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Weekly Report: Influenza Summary Update

Week ending March 30, 2002-Week 13

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The following information may be quoted:

Synopsis:

During week 13 (March 24 - 30, 2002)*, 190 (14.1%) of 1,344 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 1.6%, which is below the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 8.6%, which is above the epidemic threshold of 8.1% for week 13. One state and territorial health department reported widespread influenza activity, 13 reported regional activity, 29 reported sporadic activity, and 3 reported no influenza activity **.

Laboratory Surveillance*:

During week 13, WHO and NREVSS laboratories reported 1,344 specimens tested for influenza viruses, of which 190 (14.1%) were positive. Twenty-one (11%) were influenza A(H3N2) viruses, 1 (0.5%) was an influenza A(H1N1) virus, 59 (31%) were unsubtyped influenza A viruses, and 109 (57%) were influenza B viruses. During the past 3 weeks (weeks 11-13), the percentage of respiratory specimens testing positive for influenza ranged from 22% to 24% in the East North Central, New England, West North Central, and South Atlantic regions. In the Mountain, West South Central, East South Central, and Mid-Atlantic regions the percentage of respiratory specimens testing positive for influenza ranged from 13% to 17% and in the Pacific region 10% of specimens tested were positive for influenza.

Since September 30, WHO and NREVSS laboratories have tested a total of 77,199 specimens for influenza viruses and 12,780 (16.6%) were positive. Of the 12,780 isolates identified, 12,119 (95%) were influenza A viruses and 661 (5%) were influenza B viruses. Three thousand six hundred and forty-seven (30%) of the 12,119 influenza A viruses identified have been subtyped; 3,592 (98%) were H3 viruses and 55 (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 57% for the week ending March 30. For the week ending March 30, influenza B viruses were the predominant viruses (57% to 89%) identified in 6 of 9 surveillance regions.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 391 influenza viruses collected from U.S. laboratories since October 1: 279 influenza A(H3N2) viruses, 16 influenza A H1 viruses, and 96 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. Nine of the H1 viruses are H1N2 viruses collected in Wisconsin during December, January, and February. 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 Canada, China, Hong Kong, India, Italy, Japan, the Netherlands, Oman, the Philippines, Thailand, and the United States (California, Iowa, Hawaii, Maryland, Massachusetts, Michigan, New Jersey, New York, and Wisconsin). CDC has antigenically characterized 96 influenza B viruses received from U.S. laboratories and collected since October 1; 53 belonged to the B/Yamagata lineage and 43 belonged to the B/Victoria lineage. Of the 53 B/Yamagata lineage viruses, 22 were similar to the vaccine strain, B/Sichuan/379/99, and 31 showed somewhat reduced titers to ferret antisera produced against B/Sichuan/379/99. The B component of the current influenza vaccine is expected to provide lower levels of protection against viruses of the B/Victoria lineage.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 13, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 8.6%. This percentage is above the epidemic threshold of 8.1% for week 13.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 13, 1.6% 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 0.5% to 5.9%.*** 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 Arizona and regional in 13 states (California, Idaho, Minnesota, Missouri, Montana, New York, Ohio, Pennsylvania, South Carolina, Tennessee, Vermont, Washington, and Wisconsin). Twenty-nine states, New York City, and Washington D.C. reported sporadic influenza activity, and 3 states reported no influenza activity. Four states 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 April 4, 2002

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