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INFLUENZA SUMMARY UPDATE
(Week ending March 24, 2001-Week 12)
The next report will be available in two weeks, April 13, 2001.
The following information may be quoted:
Synopsis: During week 12 (March 18-24, 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 all 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 7.7%. This percentage is below the epidemic threshold for this time of year. Two state and territorial health departments reported regional influenza activity**, 36 reported sporadic influenza activity, and 11 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 12, WHO and NREVSS laboratories reported 987 specimens tested for influenza viruses, of which 56 (6%) were positive. Two (4%) were influenza A (H3N2) viruses, 12 (21%) were unsubtyped influenza A viruses, and 42 (75%) were influenza B viruses. During the past 3 weeks (weeks 10-12), the percentage of isolates nationwide that were influenza B was 78% and during those weeks influenza B viruses have predominated (range 56% to 100%) in all 9 surveillance regions.*** In the East North Central and South Atlantic regions, 10% of the specimens tested for influenza over the past 3 weeks were positive. Between 5% and 9% of specimens tested for influenza over the past 3 weeks in the New England, West South Central, Mountain, and Mid-Atlantic regions were positive. Less than 5% of specimens tested for influenza over the past 3 weeks in the West North Central, East South Central, and Pacific regions were positive.
Since October 1, WHO and NREVSS laboratories have tested a total of 71,209 specimens for influenza viruses, and 8,865 (12%) were positive. Of the 8,865 isolates identified, 5,015 (57%) were influenza type A and 3,850 (43%) were influenza type B. One thousand eight hundred and sixty-six (37%) of the 5,015 influenza A viruses identified have been subtyped; 1,780 (95%) were A (H1N1) and 86 (5%) were A (H3N2). Influenza A viruses have predominated (range 54% to 66%) 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 53% to 60%) in the Mountain, Pacific, and Mid-Atlantic regions. The New England region has 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 485 influenza viruses received from U.S. laboratories since October 1. Of the 275 influenza A (H1N1) isolates that have been characterized, 262 (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 20 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 190 influenza B viruses characterized, 29 (15%) were similar to the vaccine strain, B/Beijing/184/93, and 161 (85%) 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 12, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 7.7%. This percentage is below the epidemic threshold of 8.6% for week 12.
Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 12, 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 2 states (Arizona and Ohio) and sporadic in 36 states. Eleven states 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: March 28, 2001
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