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Weekly Report: Influenza Summary Update

Week ending May 17, 2003-Week 20

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This is the final report of the 2002 - 2003 season.

The following information may be quoted:

Synopsis

During week 20 (May 11-17, 2003)*, 4 (0,8 %) of the specimens 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 0,9 %, which is below the national baseline of 1,9 %. The proportion of deaths attributed to pneumonia and influenza was 6,3 %. One state and territorial health department reported regional influenza activity, 21 reported sporadic activity, and 24 reported no influenza activity**.

Laboratory Surveillance*

During week 20, U.S. WHO and NREVSS collaborating laboratories reported 511 specimens tested for influenza viruses, of which 4 (0,8 %) were positive. Two influenza A (H3N2) viruses, one unsubtyped influenza A virus, and one influenza B virus were identified.

Since September 29, WHO and NREVSS laboratories have tested a total of 94 966 specimens for influenza viruses and 11 027 (11,6 %) were positive. Of the 11 027 viruses identified, 6 324 (57 %) were influenza A viruses and 4 703 (43 %) were influenza B viruses. Three thousand three hundred and eighty-one (53 %) of the 6 324 influenza A viruses have been subtyped; 2 534 (75 %) were influenza A (H1)† viruses and 847 (25 %) were influenza A (H3N2) viruses. Influenza A viruses were reported more frequently than influenza B viruses (range 58 % - 86 %) in the New England, East North Central, Pacific, Mountain, and Mid-Atlantic regions, and influenza B viruses were reported more frequently than influenza A viruses (range 53 % - 78 %) in the West North Central, West South Central, South Atlantic, and East South Central regions.

 Influenza Virus Isolated

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Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 588 influenza viruses submitted by U.S. laboratories since September 29: two hundred and twenty-nine influenza A (H1)† viruses, 105 influenza A (H3N2) viruses, and 254 influenza B viruses. One hundred and seventy-three of the influenza A (H1) viruses had the N1 neuraminidase, 55 had the N2 neuraminidase, and the neuraminidase typing for one H1 virus is pending. The hemagglutinin proteins of all 229 influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99 (H1N1). Of the 105 influenza A (H3N2) isolates that have been characterized, 98 (93 %) were similar to A/Panama/2007/99, the H3N2 component of the 2002-03 influenza vaccine, and 7 (7 %) showed reduced titers to ferret antisera produced against A/Panama/2007/99. Of the 254 influenza B viruses that have been characterized, 253 belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/01 and one belonged to the B/Yamagata lineage and was similar to B/Shizuoka/15/01, a B/Sichuan/379/99-like virus.

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

Composition of the 2003-04 Influenza Vaccine:

The Food and Drug Administration's Vaccine and Related Biological Products Advisory Committee recommended that the 2003-04 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Hong Kong/330/2001-like viruses. All 3 components (H1N1, H3N2, and B) are the same as those for the 2002-03 season vaccine. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.

Pneumonia and Influenza (P&I) Mortality Surveillance: During week 20, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 6,3 %. This percentage is below the epidemic threshold of 7,5 % for week 20.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *

During week 20, 0,9 % of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline of 1,9 %. On a regional level***, the percentage of visits for ILI ranged from 0,2 % to 1,6 %. 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

View Chart Data

Influenza Activity as Assessed by State and Territorial Epidemiologists**

Influenza activity was reported as regional in Massachusetts. Twenty-one states, New York City, and Washington D.C. reported sporadic activity, and 24 states reported no influenza activity. Four states did not report.

Usmap for Weekly Influenza Activity


* 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 May 22, 2003

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