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Weekly Report: Influenza Summary Update

Week ending April 17, 2004-Week 15

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of April 10-17, 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 15, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (7.9%). Nine (1.4%) of 622 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. New York reported regional activity, twelve states, New York City, Guam, and Puerto Rico reported sporadic activity, and 35 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 15, WHO and NREVSS laboratories reported 622 specimens tested for influenza viruses, and 9 (1.4%) were positive. Of these, 3 were influenza A viruses that were not subtyped, and 6 were influenza B viruses.

Since September 28, 2003, WHO and NREVSS laboratories have tested 121,183 specimens for influenza viruses, of which 24,536 (20.2%) were positive. Of these, 24,328 (99.2%) were influenza A viruses, and 208 (0.8%) were influenza B viruses. Of the 24,328 influenza A viruses, 7,169 (29.5%) have been subtyped; 7,167 (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 923 influenza viruses collected by U.S. laboratories since October 1, 2003: three influenza A (H1) viruses, 885 influenza A (H3N2) viruses, and 35 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 885 influenza A (H3N2) isolates that have been characterized, 106 (12.0%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 779 (88.0%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Thirty-two 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 15, 7.0% 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 7.9% for week 15.

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 0.8%** during week 15. 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 1.3%. 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**:

New York reported regional activity due to three nosocomial outbreaks of influenza B in two counties. Sporadic activity was reported in 12 states (Alaska, Arizona, Colorado, Hawaii, Massachusetts, Minnesota, Missouri, Nevada, New Mexico, Ohio, Utah, and West Virginia), New York City, Guam, and Puerto Rico. All other states and the District of Columbia reported no influenza activity. Connecticut and Rhode Island did not report.

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 22, 2004

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