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Weekly Report: Influenza Summary Update
Week ending February 17, 2001-Week 07

The following information may be quoted:


During week 7 (February 11-17, 2001)*, 14% 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 7.8%. This percentage is below the epidemic threshold for this time of year. Seven state and territorial health departments reported widespread influenza activity**, 27 reported regional influenza activity, and 14 reported sporadic 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 (weeks 3-5). 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 7, WHO and NREVSS laboratories reported 1,595 specimens tested for influenza viruses, of which 227 (14%) were positive. Forty (18%) were influenza A (H1N1) viruses, 1 (0.4%) was an influenza A (H3N2) virus, 69 (30%) were unsubtyped influenza A viruses, and 117 (52%) were influenza B viruses. During the past 3 weeks (weeks 5-7), the percentage of influenza B isolates nationwide has increased from 37% to 52% and during those weeks influenza B viruses have predominated in 5 of 9 surveillance regions (Mountain, New England, Pacific, Mid-Atlantic, and West South Central). Thirty-four percent of specimens tested for influenza over the past 3 weeks in the New England region*** were positive. Between 24% and 27% of specimens tested for influenza over the past 3 weeks in the East South Central, West North Central, and South Atlantic regions were positive. In the remaining 5 regions (Pacific, West South Central, East North Central, Mountain, and Mid-Atlantic), the percentage of specimens testing positive for influenza in the past 3 weeks ranged from 9% to 19%.

Since October 1, WHO and NREVSS laboratories have tested a total of 50,015 specimens for influenza viruses, and 6,274 (13%) were positive. Of the 6,274 isolates identified, 4,099 (65%) were influenza type A and 2,175 (35%) were influenza type B. One thousand four hundred and ninety-seven (37%) of the 4,099 influenza A viruses identified have been subtyped; 1,454 (97%) were A (H1N1) and 43 (3%) were A (H3N2). Influenza A viruses have predominated (range 58% to 82%) in 7 regions (New England, East South Central, Mountain, West South Central, South Atlantic, West North Central, and East North Central), and influenza B viruses have predominated (58%) in the Mid-Atlantic region. The Pacific region has reported approximately equal numbers of influenza A and influenza B isolates this season.

 Influenza Virus Isolated

Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 389 influenza viruses received from U.S. laboratories since October 1. Of the 264 influenza A (H1N1) isolates that have been characterized, 257 (97%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 7 (3%) 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 109 influenza B viruses characterized, 31 (28%) were similar to the vaccine strain, B/Beijing/184/93, and 78 (72%) 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 7, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 7.8%. This percentage is below the epidemic threshold of 8.7% for week 7.

Pneumonia And Influenza Mortality

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Influenza Morbidity Reports from U.S. Sentinel Physicians*:

During week 7, 3% 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%).

Bar Chart for Influenza-like Illness

Influenza Activity as Assessed by State and Territorial Epidemiologists**:

Influenza activity was reported as widespread in 7 states (Colorado, New Jersey, North Carolina, Oklahoma, Rhode Island, Tennessee, and Utah) and regional in 27 states (Alabama, Arizona, Connecticut, Delaware, Georgia, Hawaii, Illinois, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New York, Ohio, Oregon, Pennsylvania, South Dakota, Texas, Vermont, Washington, Wisconsin, and Wyoming). Fourteen states reported sporadic influenza activity and 2 states did not report.

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)

† These numbers represent a correction of those reported in the weekly report for weeks 4 and 5.

Report prepared: February 22, 2001


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