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Weekly Report: Influenza Summary Update

Week ending April 31, 2004-Week 17

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of April 25 - May 1, 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 17, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (7.8%). Two (0.6%) of 311 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. Seven states, New York City, and Puerto Rico reported sporadic activity; and 43 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 17, WHO and NREVSS laboratories reported 311 specimens tested for influenza viruses, and 2 (0.6%) were positive; both were influenza B viruses.

Since September 28, 2003, WHO and NREVSS laboratories have tested 123,020 specimens for influenza viruses, of which 24,575 (20.0%) were positive. Of these, 24,357 (99.1%) were influenza A viruses, and 218 (0.9%) were influenza B viruses. Of the 24,357 influenza A viruses, 7,188 (29.5%) have been subtyped; 7,186 (99.9%) were influenza A (H3N2) viruses and 2 (0.1%) were influenza A (H1) viruses.

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Antigenic Characterization:

CDC has antigenically characterized 934 influenza viruses collected by U.S. laboratories since October 1, 2003: three influenza A (H1) viruses, 886 influenza A (H3N2) viruses, and 45 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 886 influenza A (H3N2) isolates that have been characterized, 106 (12.0%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 780 (88.0%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Forty-one of the influenza B viruses belonged to the B/Yamagata lineage and were similar antigenically to B/Sichuan/379/99. Four 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 17, 7.1% 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 7.8% for week 17.

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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 0.6%** during week 17. 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 0.8%. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

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Influenza Activity as Assessed by State and Territorial Epidemiologists**:

Sporadic activity was reported in 7 states (Arizona, Colorado, Delaware, Hawaii, New York, Ohio, and Utah), New York City, and Puerto Rico. All other states, Guam, and the District of Columbia reported no 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 May 7, 2004

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