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Weekly Report: Influenza Summary Update
Week ending March 3, 2001-Week 09

The following information may be quoted:


During week 9 (February 25 – March 3, 2001)*, 12% 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 7.3%. This percentage is below the epidemic threshold for this time of year. One state and territorial health department reported widespread influenza activity**, 18 reported regional influenza activity, and 31 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. 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 9, WHO and NREVSS laboratories reported 1,100 specimens tested for influenza viruses, of which 137 (12%) were positive. Fourteen (10%) were influenza A (H1N1) viruses, 2 (2%) were influenza A (H3N2) viruses, 37 (27%) were unsubtyped influenza A viruses, and 84 (61%) were influenza B viruses. During the past 3 weeks (weeks 7-9), the percentage of isolates nationwide that were influenza B was 63% and during those weeks influenza B viruses have predominated (range 61% to 80%) in 7 of 9 surveillance regions*** (West North Central, New England, East North Central, Mid-Atlantic, Mountain, Pacific, and West South Central). Between 15% and 20% of specimens tested for influenza over the past 3 weeks in the East South Central, Mountain, West North Central, East North Central, New England, and South Atlantic regions were positive. In the West South Central and Mid-Atlantic regions the percentage of specimens testing positive for influenza in the past 3 weeks ranged from 10% to 14%. In the Pacific region, 5% of specimens tested for influenza in the past 3 weeks were positive.

Since October 1, WHO and NREVSS laboratories have tested a total of 59,052 specimens for influenza viruses, and 7,819 (13%) were positive. Of the 7,819 isolates identified, 4,728 (60%) were influenza type A and 3,091 (40%) were influenza type B. One thousand seven hundred and sixty-five (37%) of the 4,728 influenza A viruses identified have been subtyped; 1,697 (96%) were A (H1N1) and 68 (4%) were A (H3N2). Influenza A viruses have predominated (range 52% to 72%) in 6 regions (Mountain, East South Central, West South Central, West North Central, South Atlantic, and East North Central), and influenza B viruses have predominated (range 54% to 58%) in the 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

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Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 393 influenza viruses received from U.S. laboratories since October 1. Of the 216 influenza A (H1N1) isolates that have been characterized, 205 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 11 (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 9, 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 8.4% for week 9.

Pneumonia And Influenza Mortality

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

During week 9, 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%).

Bar Chart for Influenza-like Illness

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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 18 states (Alabama, Arizona, Colorado, Connecticut, Delaware, Kentucky, Maryland, Michigan, Minnesota, Nebraska, New York, North Carolina, Ohio, South Dakota, Tennessee, Utah, Vermont, and Wyoming). Thirty-one states reported sporadic 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: March 8, 2001


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