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Weekly Report: Influenza Summary Update

Week ending October 6, 2001-Week 40

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The following information may be quoted:

Synopsis:

During week 40 (September 30 - October 6, 2001), two 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 7.0%. Seven state and territorial health departments reported sporadic influenza activity and 38 reported no influenza activity.

During week 40, WHO and NREVSS laboratories reported 295 specimens tested for influenza viruses. Two influenza A isolates were identified. These isolates were collected in September from residents of a New York nursing home.

Laboratory Surveillance*:

During September 1-30, 2001, laboratory-confirmed influenza A(H3N2) infections were reported from Arizona, Florida, and Hawaii. Hawaii also reported a laboratory-confirmed influenza A(H1N1) infection. Unsubtyped influenza A infections were reported from Texas and Nevada. Laboratory-confirmed influenza B infections were reported from California and Maryland.

September Isolates from Other Sources: During week 40, 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 above the epidemic threshold of 6.7% for week 40 but is based on incomplete data and should not cause concern.

Influenza Virus Isolated
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Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 40, 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 above the epidemic threshold of 6.7% for week 40 but is based on incomplete data and should not cause concern.

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 return to the CDC model for estimating the P&I mortality baseline and epidemic threshold.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 40, 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 2.9%.

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 7 states (Alaska, Kansas, Kentucky, Missouri, New York, Texas, and West Virginia) and New York City. Thirty-eight states reported no influenza activity and 5 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 11, 2001

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