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Weekly Report: Influenza Summary Update

Week ending April 3, 2004-Week 13

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of March 28-April 3, 2004. The percentage of patient visits for influenza-like illness (ILI) has remained below the national baseline (2,5 %) since the week ending January 17. During week 13, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (8,1 %). Seven (1 %) of 693 specimens collected from throughout the United States and tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. Thirteen states, New York City, and Guam reported sporadic activity and 37 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 13, WHO and NREVSS laboratories reported 693 specimens tested for influenza viruses, and 7 (1 %) were positive. Of these, 1 was an influenza A (H3N2) virus, 2 were influenza A viruses that were not subtyped, and 4 were influenza B viruses.

Since September 28, 2003, WHO and NREVSS laboratories have tested 117 702 specimens for influenza viruses, of which 24 240 (20,6 %) were positive. Of these, 24 051 (99,2 %) were influenza A viruses, and 189 (0,8 %) were influenza B viruses. Of the 24 051 influenza A viruses, 6 887 (28,6 %) have been subtyped; 6 885 (99,9 %) were influenza A (H3N2) viruses and 2 (0,1 %) were influenza A (H1) viruses.

Influenza Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 896 influenza viruses collected by U.S. laboratories since October 1, 2003: three influenza A (H1) viruses, 863 influenza A (H3N2) viruses, and 30 influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. Of the 863 influenza A (H3N2) isolates that have been characterized, 106 (12,3 %) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 757 (87,7 %) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Twenty-seven of the influenza B viruses belonged to the B/Yamagata lineage and were similar antigenically to B/Sichuan/379/99. Three influenza B viruses belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/2001.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 13, 7,4 % of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is below the epidemic threshold of 8,1 % for week 13.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

The percentage of patient visits to approximately 1 000 U.S. sentinel providers nationwide for ILI was 0,9 %** during week 13. This indicator has been below the national baseline of 2,5 % since the week ending January 17. The percentage of patient visits for ILI in each of the nine surveillance regions*** was below 1,6 %. Due to wide variability in regional level data, 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**:

There were no reports of widespread, regional, or local activity for week 13. Sporadic activity was reported in 13 states (Arizona, Colorado, Hawaii, Kentucky, Maine, Massachusetts, Michigan, Missouri, New York, Ohio, Utah, West Virginia, and Wyoming), New York City, and Guam. All other states and the District of Columbia reported no influenza activity.

U. S. map for Weekly Influenza Activity
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* Reporting is incomplete for this week. Numbers may change as more reports are received.

** The national and regional percentage of patient visits for ILI is weighted on the basis of state population.

*** 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 April 8, 2004

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