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Weekly Report: Influenza Summary Update

Week ending January 24, 2004-Week 3

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The following information may be quoted:

Synopsis:

All four surveillance components reflected decreases in influenza activity during week 3 (January 18-24, 2004). The percentage of specimens testing positive for influenza decreased, and the percentage of patient visits for influenza-like illness (ILI) remained below the national baseline of 2.5%. Mortality due to pneumonia and influenza (9.7%) appears to have peaked, but remained above the epidemic threshold during week 3 (8.2%). Eighty-five (7.5%) of 1,136 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. One state health department reported widespread influenza activity, 20 states and New York City reported regional activity, 19 states reported local activity, and 9 states, the District of Columbia, Guam, and Puerto Rico reported sporadic activity.

Laboratory Surveillance*:

During week 3, WHO and NREVSS laboratories reported 1,136 specimens tested for influenza viruses, and 85 (7.5%) were positive. Of these, 23 were influenza A(H3N2) viruses and 62 were influenza A viruses that were not subtyped.

Since September 28, WHO and NREVSS laboratories have tested 83,218 specimens for influenza viruses, of which 21,599 (26.0%) were positive. Of these, 21,471 (99.4%) were influenza A viruses, and 128 (0.6%) were influenza B viruses. Of the 21,471 influenza A viruses, 5,320 (24.8%) have been subtyped; 5,319 (99.9%) were influenza A (H3N2) viruses and one (0.1%) was an influenza A (H1) virus.

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Antigenic Characterization:

CDC has antigenically characterized 573 influenza viruses collected by U.S. laboratories since October 1: two influenza A (H1) viruses, 565 influenza A (H3N2) viruses, and six influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. Of the 565 influenza A (H3N2) isolates that have been characterized, 106 (18.8%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 459 (81.2%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Five influenza B viruses characterized were similar antigenically to B/Sichuan/379/99 and 1 was similar antigenically to B/Hong Kong/330/2001.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 3, 9.7% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is above the epidemic threshold of 8.2% for week 3.

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Influenza-like Illness Surveillance *:

The percentage of patient visits to approximately 1,000 U.S. sentinel providers nationwide for ILI remained at 2.0%** during week 3, which is below the national baseline of 2.5%. The percentage of patient visits for ILI in all nine surveillance regions*** ranged from 3.1% in the South Atlantic region to 1.0% in the West North Central region. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

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

Delaware reported widespread influenza activity. Regional activity was reported in 20 states (Colorado, Connecticut, Florida, Georgia, Idaho, Iowa, Kentucky, Louisiana, Massachusetts, Minnesota, Mississippi, Montana, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, West Virginia, and Wyoming). Local activity was reported in 19 states (Alaska, Arizona, California, Hawaii, Illinois, Indiana, Kansas, Maine, Maryland, Michigan, Missouri, Nebraska, New Hampshire, Oregon, Tennessee, Vermont, Virginia, Washington, and Wisconsin) and New York City. Sporadic activity was reported in 9 states (Alabama, Arkansas, Nevada, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, and Utah), the District of Columbia, Guam, and Puerto Rico. North Carolina did not report.

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* Reporting is incomplete for this week. Numbers may change as more reports are received.

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

*** 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 28, 2004

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