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INFLUENZA SUMMARY UPDATE
(Week ending March 10, 2001-Week 10)
The following information may be quoted:
Synopsis: During week 10 (March 4-10, 2001)*, 6% 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 8 of 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 8.0%. This percentage is below the epidemic threshold for this time of year. One state and territorial health department reported widespread influenza activity**, 12 reported regional influenza activity, 35 reported sporadic activity, and one reported no influenza activity.
For the current season, the overall national percentage of respiratory specimens positive for influenza appears to have 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 appears to have peaked at 4% in mid to late January. 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 10, WHO and NREVSS laboratories reported 1,464 specimens tested for influenza viruses, of which 94 (6%) were positive. Seven (7%) were influenza A (H1N1) viruses, 27 (29%) were unsubtyped influenza A viruses, and 60 (64%) were influenza B viruses. During the past 3 weeks (weeks 8-10), the percentage of isolates nationwide that were influenza B was 70% and during those weeks influenza B viruses have predominated (range 61% to 93%) in 8 of 9 surveillance regions*** (New England, East South Central, West North Central, Mid-Atlantic, Mountain, East North Central, West South Central, and Pacific). Twenty-two percent of specimens tested for influenza over the past 3 weeks in the New England region were positive. Between 10% and 16% of specimens tested for influenza over the past 3 weeks in the Mid-Atlantic, Mountain, West North Central, South Atlantic, and East North Central regions were positive. In the remaining 3 regions (Pacific, West South Central, and East South Central), the percentage of specimens testing positive for influenza in the past 3 weeks was less than 10%.
Since October 1, WHO and NREVSS laboratories have tested a total of 64,840 specimens for influenza viruses, and 8,386 (13%) were positive. Of the 8,386 isolates identified, 4,885 (58%) were influenza type A and 3,501 (42%) were influenza type B. One thousand eight hundred and twenty-six (37%) of the 4,885 influenza A viruses identified have been subtyped; 1,746 (96%) were A (H1N1) and 80 (4%) were A (H3N2). Influenza A viruses have predominated (range 56% to 69%) in the East South Central, West South Central, West North Central, South Atlantic, and East North Central regions, and influenza B viruses have predominated (range 55% to 59%) in the Pacific and Mid-Atlantic regions. The New England and Mountain regions have reported approximately equal numbers of influenza A and influenza B isolates this season.
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Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 436 influenza viruses received from U.S. laboratories since October 1. Of the 259 influenza A (H1N1) isolates that have been characterized, 246 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 13 (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 16 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 161 influenza B viruses characterized, 29 (18%) were similar to the vaccine strain, B/Beijing/184/93, and 132 (82%) 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 10, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 8.0%. This percentage is below the epidemic threshold of 8.7% for week 10.
Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 10, 2% of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of patient visits for ILI was above baseline levels of 0% to 3% only in the Pacific region (4%).
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Influenza Activity as Assessed by State and Territorial Epidemiologists**: Influenza activity was reported as widespread in Rhode Island and regional in 12 states (Alabama, Arizona, Colorado, Delaware, Kentucky, Maryland, Michigan, Minnesota, Nebraska, New York, Ohio, and South Dakota). Thirty-five states reported sporadic activity, one state reported no influenza activity, and one 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: Report prepared: March 15, 2001
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