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(Week ending April 21, 2001-Week 16)

The next report will be available in two weeks (May 11, 2001).

The following information may be quoted:

Synopsis: During week 16 (April 15-21, 2001)*, 3% 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 7.3%. This percentage is below the epidemic threshold for this time of year. Nineteen state and territorial health departments reported sporadic influenza activity** and 31 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 16, WHO and NREVSS laboratories reported 516 specimens tested for influenza viruses, of which 17 (3%) were positive. All 17 isolates reported for week 16 were influenza B viruses and during the past 3 weeks (weeks 14-16), 89% of isolates reported were influenza B viruses. Less than 10% (range 0% to 6%) of specimens tested for influenza during the past 3 weeks in each of 9 surveillance regions*** were positive.

Since October 1, WHO and NREVSS laboratories have tested a total of 80,686 specimens for influenza viruses, and 9,563 (12%) were positive. Of the 9,563 isolates identified, 5,213 (55%) were influenza type A and 4,350 (45%) were influenza type B. Two thousand and nine (39%) of the 5,213 influenza A viruses identified have been subtyped; 1,912 (95%) were A (H1N1) and 97 (5%) were A (H3N2). Influenza A viruses have predominated (range 52% to 63%) in the East South Central, West South Central, West North Central, East North Central, and South Atlantic regions, and influenza B viruses have predominated (range 53% to 62%) 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.

 Influenza Virus Isolated View Chart Data

Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 603 influenza viruses received from U.S. laboratories since October 1. Of the 325 influenza A (H1N1) isolates that have been characterized, 309 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 16 (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 22 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 256 influenza B viruses characterized, 29 (11%) were similar to the vaccine strain, B/Beijing/184/93, and 227 (89%) 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 16, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 7.3%. This percentage is below the epidemic threshold of 8.3% for week 16.

Pneumonia And Influenza Mortality 





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Influenza Morbidity Reports from U.S. Sentinel Physicians*: During week 16, 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.

Bar Chart for Influenza-like Illness View Chart Data

Influenza Activity as Assessed by State and Territorial Epidemiologists**: Influenza activity was reported as sporadic in 19 states (Arizona, Arkansas, Florida, Hawaii, Indiana, Kentucky, Michigan, Missouri, New Mexico, New York, Ohio, Pennsylvania, Rhode Island, South Dakota, Tennessee, Virginia, West Virginia, Wisconsin, and Wyoming). Thirty-one 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.

*** 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: April 26, 2001

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