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Weekly Report: Influenza Summary Update

Week ending December 18, 2004-Week 50

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The following information may be quoted:

Synopsis:

During week 50 (December 12-18, 2004)*, influenza activity continued to slowly increase, but overall remained low in the United States. Ninety-three (5,5 %) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza viruses. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) and the proportion of deaths attributed to pneumonia and influenza were below national baseline values. New York State and New York City reported widespread influenza activity, 4 states reported regional influenza activity, and 10 states reported local activity. Thirty-four states, the District of Columbia, and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 50, WHO and NREVSS laboratories reported testing 1 703 specimens for influenza viruses, of which 93 (5,5 %) were positive. Of these, 26 were influenza A (H3N2) viruses, 60 were influenza A viruses that were not subtyped, and 7 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories have tested a total of 26 498 specimens for influenza viruses and 569 (2,1 %) were positive. Among the 569 influenza viruses, 430 (75,6 %) were influenza A viruses and 139 (24,4 %) were influenza B viruses. One hundred eighty-five (43 %) of the 430 influenza A viruses have been subtyped and all were influenza A (H3N2) viruses. Forty states from all surveillance regions** have reported laboratory-confirmed influenza this season.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 60 influenza viruses collected by U.S. laboratories since October 1, 2004: forty-two influenza A (H3N2) viruses and 18 influenza B viruses. All of the influenza A (H3N2) isolates were characterized as A/Fujian/411/2002-like (H3N2), which is the influenza A (H3N2) component recommended for the 2004-05 influenza vaccine.

Influenza B viruses currently circulating can be divided into two antigenically and genetically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata lineage viruses circulated widely between 1990 and 2001 during which time B/Victoria-like viruses were not identified outside of Asia. However, between March 2001 and October 2003, B/Victoria-like viruses predominated in many countries, including the United States and the vaccine strains were changed accordingly. While both Victoria-lineage and Yamagata lineage viruses have been reported worldwide during the past year, Yamagata lineage viruses have once again become predominant, Sixteen of the influenza B viruses isolated this season belong to the B/Yamagata lineage and were characterized as B/Shanghai/361/2002-like, which is the influenza B component recommended for the 2004-05 influenza vaccine. Two influenza B viruses belong to the B/Victoria lineage and were characterized as B/Hong Kong/330/2001-like.

Pneumonia and Influenza (P&I) Mortality Surveillance*:

During week 50, 6,1 % of all deaths reported by the vital statistics offices of 122 U.S. cities were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7,7 % for week 50.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

As of week 50, no influenza-associated pediatric deaths have been reported to CDC.

Influenza-Associated Pediatric Hospitalizations:

The New Vaccine Surveillance Network consists of three sites in Cincinnati OH, Nashville TN, and Rochester NY. The sites conduct population-based surveillance for laboratory-confirmed influenza among children 0-4 years of age who are admitted to the hospital with fever or acute respiratory illnesses. During October 3-December 11, 2004, there were no laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old. Three influenza-associated hospitalizations previously reported for the 2004-05 season have since been removed after the laboratory results were shown to be false-positives. Hospitalization rate estimates are preliminary and may change as data continue to be collected. In years 2000-2003, the end-of-season hospitalization rate ranged from 3.7 to 12 per 10 000 children.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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The Emerging Infections Program (EIP) is collecting data in 10 sites on hospitalizations of children <18 years of age with illnesses associated with laboratory-confirmed influenza. Hospitalization data from the EIP sites will be included as they become available.

Influenza-like Illness Surveillance*:

During week 50, 2,2 %*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 2,5 %. On a regional level**, the percentage of visits for ILI ranged from 0,7 % in the West North Central region to 3,4 % in the Pacific region. 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*:

During week 50, New York State and New York City reported widespread activity and Alaska, Delaware, Kentucky, and Minnesota reported regional influenza activity. Ten states (Colorado, Connecticut, Kansas, Maryland, Massachusetts, New Hampshire, North Carolina, South Carolina, Vermont, and Virginia) reported local activity. Thirty-four states (Alabama, Arizona, Arkansas, California, Florida, Georgia, Hawaii, Idaho, Illinois, Indiana, Iowa, Maine, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Utah, Washington, West Virginia, Wisconsin, and Wyoming), the District of Columbia, and Puerto Rico reported sporadic influenza activity. Louisiana did not report.

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.

** 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: December 22, 2004

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