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Weekly Report: Influenza Summary Update

Week ending February 7, 2004-Week 5

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The following information may be quoted:

Synopsis:

Influenza activity remained low during the week of February 1-7, 2004. The percentage of patient visits for influenza-like illness (ILI) continued to decline, and remained below the national baseline of 2.5% for the fourth consecutive week (1.5%). Mortality due to pneumonia and influenza (P&I) has peaked, but remained above the epidemic threshold during week 5 (8.7%). Forty-eight (3.6%) of 1,322 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 5. Four states reported regional activity, 14 states reported local activity, 30 states, New York City, the District of Columbia, Guam, and Puerto Rico reported sporadic activity, and one state reported no activity. Reports of influenza-associated deaths in children continue to be collected from state health departments.

Laboratory Surveillance*:

During week 5, WHO and NREVSS laboratories reported 1,322 specimens tested for influenza viruses, and 48 (3.6%) were positive. Of these, 25 were influenza A (H3N2) viruses, 22 were influenza A viruses that were not subtyped, and 1 was an influenza B virus.

Since September 28, WHO and NREVSS laboratories have tested 92,159 specimens for influenza viruses, of which 22,419 (24.3%) were positive. Of these, 22,286 (99.4%) were influenza A viruses, and 133 (0.6%) were influenza B viruses. Of the 22,286 influenza A viruses, 5,862 (26.3%) have been subtyped; 5,861 (99.9%) were influenza A (H3N2) viruses and one (0.1%) was an influenza A (H1) virus.

Influenza Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 593 influenza viruses collected by U.S. laboratories since October 1: two influenza A (H1) viruses, 584 influenza A (H3N2) viruses, and 7 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 584 influenza A (H3N2) isolates that have been characterized, 106 (18.2%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 478 (81.8%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Six 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 5, 8.7% 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 5.

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 1.5%** during week 5, which is below the national baseline of 2.5%. The percentage of patient visits for ILI in all nine surveillance regions*** ranged from 2.2% in the South Atlantic region to 0.7% in the West North Central region. 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 5. Regional activity was reported in 4 states (Kentucky, Mississippi, Montana, and West Virginia). Local activity was reported in 14 states (Colorado, Connecticut, Delaware, Florida, Hawaii, Iowa, Maryland, Massachusetts, Minnesota, New Jersey, North Carolina, Ohio, Virginia, and Wisconsin). Sporadic activity was reported in 30 states (Arizona, Arkansas, California, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Maine, Michigan, Missouri, Nebraska, Nevada, New Hampshire, New Mexico, New York, North Dakota, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Washington, and Wyoming), New York City, the District of Columbia, Guam, and Puerto Rico. Alabama reported no activity. Alaska did not report.

Influenza-associated deaths in children aged <18 years:

As of February 12, CDC had received reports of 133 influenza-associated deaths in U.S. residents aged <18 years. These reports are preliminary and subject to change as more data become available. All patients had evidence of influenza virus infection detected by rapid-antigen testing or other laboratory tests. Among reported deaths, 68 (51.9%) were male; the gender for two children was not reported. The median age was 3.4 years (range 2 weeks--17 years). Of the 81 children aged <5 years, 38 were aged 6--23 months. Thirty-two children had medical conditions that put them at increased risk for complications due to influenza. Of the children whose influenza vaccination status was reported, 3 were vaccinated according to current recommendations (1), and 76 were not vaccinated.

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)

1. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2003;52(No. RR-8).

Report prepared February 12, 2004

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