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Weekly Report: Influenza Summary Update

Week ending March 23, 2002-Week 12

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The following information may be quoted:

Synopsis:

During week 12 (March 17-23, 2002)*, 211 (19.9%) of 1,058 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 1.7%, which is below the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 8.8%, which is above the epidemic threshold of 8.2% for week 12. Three state and territorial health departments reported widespread influenza activity, 16 reported regional activity, 27 reported sporadic activity, and 2 reported no influenza activity **.

Laboratory Surveillance*:

During week 12, WHO and NREVSS laboratories reported 1,058 specimens tested for influenza viruses, of which 211 (19.9%) were positive. Thirty-five influenza A(H3N2) viruses, 119 unsubtyped influenza A viruses, and 57 influenza B viruses were identified. During the past 3 weeks (weeks 10-12), the percentage of respiratory specimens testing positive for influenza ranged from 23% to 28% in the East South Central, East North Central, New England, West North Central, and South Atlantic regions. In the West South Central, Mountain, and Mid-Atlantic regions the percentage of respiratory specimens testing positive for influenza ranged from 15% to 17% and in the Pacific region 9% of specimens tested were positive for influenza.

Since September 30, WHO and NREVSS laboratories have tested a total of 72,877 specimens for influenza viruses and 12,017 (16.5%) were positive. Of the 12,017 isolates identified, 11,599 (97%) were influenza A viruses and 418 (3%) were influenza B viruses. Three thousand four hundred and seventy-nine (30%) of the 11,599 influenza A viruses identified have been subtyped; 3,426 (98%) were H3 viruses and 53 (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 27% for the week ending March 23. Thirty-three percent of the influenza B isolates reported this season were identified in the Mid-Atlantic region.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 342 influenza viruses collected from U.S. laboratories since October 1: 279 influenza A(H3N2) viruses, 16 influenza A H1 viruses, and 47 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 (Hawaii, Maryland, New Jersey, New York, and Wisconsin). CDC has antigenically characterized 47 influenza B viruses received from U.S. laboratories and collected since October 1; 34 belonged to the B/Yamagata lineage and 13 belonged to the B/Victoria lineage. Of the 34 B/Yamagata lineage viruses, 12 were similar to the vaccine strain, B/Sichuan/379/99, and 22 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.

Composition of the 2002-03 Influenza Vaccine:

The Food and Drug Administration's Vaccine and Related Biological Products Advisory Committee recommended that the 2002-03 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Hong Kong/330/2001-like viruses. The influenza B component of the 2002-03 season vaccine is a virus of the B/Victoria lineage. The A(H1N1) and A(H3N2) components are the same as those for the 2001-02 season vaccine. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and postvaccination serologic studies in humans.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 12, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 8.8%. This percentage is above the epidemic threshold of 8.2% for week 12.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 12, 1.7% 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 0.8% to 4.4%.*** 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 3 states (Arizona, Missouri, and Vermont), and regional in 16 states (California, Idaho, Illinois, Louisiana, Michigan, Montana, Nebraska, New York, Ohio, South Carolina, South Dakota, Tennessee, Utah, Virginia, Washington, and Wisconsin). Twenty-seven states, New York City, and Washington D.C. reported sporadic influenza activity, and Alaska and Georgia reported no influenza activity. Two 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 March 28, 2002

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