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Weekly Report: Influenza Summary Update

Week ending November 29, 2003-Week 48

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The following information may be quoted:

Synopsis

Influenza activity in the United States continued to increase during week 48 (November 23 - 29, 2003). One thousand three hundred nine (39.1%) of 3,350 specimens collected from throughout the United States and tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 5.1%, which is above the national baseline of 2.5%. The proportion of deaths attributed to pneumonia and influenza was 6.5%, which is below the epidemic threshold for the week. Thirteen state health departments reported widespread influenza activity, 16 states and New York City reported regional activity, 6 states reported local influenza activity, 13 states, Guam, and Puerto Rico reported sporadic influenza activity, and 1 state and the District of Columbia reported no influenza activity.

Laboratory Surveillance*

During week 48, WHO and NREVSS laboratories reported 3,350 specimens tested for influenza viruses, and 1,309 (39.1%) were positive. Of these, 105 were influenza A(H3N2) viruses, 1,197 were influenza A viruses that were not subtyped, and 7 were influenza B viruses.

Since September 28, WHO and NREVSS laboratories have tested a total of 19,469 specimens for influenza viruses and 4,992 (25.6%) were positive. Among the 4,992 influenza viruses, 4,973 (99.6%) were influenza A viruses and 19 (0.4%) were influenza B viruses. One thousand sixteen (20.4%) of the 4,973 influenza A viruses have been subtyped; 1,015 (99.9%) were influenza A (H3N2) viruses and 1 (0.1%) was an A (H1) virus. Forty-one states and all 9 surveillance regions** have reported laboratory-confirmed influenza this season. Two thousand one hundred ninety-four (44.0%) of the 4,992 isolates were reported from the West South Central region, and 1,793 (35.9%) were from the Mountain region.

Influenza Virus Isolated


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Antigenic Characterization

CDC has antigenically characterized 157 influenza A (H3N2) viruses collected by U.S. laboratories since October 1 and found that 45 (29%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 112 (71%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). The A/Fujian strain predominated in Australia and New Zealand during the recent Southern Hemisphere influenza season and is a drift variant related to the vaccine strain, A/Panama/2007/99. Antibodies produced against the vaccine virus cross-react with A/Fujian/411/2002-like viruses, but at a lower level than against A/Panama/2007/99 (H3N2). Vaccine effectiveness depends, in part, on the match between vaccine strains and circulating viruses and cannot be determined by laboratory testing. Although vaccine effectiveness against A/Fujian/411/2002-like viruses may be less than that against A/Panama/2007/99-like viruses, it is expected that the current U.S. vaccine will offer some cross-protective immunity against the A/Fujian/411/2002-like viruses and reduce the severity of disease. One influenza A(H1N1) virus was antigenically characterized and was similar to the vaccine strain A/New Caledonia/20/99.

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 48, 6.5% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is below the epidemic threshold of 7.5% for week 48.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *

During week 48, 5.1%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 2.5%. The percentage of patient visits for ILI increased in all regions. On a regional level**, the percentage of visits for ILI was highest in the West South Central region (14.6%), followed by the Pacific (7.2%), Mountain (5.8%), and South Atlantic (4.2%) regions. All other regions were below 4%. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

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 13 states (Alaska, Arkansas, Colorado, Idaho, Nebraska, Nevada, New Mexico, Pennsylvania, Tennessee, Texas, Utah, Washington, and Wyoming), regional in 16 (Arizona, California, Florida, Georgia, Indiana, Iowa, Maryland, Mississippi, Montana, New York, North Carolina, North Dakota, Oklahoma, Oregon, Rhode Island, and South Carolina) and New York City, and local in 6 states (Illinois, Kansas, Louisiana, Minnesota, Missouri, and Virginia). Sporadic influenza activity was reported in 13 states (Alabama, Connecticut, Delaware, Hawaii, Kentucky, Maine, Michigan, New Hampshire, Ohio, South Dakota, Vermont, West Virginia, and Wisconsin) Guam, and Puerto Rico. Massachusetts and the District of Columbia reported no influenza activity, and 1 state did not report.

U. S. map for Weekly Influenza Activity
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* Reporting is incomplete for this week. Numbers may change as more reports are received.

** 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)

*** The national and regional percentage of patient visits for ILI is weighted on the basis of state population.

Report prepared December 5, 2003

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