The following information may be quoted:
During week 6 (February 4-10, 2001)*, 22% 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 5 of 9 surveillance regions. The proportion of deaths attributed to pneumonia and influenza was 7.7%. This percentage is below the epidemic threshold for this time of year. Ten state and territorial health departments reported widespread influenza activity**, 26 reported regional influenza activity, and 13 reported sporadic activity.
U.S. World Health Organization (WHO) and National Respiratory And Enteric Virus Surveillance System(NREVSS)Collaborating Laboratory Reports *:
During week 6, WHO and NREVSS laboratories reported 1,668 specimens tested for influenza viruses, of which 368 (22%) were positive. Sixty-four (17%) were influenza A (H1N1) viruses, 1 (0.3%) was an influenza A (H3N2) virus, 136 (37%) were unsubtyped influenza A viruses, and 167 (45%) were influenza B viruses. Thirty-four percent of specimens tested for influenza over the past 3 weeks (weeks 4-6) 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, Mountain, East North Central, West North Central, and New England regions were positive. In the remaining 3 regions (Pacific, Mid-Atlantic, and West South Central), the percentage of specimens testing positive for influenza in the past 3 weeks ranged from 11% to 18%.
Since October 1, WHO and NREVSS laboratories have tested a total of 43,427 specimens for influenza viruses, and 5,307 (12%) were positive. Of the 5,307 isolates identified, 3,629 (68%) were influenza type A and 1,678 (32%) were influenza type B. One thousand two hundred and sixty-nine (35%) of the 3,629 influenza A viruses identified have been subtyped; 1,233 (97%) were A (H1N1) and 36 (3%) were A (H3N2). Influenza A viruses have predominated (range 58% to 84%) in 7 regions (East South Central, New England, Mountain, West South Central, West North Central, South Atlantic, and East North Central), and influenza B viruses have predominated (56%) in the Mid-Atlantic region. The Pacific region has reported approximately equal numbers of influenza A and influenza B isolates this season.
Antigenic Characterization of Viral Isolates:
CDC has antigenically characterized 250 influenza viruses received from U.S. laboratories since October 1. Of the 125 influenza A (H1N1) isolates that have been characterized, 118 (94%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 7 (6%) 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 6, the percentage of all deaths due to P&I as reported by the vital statistics offices of 122 U.S. cities was 7.7%. This percentage is below the epidemic threshold of 8.7% for week 6.
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Influenza Morbidity Reports from U.S. Sentinel Physicians*:
During week 6, 3% 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 4 of the 9 surveillance regions (East South Central, New England, Pacific, and West South Central) and ranged from 4% to 6%.
Influenza Activity as Assessed by State and Territorial Epidemiologists**:
Influenza activity was reported as widespread in 10 states (Colorado, Minnesota, New Jersey, New York, North Carolina, Oklahoma, Rhode Island, Tennessee, Utah, and Virginia) and regional in 26 states (Alabama, Arizona, Connecticut, Delaware, Georgia, Idaho, Illinois, Iowa, Kentucky, Louisiana, Maryland, Massachusetts, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, North Dakota, Ohio, Oregon, Pennsylvania, South Dakota, Texas, Vermont, and Wyoming). Thirteen states reported sporadic influenza activity and 1 state did not report.
* 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 15, 2001