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Weekly Report: Influenza Summary Update

Week ending February 23, 2002-Week 8

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The following information may be quoted:

Synopsis:

During week 8 (February 17-23, 2002)*, 752 (25.9%) of 2,902 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 3.5%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 8.1%, which is below the epidemic threshold of 8.3% for week 8. Fourteen state and territorial health departments reported widespread influenza activity, 24 reported regional activity, and 11 reported sporadic influenza activity **.

Laboratory Surveillance*:

During week 8, WHO and NREVSS laboratories reported 2,902 specimens tested for influenza viruses, of which 752 (25.9%) were positive. One hundred and sixty-four influenza A(H3N2) viruses, 575 unsubtyped influenza A viruses, and 13 influenza B viruses were identified. During the past 3 weeks (weeks 6-8), between 30% and 36% of the specimens tested for influenza in the New England, East North Central, South Atlantic, and West South Central regions were positive. The percentage of respiratory specimens testing positive for influenza in the East South Central and West North Central regions ranged from 24% to 26%. In the Mid-Atlantic and Mountain regions, 17% of specimens tested for influenza were positive and in the Pacific region 9% of specimens tested were positive.

Since September 30, WHO and NREVSS laboratories have tested a total of 55,876 specimens for influenza viruses and 7,499 (13.4%) were positive. Of the 7,499 isolates identified, 7,402 (99%) were influenza A viruses and 97 (1%) were influenza B viruses. Two thousand one hundred and eighty-eight (30%) of the 7,402 influenza A viruses identified have been subtyped; 2,162 (99%) were H3 viruses and 26 (1%) were H1 viruses.

Influenza Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 258 influenza viruses collected from U.S. laboratories since October 1: 225 influenza A(H3N2) viruses, 5 influenza A H1 viruses, and 28 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. One of the H1 viruses is an H1N2 virus from a patient specimen collected in Wisconsin in December. 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, and New York). CDC has antigenically characterized 28 influenza B viruses received from U.S. laboratories and collected since October 1; 24 belonged to the B/Yamagata lineage and 4 belonged to the B/Victoria lineage. Of the 24 B/Yamagata lineage viruses, 7 were similar to the vaccine strain, B/Sichuan/379/99, and 17 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. A final decision has not yet been made in the United States regarding the influenza B component of the 2002-03 vaccine.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 8, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 8.1%. This percentage is below the epidemic threshold of 8.3% for week 8.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 8, 3.5% 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 1.2% to 5.0%.*** 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 widespread in 14 states (Arizona, Colorado, Minnesota, Missouri, Nebraska, New Hampshire, New York, Pennsylvania, South Dakota, Tennessee, Texas, Utah, Virginia, and Wisconsin), and regional in 24 states (Arkansas, California, Connecticut, Delaware, Georgia, Idaho, Indiana, Iowa, Kansas, Louisiana, Maryland, Massachusetts, Michigan, Mississippi, Montana, Nevada, New Mexico, North Carolina, North Dakota, Ohio, Oklahoma, South Carolina, Vermont, and Wyoming). Eleven states and New York City reported sporadic 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 28, 2002

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