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Weekly Report: Influenza Summary Update

Week ending February 1, 2003-Week 5

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The following information may be quoted:

Synopsis:

During week 5 (January 25 – February 1, 2003)*, 166 (14.3%) of the specimens tested by U.S. World Health Organization (WHO) collaborating laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 2.5%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.2%. Six state and territorial health departments reported widespread influenza activity, 18 reported regional activity, 24 reported sporadic activity, and 2 reported no influenza activity**.

Laboratory Surveillance*:

During week 5, WHO laboratories reported 1,157 specimens tested for influenza viruses, of which 166 (14.3%) were positive. Twenty-two influenza A (H1)† viruses, seven influenza A (H3N2) viruses, 20 unsubtyped influenza A viruses, and 117 influenza B viruses were identified. During the past 3 weeks (weeks 3-5), between 28.5% and 33.5% of the specimens tested for influenza in the West South Central, New England, and East North Central regions*** were positive. Between 14.6% and 15.6% of specimens tested for influenza during the past 3 weeks in the South Atlantic, East South Central, and West North Central regions were positive, and between 3.8% and 8.7% of the specimens tested in the Pacific, Mountain, and Mid-Atlantic regions were positive for influenza. Data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories for week 5 were not available at the time this report was written; therefore, numbers may change substantially next week.

Since September 29, WHO and NREVSS laboratories have tested a total of 36,753 specimens for influenza viruses and 1,714 (4.7%) were positive. Of the 1,714 viruses identified, 543 (32%) were influenza A viruses and 1,171 (68%) were influenza B viruses. Two hundred and thirty-seven (44%) of the 543 influenza A viruses have been subtyped; 192 (81%) were influenza A (H1)† viruses and 45 (19%) were influenza A (H3N2) viruses. Seven hundred and forty-nine (64%) of the 1,171 influenza B viruses were identified in Texas and Missouri. Forty-seven states have reported laboratory-confirmed influenza. Influenza A viruses were reported more frequently than influenza B viruses (range 63% - 85%) in the Mountain, New England, East North Central, Pacific, and Mid-Atlantic regions, and influenza B viruses were reported more frequently than influenza A viruses (range 59% - 92%) in the South Atlantic, West South Central, East South Central, and West North Central regions.

 Influenza Virus Isolated

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Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 119 influenza viruses submitted by U.S. laboratories since September 29: forty-seven influenza B viruses, 23 influenza A (H3N2) viruses, and 49 influenza A (H1)† viruses. Thirty-four of the influenza A (H1) viruses had the N1 neuraminidase and 15 had the N2 neuraminidase. The influenza B viruses, the A (H3N2) viruses, and the hemagglutinin proteins of the A (H1N1) and A (H1N2) viruses were similar antigenically to those of the corresponding vaccine strains B/Hong Kong/330/01, A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively.

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

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 5, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.2%. This percentage is below the epidemic threshold of 8.2% for week 5.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 5, 2.5% of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 1.9%. On a regional level***, the percentage of visits for ILI ranged from 1.3% to 4.9%. Due to wide variability in regional level data, 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 6 states (Alabama, Colorado, Missouri, Tennessee, Texas, and Virginia) and regional in 18 states. Twenty-four states and New York City reported sporadic activity, and 2 states reported no influenza activity.


Usmap 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.

†Includes both the A (H1N1) and A (H1N2) influenza virus subtypes. The influenza A (H1N2) strain appears to have resulted from the reassortment of the genes of currently circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin proteins of the A (H1N2) viruses are similar to those of the currently circulating A (H1N1) viruses and the neuraminidase proteins are similar to those of the currently circulating A (H3N2) viruses, the 2002-03 influenza vaccine should provide protection against A (H1N2) viruses.

*** 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 6, 2003

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Report prepared February 6, 2003