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INFLUENZA SUMMARY UPDATE
(Week ending May 19, 2001-Week 20)
This is the final report of the 2000-2001 season.
The following information may be quoted:
Synopsis: During week 20 (May 13-19, 2001)*, 1% of the specimens tested by WHO and NREVSS laboratories were positive for influenza. The proportion of patient visits to sentinel physicians for influenza-like illness was within baseline levels of 0% to 3% in the United States overall and in all 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 6.8%. This percentage is below the epidemic threshold for this time of year. One state and territorial health department reported regional influenza activity**, 9 reported sporadic influenza activity, and 40 reported no influenza activity.
For the current season, the overall national percentage of respiratory specimens positive for influenza peaked at 24% at the end of January (week 4). During the past 3 seasons, the peak percentages of respiratory specimens positive for influenza viruses have ranged from 28% to 33%. For this season, the percentage of patient visits to sentinel physicians for influenza-like illness peaked at 4% between mid-January and mid-February. During the past 3 seasons, the peak percentages for such visits ranged between 5% and 6%.
U.S. World Health Organization (WHO) and National Respiratory And Enteric Virus Surveillance System(NREVSS) Collaborating Laboratory Reports *: During week 20, WHO and NREVSS laboratories reported 452 specimens tested for influenza viruses, of which 3 (1%) were positive. All 3 isolates reported for week 20 were influenza B viruses. During the past 3 weeks (weeks 18-20), 7 (0.3%) of the specimens tested for influenza were positive and 6 (86%) were influenza B viruses. The percentage of specimens testing positive for influenza during the past 3 weeks in each of 9 surveillance regions*** ranged from 0% to 6%.
Since October 1, WHO and NREVSS laboratories have tested a total of 86,534 specimens for influenza viruses, and 9,900 (11%) were positive. Of the 9,900 isolates identified, 5,317 (54%) were influenza type A and 4,583 (46%) were influenza type B. Two thousand one hundred and twenty-three (40%) of the 5,317 influenza A viruses identified have been subtyped; 2,028 (96%) were A (H1N1) and 95 (4%) were A (H3N2). Influenza A viruses have predominated (range 55% to 63%) in the West South Central, West North Central, East North Central, and South Atlantic regions, and influenza B viruses have predominated (range 53% to 63%) in the Mountain, Pacific, and Mid-Atlantic regions. The New England and East South Central regions have reported approximately equal numbers of influenza A and influenza B isolates this season.
View Chart Data
Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 687 influenza viruses received from U.S. laboratories since October 1. Of the 353 influenza A (H1N1) isolates that have been characterized, 334 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 19 (5%) were similar to A/Bayern/07/95. Although A/Bayern-like viruses are antigenically distinct from the A/New Caledonia-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses. Of the 30 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 304 influenza B viruses characterized, 29 (10%) were similar to the vaccine strain, B/Beijing/184/93, and 275 (90%) were more closely related antigenically to the B/Sichuan/379/99 reference strain. It should be noted that the B/Sichuan virus exhibits cross-reactivity with the vaccine strain.
Pneumonia and Influenza (P&I) Mortality: During week 20, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 6.8%. This percentage is below the epidemic threshold of 8.0% for week 20.
Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 20, less than 1% of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of patient visits for ILI was within baseline levels of 0% to 3% in all 9 surveillance regions.
View Chart Data
Influenza Activity as Assessed by State and Territorial Epidemiologists**: Influenza activity was reported as regional in Hawaii and sporadic in 9 states (Alaska, Kentucky, Missouri, New Mexico, Ohio, Oklahoma, Virginia, West Virginia, and Wyoming). Forty states reported no 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: May 29, 2001
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