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Weekly Report: Influenza Summary Update

Week ending February 14, 2004-Week 6

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of February 8-14, 2004. The percentage of patient visits for influenza-like illness (ILI) remained below the national baseline (2.5%). Mortality due to pneumonia and influenza (P&I) continued to decline (8.6%), but remained above the epidemic threshold for week 6 (8.3%). Twenty-seven (2.0%) of 1,328 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. There were no reports of widespread influenza activity from state and territorial epidemiologists during week 6. Two states reported regional activity, and 11 states reported local activity. Thirty-three states, New York City, the District of Columbia, and Guam, reported sporadic activity, and 4 states reported no activity.

Laboratory Surveillance*:

During week 6, WHO and NREVSS laboratories reported 1,328 specimens tested for influenza viruses, and 27 (2.0%) were positive. Of these, 21 were influenza A (H3N2) viruses, 5 were influenza A viruses that were not subtyped, and 1 was an influenza B virus.

Since September 28, WHO and NREVSS laboratories have tested 95,778 specimens for influenza viruses, of which 22,702 (23.7%) were positive. Of these, 22,562 (99.4%) were influenza A viruses, and 140 (0.6%) were influenza B viruses. Of the 22,562 influenza A viruses, 5,995 (26.6%) have been subtyped; 5,993 (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 624 influenza viruses collected by U.S. laboratories since October 1: two influenza A (H1) viruses, 614 influenza A (H3N2) viruses, and 8 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 614 influenza A (H3N2) isolates that have been characterized, 106 (17.3%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 508 (82.7%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Seven influenza B viruses characterized were similar antigenically to B/Sichuan/379/99 and 1 was similar antigenically to B/Hong Kong/330/2001.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 6, 8.6% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is above the epidemic threshold of 8.3% for week 6.

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 1.3%** during week 6, which is below the national baseline of 2.5%. The percentage of patient visits for ILI in all nine surveillance regions*** ranged from 2.3% in the West South Central region to 0.7% in the New England and West North Central regions. 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 activity for week 6. Mississippi and Rhode Island reported regional activity. Local activity was reported in 11 states (Connecticut, Florida, Hawaii, Idaho, Kentucky, Massachusetts, Minnesota, New Jersey, Ohio, West Virginia, and Wyoming). Sporadic activity was reported in 33 states (Arizona, Arkansas, California, Colorado, Georgia, Illinois, Indiana, Iowa, Kansas, Louisiana, Maine, Maryland, Michigan, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Mexico, New York, North Carolina, North Dakota, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, and Wisconsin), New York City, the District of Columbia, and Guam. Alabama, Alaska, Delaware, and Oklahoma reported no 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 February 19, 2004

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