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Weekly Report: Influenza Summary Update

Week ending April 24, 2004-Week 16

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of April 18-24, 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 16, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (7.9%). Thirteen (3.9%) of 333 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 for the second consecutive week; 5 states, New York City, Guam, and Puerto Rico reported sporadic activity; and 43 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 16, WHO and NREVSS laboratories reported 333 specimens tested for influenza viruses, and 13 (3.9%) were positive. Of these, 1 was an influenza A (H3N2) virus, 8 were influenza A viruses that were not subtyped, and 4 were influenza B viruses.

Since September 28, 2003, WHO and NREVSS laboratories have tested 122,040 specimens for influenza viruses, of which 24,564 (20.1%) were positive. Of these, 24,353 (99.1%) were influenza A viruses, and 211 (0.9%) were influenza B viruses. Of the 24,353 influenza A viruses, 7,186 (29.5%) have been subtyped; 7,184 (99.9%) were influenza A (H3N2) viruses and 2 (0.1%) were influenza A (H1) viruses.

Influenza Virus Isolated


View WHO-NREVSS Regional Bar Charts | View Chart Data

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 16, 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 16.

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 0.7%** during week 16. 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 1.1%. 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
View Sentinel Providers Regional Charts | View Chart Data | View Full Screen

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

New York reported regional activity for the second consecutive week due to reports of nosocomial outbreaks of influenza B. Sporadic activity was reported in 5 states (Colorado, Hawaii, New Mexico, Ohio, and Utah), New York City, Guam, and Puerto Rico. All other states and the District of Columbia reported no influenza activity. Alabama 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 29, 2004

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