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Weekly Report: Influenza Summary Update

Week ending April 27, 2002-Week 17

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The following information may be quoted:

Synopsis:

During week 17 (April 21-27, 2002)*, 35 (6.1%) of 573 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 0.8%, which is below the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.0%, which is below the epidemic threshold of 7.8% for week 17. One state and territorial health department reported widespread influenza activity, 4 reported regional activity, 23 reported sporadic activity, and 16 reported no influenza activity **.

Laboratory Surveillance*:

During week 17, WHO and NREVSS laboratories reported 573 specimens tested for influenza viruses, of which 35 (6.1%) were positive. Three (9%) were influenza A(H3N2) viruses, 3 (9%) were unsubtyped influenza A viruses, and 29 (83%) were influenza B viruses. During the past 3 weeks (weeks 15-17), 21% of respiratory specimens in the New England region were positive for influenza. In the East North Central, Mountain, and West South Central regions the percentage of respiratory specimens testing positive for influenza ranged from 12% to 16% and in the Mid-Atlantic, East South Central, West South Central, Pacific and South Atlantic regions the percentage of respiratory specimens testing positive for influenza ranged from 3% to 8%.

Since September 30, WHO and NREVSS laboratories have tested a total of 92,172 specimens for influenza viruses and 14,635 (15.9%) were positive. Of the 14,635 isolates identified, 13,321 (91%) were influenza A viruses and 1,314 (9%) were influenza B viruses. Three thousand nine hundred and sixty-one (30%) of the 13,321 influenza A viruses identified have been subtyped; 3,893 (98%) were H3 viruses and 68 (2%) were H1 viruses. The percentage of influenza isolates that are influenza type B has increased from 3% for the week ending February 23 to 83% for the week ending April 27. For the week ending April 27, influenza B viruses accounted for at least 50% of the influenza viruses identified in the 7 surveillance regions that reported influenza isolates.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 564 influenza viruses collected since October 1 from U.S. laboratories: 350 influenza A(H3N2) viruses, 19 influenza A H1 viruses, and 195 influenza B viruses. The influenza A (H3N2) and A H1 viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2) and A/New Caledonia/ 20/99 (H1N1), respectively. Ten of the H1 viruses are H1N2 viruses collected in Wisconsin (December 2001 through February 2002) and Pennsylvania (February 2002). Two additional H1N2 viruses have been identified from patient specimens collected during July and September in Texas and Nevada, respectively.

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

Influenza B viruses currently circulating worldwide can be divided into 2 antigenically distinct lineages, B/Yamagata/16/88 and B/Victoria/2/87. Viruses of the B/Yamagata lineage have circulated widely since 1990. The B component of the current influenza vaccine belongs to the B/Yamagata lineage. Viruses of the B/Victoria lineage had not been identified outside of Asia between 1991 and March 2001. Since March 2001, B/Victoria lineage viruses have been identified in Africa, Asia, Europe, and North America. CDC has antigenically characterized 195 influenza B viruses received from U.S. laboratories and collected since October 1; 61 belonged to the B/Yamagata lineage and 134 belonged to the B/Victoria lineage. Of the 61 B/Yamagata lineage viruses, 13 were similar to the vaccine strain, B/Sichuan/379/99, and 48 showed somewhat reduced titers to ferret antisera produced against B/Sichuan/379/99. Most of the viruses that showed somewhat reduced titers to ferret antisera produced against B/Sichuan/379/99 are closely related to B/Shizuoka/15/2001, a minor antigenic variant of B/Sichuan/379/99.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 17, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.0%. This percentage is below the epidemic threshold of 7.8% for week 17.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 17, 0.8% of patient visits to U.S. sentinel physicians were due to ILI. This is below the national baseline of 1.9%. On a regional level, the percentage of visits for ILI ranged from 0% to 2.9%.*** 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 Alaska and regional in 4 states (Arizona, Missouri, Nebraska, and Texas). Twenty-three states reported sporadic influenza activity, and 16 states and New York City reported no influenza activity. Six states and Washington D.C. 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 May 2, 2002

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