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Weekly Report: Influenza Summary Update

Week ending May 22, 2004-Week 20

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of May 16-22, 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 20, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (7.3%). One (0.2%) of the 411 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 was positive for influenza. Five states and New York City reported sporadic activity, and 34 states reported no influenza activity.

Laboratory Surveillance*:

During week 20, WHO and NREVSS laboratories reported 411 specimens tested for influenza viruses, and 1 (0.2%) was positive for an influenza A virus that was not subtyped.

Since September 28, 2003, WHO and NREVSS laboratories have tested 127,316 specimens for influenza viruses, of which 24,612 (19.3%) were positive. Of these, 24,380 (99.1%) were influenza A viruses, and 232 (0.9%) were influenza B viruses. Of the 24,380 influenza A viruses, 7,190 (29.5%) have been subtyped; 7,188 (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 991 influenza viruses collected by U.S. laboratories since October 1, 2003: three influenza A (H1) viruses, 918 influenza A (H3N2) viruses, and 70 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 918 influenza A (H3N2) isolates that have been characterized, 106 (11.5%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 812 (88.5%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Sixty-five of the influenza B viruses belonged to the B/Yamagata lineage and were similar antigenically to B/Sichuan/379/99. Five influenza B viruses belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/2001.

Composition of the 2004-05 Influenza Vaccine: The Food and Drug Administration's Vaccine and Related Biological Products Advisory Committee recommended that the 2004-05 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/Fujian/411/2002-like (H3N2), and B/Shanghai/361/2002-like viruses. Both the influenza A (H3N2) and influenza B components have been changed from the 2003-04 season vaccine components. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 20, 6.6% 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 7.3% for week 20.

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 20. 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 1.5%. 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**:

Sporadic activity was reported in 5 states (Hawaii, Nevada, New York, Ohio, and Utah) and New York City. All other states reported no influenza activity. Alabama, Arkansas, California, Iowa, Kentucky, Nebraska, New Mexico, Oklahoma, Rhode Island, South Dakota, Wisconsin, and the District of Columbia 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 May 20, 2004

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