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Weekly Report: Influenza Summary Update

Week ending November 30, 2002-Week 48

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The following information may be quoted:

Synopsis:

During week 48 (November 24-30, 2002)*, 7 (1.3%) of the specimens tested by World Health Organization (WHO) laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 1.4%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.1%. Three state and territorial health departments reported regional influenza activity, 18 reported sporadic activity, and 28 reported no influenza activity**.

Laboratory Surveillance*:

During week 48, WHO laboratories reported 519 specimens tested for influenza viruses, of which 7 (1.3%) were positive. One influenza A (H1)† virus, 2 unsubtyped influenza A viruses, and 4 influenza B viruses were identified. Data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories for week 48 were not available at the time this report was written; therefore, numbers may change substantially next week.

Since September 29, WHO and NREVSS laboratories have tested a total of 10,691 specimens for influenza viruses and 55 (0.5%) were positive. Of the 55 viruses identified, 24 (44%) were influenza A viruses and 31 (56%) were influenza B viruses. Eight (33%) of the 24 influenza A viruses have been subtyped; 7 were influenza A (H1)† viruses and 1 was an influenza A (H3N2) virus. Eleven states have reported laboratory-confirmed influenza. Influenza A viruses have been identified in Florida, Hawaii, Louisiana, Nebraska, New York, North Carolina, Oregon, South Carolina, Texas, and Virginia. Influenza B viruses have been identified in Louisiana, New York, Oklahoma, South Carolina, and Texas. Of the 55 viruses reported, 26 (47%) were identified in the South Atlantic region*** and 23 (42%) were identified in the West South Central region***. The New England, East North Central, East South Central, and Mountain regions have reported no influenza isolates.

Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 11 influenza viruses submitted by U.S. laboratories since September 29: five influenza B viruses, one influenza A (H3N2) virus, and 5 influenza A (H1)† viruses. Three of the influenza A (H1) viruses contained the N1 neuraminidase, one contained the N2 neuraminidase, and the subtyping of the neuraminidase for one influenza (H1) virus is pending. The influenza B viruses, the A (H3N2) virus, 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 48, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.1%. This percentage is below the epidemic threshold of 7.5% for week 48.

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Influenza-like Illness Surveillance *:

During week 48, 1.4% of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 1.9%. On a regional level***, the percentage of visits for ILI ranged from 0.2% to 2.4%. 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 3 states (Colorado, Tennessee, and Texas), and sporadic in 18 states (Alabama, Florida, Georgia, Indiana, Kansas, Louisiana, Maine, Michigan, Missouri, Nebraska, Nevada, New Mexico, New York, Oklahoma, Oregon, Utah, Vermont, and West Virginia) and New York City. Twenty-eight states reported no influenza activity, and 1 state 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 December 5, 2002

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