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Weekly Report: Influenza Summary Update

Week ending December 28, 2002-Week 52

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The following information may be quoted:

Synopsis:

During week 52 (December 22-28, 2002)*, 18 (1.9%) of the specimens tested by World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 2.0%, which is greater than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.4%. Five state and territorial health departments reported regional influenza activity, 22 reported sporadic activity, and 19 reported no influenza activity**.

Laboratory Surveillance*:

During week 52, WHO and NREVSS laboratories reported 924 specimens tested for influenza viruses, of which 18 (1.9%) were positive. One influenza A (H1) virus, four unsubtyped influenza A viruses, and 13 influenza B viruses were identified.

Since September 29, WHO and NREVSS laboratories have tested a total of 21,685 specimens for influenza viruses and 273 (1.3%) were positive. Of the 273 viruses identified, 75 (27%) were influenza A viruses and 198 (73%) were influenza B viruses. Nineteen (25%) of the 75 influenza A viruses have been subtyped; 14 were influenza A (H1)† viruses and 5 were influenza A (H3N2) viruses. Twenty-one states have reported laboratory-confirmed influenza. Influenza A viruses have been identified in Florida, Hawaii, Louisiana, Massachusetts, Missouri, Nebraska, New Jersey, New York, North Carolina, Oregon, South Carolina, South Dakota, Texas, Virginia, Washington, and Wisconsin. Influenza B viruses have been identified in Arkansas, Arizona, Indiana, Louisiana, Missouri, Nebraska, Nevada, New York, North Carolina, Oklahoma, South Carolina, and Texas. One hundred and eighty-six (68%) of the 273 viruses were identified in the West South Central region***.

Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 35 influenza viruses submitted by U.S. laboratories since September 29: twenty-six influenza B viruses, four influenza A (H3N2) viruses, and five influenza A (H1)† viruses. Four of the influenza A (H1) viruses had the N1 neuraminidase and one had the N2 neuraminidase. The influenza B viruses, the A (H3N2) viruses, and the hemagglutinin protein of the A (H1) viruses were similar antigenically to the corresponding vaccine strains B/Hong Kong/330/01, A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively.

Click here for more information about influenza A (H1N2) viruses

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 52, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.4%. This percentage is below the epidemic threshold of 7.9% for week 52.

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Influenza-like Illness Surveillance *:

During week 52, 2.0% of patient visits to U.S. sentinel providers were due to ILI. This percentage is greater than the national baseline of 1.9%. On a regional level***, the percentage of visits for ILI ranged from 0.4% to 6.7%. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

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

Influenza activity was reported as regional in Kansas, Oklahoma, Tennessee, Texas, and Virginia. Twenty-two states, New York City, and Washington D.C reported sporadic activity, 19 states reported no influenza activity, and 4 states did not report.

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* 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.

†Includes both the A (H1N1) and A (H1N2) influenza virus subtypes. The influenza A (H1N2) strain appears to have resulted from the reassortment of the genes of currently circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin proteins of the A (H1N2) viruses are similar to those of the currently circulating A (H1N1) viruses and the neuraminidase proteins are similar to those of the currently circulating A (H3N2) viruses, the 2002-03 influenza vaccine should provide protection against A (H1N2) viruses.

*** 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 2, 2003

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