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Weekly Report: Influenza Summary Update

Week ending November 3, 2001-Week 44

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The following information may be quoted:

Synopsis:

During week 45 (November 4-10, 2001)*, two of 414 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 1.2%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 6.7%, which is below the epidemic threshold of 7.2% for week 45. Twenty-two state and territorial health departments reported sporadic influenza activity and 26 reported no influenza activity**.

Prior to the 1999-2000 season, a new case definition for a P&I 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.

Laboratory Surveillance*:

During week 45, WHO and NREVSS laboratories reported 414 specimens tested for influenza viruses. Two influenza A isolates were identified.

Since September 30, WHO and NREVSS laboratories have tested a total of 4,891 specimens for influenza viruses and 39 (1%) were positive. Of the 39 isolates identified, 38 (97%) were influenza A viruses and 1 (3%) was an influenza B virus. Fourteen (37%) of the 38 influenza A viruses identified have been subtyped and all were influenza A (H3N2) viruses. Influenza A isolates have been identified in Alaska, Arizona, Florida, Kentucky, Louisiana, New York, Texas, and Utah. The influenza B isolate was identified in Texas.

Influenza Virus Isolated
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Antigenic Characterization:

CDC antigenically characterized 10 influenza viruses collected in September and 5 collected in October: 12 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 45, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6.7%. This percentage is below the epidemic threshold of 7.2% for week 45.

Prior to the 1999-2000 season, a new case definition for a P&I 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 45, 1.2% of patient visits to U.S. sentinel physicians were due to ILI. On a regional level,*** the percentage of visits for ILI ranged from 0.7% to 1.8%.

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 22 states (Colorado, Florida, Georgia, Indiana, Kansas, Kentucky, Louisiana, Maine, Michigan, Missouri, Nebraska, Nevada, New Mexico, New York, North Dakota, Ohio, Pennsylvania, Tennessee, Texas, Utah, West Virginia, and Wyoming), New York City, and Washington, DC. Twenty-six states reported no influenza 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 November 15, 2001

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