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Weekly Report: Influenza Summary Update

Week ending December 13, 2003-Week 50

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The following information may be quoted:

Synopsis:

Influenza activity in the United States continued to increase during week 50 (December 7 - 13, 2003). One thousand three hundred sixty-five (35.8%) of 3,814 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 7.4%, which is above the national baseline of 2.5%. The proportion of deaths attributed to pneumonia and influenza was 7.2%, which is below the epidemic threshold for the week. Thirty-six state health departments reported widespread influenza activity, 12 states and New York City reported regional activity, 1 state and the District of Columbia reported local influenza activity, and 1 state and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 50, WHO and NREVSS laboratories reported 3,814 specimens tested for influenza viruses, and 1,365 (35.8%) were positive. Of these, 262 were influenza A(H3N2) viruses, 1,080 were influenza A viruses that were not subtyped, and 23 were influenza B viruses.

Since September 28, WHO and NREVSS laboratories have tested a total of 32,854 specimens for influenza viruses and 9,464 (28.8%) were positive. Among the 9,464 influenza viruses, 9,395 (99.3%) were influenza A viruses and 69 (0.7%) were influenza B viruses. Two thousand one hundred thirteen (22.5%) of the 9,395 influenza A viruses have been subtyped; 2,112 (99.9%) were influenza A (H3N2) viruses and 1 (0.1%) was an A (H1) virus. All 50 states have reported laboratory-confirmed influenza this season. Two thousand seven hundred eighty-two (29.4%) of the 9,464 isolates were reported from the West South Central region, 2,678 (28.3%) were from the Mountain region, and 2,085 (22.0%) were from the South Atlantic region.

Influenza Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 269 influenza viruses collected by U.S. laboratories since October 1: two influenza A (H1) viruses, 265 influenza A (H3N2) viruses, and 2 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 265 influenza A (H3N2) isolates that have been characterized, 62 (23%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 203 (77%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Both influenza B viruses characterized were similar antigenically to B/Sichuan/379/99. 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.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 50, 7.2% 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 7.7% for week 50.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 50, 7.4%*** 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 8 of the 9 surveillance regions, but has continued to decline in the West South Central region (8.0% for week 50 compared with 11.3% during week 47). On a regional level**, the percentage of visits for ILI was highest in the Pacific region (10.6%), followed by the West South Central region (8.0%), West North Central region (7.9%), Mountain region (7.7%), East North Central region (7.6%), South Atlantic region (6.9%), East South Central region (6.3%), and the New England and Mid-Atlantic regions (4.7%). 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 36 states (Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Massachusetts, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, New York, North Carolina, North Dakota, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, and Wyoming). Regional activity was reported by 12 states (Alabama, Alaska, Florida, Louisiana, Maine, Maryland, Michigan, Minnesota, New Jersey, Ohio, South Dakota, and Wisconsin) and New York City. Hawaii and the District of Columbia reported local activity, and New Hampshire and Puerto Rico reported sporadic activity.

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.

** Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

*** 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 17, 2003

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