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Weekly Report: Influenza Summary Update

Week ending November 15, 2003-Week 46

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The following information may be quoted:

Synopsis

Influenza activity in the United States continued to increase during week 46 (November 9 - 15, 2003). Six hundred seventy-one (30.7%) of 2,185 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 3.3%, which is above the national baseline of 2.5%. The proportion of deaths attributed to pneumonia and influenza was 6.1%, which is below the epidemic threshold for the week. Two state health departments reported widespread influenza activity, 6 states reported regional activity, 11 states reported local influenza activity, 20 states, New York City, Guam, and Puerto Rico reported sporadic influenza activity, and 10 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 46, WHO and NREVSS laboratories reported 2,185 specimens tested for influenza viruses, and 671 (30.7%) were positive. Eighty-two influenza A(H3N2) viruses, 10 influenza A(H1) viruses, and 579 unsubtyped influenza A viruses were identified. The percentage of specimens testing positive for influenza first exceeded 10% (an indicator of increasing influenza activity) during the week ending November 1, earlier than any other year for which CDC has this information (1976-77 season to present).

Since September 28, WHO and NREVSS laboratories have tested a total of 10,550 specimens for influenza viruses and 1,876 (17.8%) were positive. Of the 1,876 influenza viruses identified, 1,871 (99.7%) were influenza A viruses and 5 (0.3%) were influenza B viruses. Two hundred ninety-eight (16%) of the 1871 influenza A viruses have been subtyped; 288 (96.6%) were influenza A (H3N2) viruses and 10 (3.4%) were A(H1) viruses. Thirty-five states and all 9 surveillance regions** have reported laboratory-confirmed influenza this season. One thousand two hundred fifty-two (66.7%) of the 1,876 isolates were reported from the West South Central region and 482 (25.7%) were from the Mountain region.

Influenza Virus Isolated


View Chart Data

Antigenic Characterization:

CDC has antigenically characterized 77 influenza A (H3N2) viruses submitted by U.S. laboratories since September 28. Of the 77 A (H3N2) viruses tested, 17 (22%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 60 (78%) were similar to the drift variant, A/Fujian/411/2002 (H3N2), that predominated in Australia and New Zealand during the recent Southern Hemisphere influenza season. This drift variant is related to the vaccine strain, A/Panama/2007/99. Antibodies produced against the vaccine virus cross-react with A/Fujian/411/2002-like viruses, but at a lower level. Vaccine effectiveness depends, in part, on the match between vaccine strains and circulating viruses. 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 46, 6.1% 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 7.2% for week 46.

Pneumonia and Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 46, 3.3%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 2.5%. On a regional level**, the percentage of visits for ILI was highest in the West South Central region(13.9%); all other regions were below 4%. 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 2 states (Texas and Nevada), regional in 6 states (Colorado, Idaho, Tennessee, Utah, Washington, and West Virginia), and local in 11 states (Arizona, California, Georgia, Kansas, Louisiana, Montana, Nebraska, New Mexico, North Carolina, North Dakota, and Oklahoma). Sporadic influenza activity was reported in 20 states (Alabama, Florida, Hawaii, Illinois, Indiana, Kentucky, Maryland, Minnesota, Mississippi, Missouri, New York, Ohio, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Virginia, Wisconsin, and Wyoming), New York City, Guam, and Puerto Rico. Ten states and the District of Columbia reported no influenza activity, and 1 state did not report.

Usmap for Weekly Influenza Activity


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

** 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)

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

Report prepared November 21, 2003

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