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Weekly Report: Influenza Summary Update

Week ending May 11, 2002-Week 19

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The following information may be quoted:

Synopsis:

During week 19 (May 5-11, 2002)*, 32 (6.9%) of 461 respiratory specimens tested by World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories were positive for influenza. The overall proportion of patient visits to sentinel physicians for influenza-like illness (ILI) was 0.6%, which is below the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 6.8%, which is below the epidemic threshold of 7.6% for week 19. Twenty state and territorial health department reported sporadic influenza and 25 reported no influenza activity **.

Laboratory Surveillance*:

During week 19, WHO and NREVSS laboratories reported 461 specimens tested for influenza viruses, of which 32 (6.9%) were positive. One (3%) was an influenza A(H1N1) virus, 3 (9%) were unsubtyped influenza A viruses, and 28 (88%) were influenza B viruses. During the past 3 weeks (weeks 17-19), percentages of respiratory specimens testing positive for influenza were 14% in the New England region, 7% - 10% in the Pacific, West North Central, West South Central, East North Central, South Atlantic, and Mountain regions, and 0% - 3% in the Mid-Atlantic and East South Central regions.

Since September 30, WHO and NREVSS laboratories have tested a total of 95,501 specimens for influenza viruses and 14,982 (15.7%) were positive. Of the 14,982 isolates identified, 13,454 (90%) were influenza A viruses and 1,528 (10%) were influenza B viruses. Four thousand and sixty-four (30%) of the 13,454 influenza A viruses identified have been subtyped; 3,988 (98%) were H3 viruses and 76 (2%) were H1 viruses. Influenza B viruses were reported more frequently than influenza A viruses each week since the end of March.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 604 influenza viruses collected since October 1 from U.S. laboratories: 366 influenza A(H3N2) viruses, 26 influenza A H1 viruses, and 212 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 were 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 212 influenza B viruses received from U.S. laboratories and collected since October 1; 61 belonged to the B/Yamagata lineage and 151 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 19, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6.8%. This percentage is below the epidemic threshold of 7.6% for week 19.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 19, 0.6% 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 2.0%.*** 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 sporadic in 20 states (Alaska, Arizona, Arkansas, California, Colorado, Florida, Hawaii, Indiana, Minnesota, Missouri, Nebraska, Nevada, New Mexico, North Dakota, Ohio, South Dakota, Texas, Vermont, West Virginia, and Wyoming). Twenty-five states and New York City reported no influenza activity. Five 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 16, 2002

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