U.S. sentinel physicians were due to ILI. On a regional level***, the percentage of visits for ILI ranged from 0.8% to 2.1%.

On the basis of an analysis of sentinel physician data collected from the 1997-98 to 2000-01 seasons, the national baseline for sentinel physician data was determined to be 1.9%. The baseline was calculated as the mean percentage of visits for ILI during non-influenza weeks plus 2 standard deviations. Due to wide variability in regional level data, it was 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 sporadic in 13 states (Alaska, Colorado, Florida, Hawaii, Indiana, Kansas, Kentucky, Minnesota, Missouri, New Mexico, New York, Pennsylvania, and West Virginia) and Washington, DC. Thirty-four states and New York City 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 October 18, 2001

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