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Influenza Summary Update

(Week ending December 30, 2000-Week 52)

The following information may be quoted:

Synopsis: During week 52 (December 24-30, 2000)*, 11% 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 7 of 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 7.5%. This percentage is below the epidemic threshold for this time of year. Nine state and territorial health departments reported regional influenza activity**, 36 reported sporadic activity, and 4 reported no influenza activity.

U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratory Reports*During week 52, WHO and NREVSS laboratories reported 799 specimens tested for influenza viruses, of which 84 (11%) were positive. Twenty-eight were influenza A (H1N1) viruses, 24 were unsubtyped influenza A viruses, and 32 were influenza B viruses. Thirty percent of specimens tested for influenza over the past 3 weeks (weeks 50-52) in the West South Central*** region were positive. In 4 regions (East North Central, Mountain, New England, and Pacific), the percentage of specimens testing positive for influenza in the past 3 weeks ranged from 8% to 10%. In the remaining 4 regions (East South Central, Mid-Atlantic, South Atlantic, and West North Central), less than 5% of specimens tested were positive for influenza (range 3% to 4%).

Since October 1, WHO and NREVSS laboratories have tested a total of 18,121 specimens for influenza viruses, and 683 (4%) have yielded influenza virus isolates. Of the 683 isolates identified, 482 (71%) were influenza type A and 201 (29%) were influenza type B. Of the 482 influenza A viruses, 280 (58%) have been subtyped; 271 (97%) were A (H1N1) and 9 (3%) were A (H3N2). Influenza A viruses have predominanted (range 66% to 87%) in 6 regions (East North Central, Mountain, New England, South Atlantic, West North Central, and West South Central), and influenza B viruses have predominated (range 56% to 77%) in the remaining 3 regions (Mid-Atlantic, East South Central, and Pacific).

Influenza Virus

Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 64 influenza viruses received from U.S. laboratories since October 1. Of the 42 influenza A (H1N1) isolates that have been characterized, 40 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 2 (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 6 influenza A (H3N2) viruses and the 16 influenza B viruses that have been characterized, all were antigenically similar to vaccine strains A/Panama/2007/99 (H3N2) and B/Beijing/184/93, respectively.

Pneumonia and Influenza (P&I) Mortality: During week 52, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 7.5%. This percentage is below the epidemic threshold of 8.4% for week 52.

Pneumonia and Influenza Mortality 





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Influenza Morbidity Reports from U.S. Sentinel Physicians *: During week 52, 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 in the West South Central region (4%) and the Pacific region (6%).

Percentage of Visits for Influenza - like Illness

Influenza Activity as Assessed by State and Territorial Epidemiologists**: Influenza activity was reported as regional in 9 states (Arizona, Colorado, Kentucky, Maryland, Oregon, Tennessee, Texas, Utah, and Virginia). Thirty-six states reported sporadic influenza activity, 4 states reported no influenza activity, and 1 state did not report.

US Map

* 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: January 4, 2001

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