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Weekly Report: Influenza Summary Update
Week ending January 27, 2001-Week 04

The following information may be quoted:


During week 4 (January 21-27, 2001)*, 24 % 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 4 % overall in the United States, exceeding baseline levels of 0 % to 3 %, and was above the baseline 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. Thirteen state and territorial health departments reported widespread influenza activity**, 22 reported regional influenza activity, and 14 reported sporadic activity.

U.S. World Health Organization (WHO) and National Respiratory And Enteric Virus Surveillance System(NREVSS)Collaborating Laboratory Reports *:

During week 4, WHO and NREVSS laboratories reported 1 949 specimens tested for influenza viruses, of which 475 (24 %) were positive. Sixty-seven (14 %) were influenza A (H1N1) viruses, 250 (53 %) were unsubtyped influenza A viruses, and 158 (33 %) were influenza B viruses. Thirty-three percent of specimens tested for influenza over the past 3 weeks (weeks 2-4) in the South Atlantic region*** were positive. Between 20 % and 30 % of specimens tested for influenza over the past 3 weeks in the East South Central, West North Central, West South Central, East North Central, and New England regions were positive. In the remaining 3 regions (Mid-Atlantic, Pacific, and Mountain), the percentage of specimens testing positive for influenza in the past 3 weeks ranged from 10 % to 17 %.

Since October 1, WHO and NREVSS laboratories have tested a total of 34 142 specimens for influenza viruses, and 3 128 (9 %) were positive. Of the 3 128 isolates identified, 2 242 (72 %) were influenza type A and 886 (28 %) were influenza type B. Seven hundred and ninety-five (35 %) of the 2 242 influenza A viruses identified have been subtyped; 773 (97 %) were A (H1N1) and 22 (3 %) were A (H3N2). Influenza A viruses have predominated (range 63 % to 88 %) in 6 regions (East North Central, Mountain, New England, South Atlantic, West North Central, and West South Central), and influenza B viruses have predominated (58 %) in the Mid-Atlantic region. The East South Central and Pacific regions have 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 244 influenza viruses received from U.S. laboratories since October 1. Of the 121 influenza A (H1N1) isolates that have been characterized, 115 (95 %) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 6 (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 13 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 110 influenza B viruses characterized, 57 (52 %) were similar to the vaccine strain, B/Beijing/184/93, and 53 (48 %) 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 4, 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 8,6 % for week 4.

Pneumonia And Influenza Mortality

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

During week 4, 4 % of patient visits to U.S. sentinel physicians were due to influenza-like illness (ILI). The percentage of ILI was above the baseline levels of 0 % to 3 % in 7 of the 9 surveillance regions (East South Central, Mid-Atlantic, Mountain, New England, Pacific, West North Central, and West South Central) and ranged from 4 % to 11 %.

Bar Chart for Influenza-like Illness

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

Influenza activity was reported as widespread in 13 states (Arizona, Colorado, Connecticut, Iowa, Minnesota, Mississippi, North Carolina, Oregon, Rhode Island, Tennessee, Utah, Virginia, and Washington) and regional in 22 states (Alabama, Arkansas, Georgia, Hawaii, Idaho, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Missouri, Nevada, New Hampshire, New Jersey, New York, Pennsylvania, South Dakota, Texas, Vermont, West Virginia, and Wyoming). Fourteen states reported sporadic influenza activity and 1 state 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)

Report prepared: February 2, 2001


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