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Weekly Report: Influenza Summary Update
Week ending December 29, 2001-Week 52
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Synopsis:
During week 52 (December 23-29, 2001)*, 20 of 608 respiratory specimens tested by WHO and NREVSS laboratories were positive for influenza. The overall proportion of patient visits to sentinel physicians for influenza-like illness (ILI) was 2,5 %, which is above the national baseline of 1,9 %. The proportion of deaths attributed to pneumonia and influenza was 7 %, which is below the epidemic threshold of 7,9 % for week 52. One state and territorial health department reported widespread influenza activity, 3 reported regional activity, 42 reported sporadic activity, and 3 reported no influenza activity**.
Laboratory Surveillance*:
During week 52, WHO and NREVSS laboratories reported 608 specimens tested for influenza viruses. Five influenza A (H3N2) viruses and 15 unsubtyped influenza A viruses were identified.
Since September 30, WHO and NREVSS laboratories have tested a total of 17 212 specimens for influenza viruses and 356 (2,2 %) were positive. Of the 356 isolates identified, 343 (96 %) were influenza A viruses and 13 (4 %) were influenza B viruses. Two hundred and nine (61 %) of the 343 influenza A viruses identified have been subtyped; 203 were influenza A (H3N2) viruses and 6 were influenza A (H1N1) viruses. One hundred and ninety-eight (56 %) of the 356 U.S. isolates were reported from the Pacific region*** and 136 (38 %) of the 356 isolates reported were identified in Alaska. Thirty-six states have reported laboratory-confirmed influenza.
Antigenic Characterization:
CDC antigenically characterized 10 influenza viruses collected in September and 23 collected since October: 30 influenza A (H3N2) viruses, 2 influenza A (H1N1) viruses, and 1 influenza B virus. All viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2), A/New Caledonia/ 20/99 (H1N1), and B/Sichuan/379/99, respectively.
Pneumonia and Influenza (P&I) Mortality Surveillance:
During week 52, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7 %. This percentage is below the epidemic threshold of 7,9 % for week 52.
Influenza-like Illness Surveillance *:
During week 52, 2,5 % 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 0,9 % to 6 %.*** 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.
Influenza Activity as Assessed by State and Territorial Epidemiologists**:
Influenza activity was reported as widespread in Colorado, and regional in Connecticut, Pennsylvania, and Utah. Forty-two states and New York City reported sporadic activity, 3 states reported no influenza activity, and 1 state did not report.
* 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 January 4, 2002
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