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Weekly Report: Influenza Summary Update

Week ending February 9, 2002-Week 6

The following information may be quoted:

Synopsis:

During week 6 (February 2 - 9, 2002)*, 363 (17.5%) of 2,071 respiratory specimens tested by World Health Organization (WHO) and National Respiratory Virus Surveillance System (NREVSS) laboratories were positive for influenza. The overall proportion of patient visits to sentinel physicians for influenza-like illness (ILI) was 3.0%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.4%, which is below the epidemic threshold of 8.3% for week 6. Eleven state and territorial health departments reported widespread influenza activity, 27 reported regional activity, 10 reported sporadic activity, and one reported no influenza activity **.

On February 6, 2002, WHO and the Public Health Laboratory Service (PHLS) in the United Kingdom reported the recent identification of a new influenza virus strain, influenza A(H1N2), isolated from humans in England, Israel, and Egypt. In addition to the viruses reported by PHLS, the Centers for Disease Control and Prevention (CDC) has identified influenza A(H1N2) viruses from patient specimens collected during July, September, and December 2001 in Texas, Nevada, and Wisconsin, respectively. CDC has received additional influenza A H1 viruses for further antigenic characterization. The neuraminidase type of these H1 viruses has not yet been determined, but testing is under way. Influenza A(H1N2) viruses have been identified in the past. Between December 1988 and March 1989, 19 influenza A(H1N2) viruses were identified in 6 cities in China, but the virus did not spread further. International influenza surveillance conducted through WHO and U.S. surveillance conducted by CDC will continue to track the occurrence of A(H1N2) viruses.

Click here for more information about influenza A(H1N2) viruses

Laboratory Surveillance*:

During week 6, WHO and NREVSS laboratories reported 2,071 specimens tested for influenza viruses, of which 363 (17.5%) were positive. One hundred and fifty-seven influenza A(H3N2) viruses, 200 unsubtyped influenza A viruses, and 6 influenza B viruses were identified. During the past 3 weeks (weeks 4-6), 41% of the specimens tested for influenza in the West South Central region were positive. Between 24% and 29% of the specimens tested for influenza in the Mountain, South Atlantic, and New England regions were positive, and between 16% and 19% of specimens tested for influenza in the East South Central, Mid-Atlantic, West North Central, and East North Central regions were positive. Six percent of the specimens tested for influenza in the Pacific region were positive.

Since September 30, WHO and NREVSS laboratories have tested a total of 42,409 specimens for influenza viruses and 4,151 (9.8%) were positive. Of the 4,151 isolates identified, 4,091 (99%) were influenza A viruses and 60 (1%) were influenza B viruses. One thousand two hundred and ninety-nine (32%) of the 4,091 influenza A viruses identified have been subtyped; 1,284 (99%) were H3 viruses and 15 (1%) were H1 viruses. Laboratory-confirmed influenza has been reported by all 50 states.

Influenza Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 169 influenza viruses collected from U.S. laboratories since October 1: 165 influenza A(H3N2) viruses and 4 influenza A H1 viruses. All viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2) and A/New Caledonia/ 20/99 (H1N1), respectively.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 6, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.4%. This percentage is below the epidemic threshold of 8.3% for week 6.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 6, 3.0% of patient visits to U.S. sentinel physicians were due to ILI. This is above the national baseline of 1.9%. On a regional level, the percentage of visits for ILI ranged from 1.3% to 5.1%.*** Due to wide variability in regional level data, it is not possible to calculate region-specific baselines and 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**:

Influenza activity was reported as widespread in 11 states (Arizona, Colorado, Kansas, Nebraska, New Mexico, New York, Tennessee, Texas, Utah, Virginia, and Washington), and regional in 27 states (Arkansas, California, Connecticut, Delaware, Georgia, Idaho, Indiana, Iowa, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nevada, New Hampshire, North Carolina, Ohio, Oklahoma, Pennsylvania, South Dakota, Vermont, Wisconsin, and Wyoming). Ten states, New York City and Washington, DC reported sporadic activity, one state reported no influenza activity, and one state did not report.

U. S. map for Weekly Influenza Activity


* Reporting is incomplete for this week. Numbers may change as more reports are received.

** Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

*** 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 14, 2002

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