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Weekly Report: Influenza Summary Update

Week ending January 8, 2005-Week 1

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The following information may be quoted:

Synopsis:

During week 1 (January 2-8, 2005)*, influenza activity continued to increase mostly in the eastern United States. Five hundred ten (16.8%) 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. Over fifty percent of all influenza A isolates this season have been reported from the Mid-Atlantic and New England surveillance regions**. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) is at the national baseline and the proportion of deaths attributed to pneumonia and influenza is below the epidemic threshold. The first influenza-associated pediatric death was reported during week 1. Eight states and New York City reported widespread influenza activity, 12 states reported regional influenza activity, and 10 states reported local activity. Twenty states, the District of Columbia, and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 1, WHO and NREVSS laboratories in the United States reported testing 3,040 specimens for influenza viruses, of which 510 (16.8%) were positive. Of these 1 was an influenza A (H1N1), 69 were influenza A (H3N2) viruses, 383 were influenza A viruses that were not subtyped, and 57 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 39,643 specimens for influenza viruses and 2,190 (5.5%) were positive. Among the 2,190 influenza viruses, 1,851 (84.5%) were influenza A viruses and 339 (15.5%) were influenza B viruses. Five hundred seventy-seven (31.2%) of the 1,851 influenza A viruses have been subtyped; 574 (99.5%) were influenza A (H3N2) and 3 (0.5%) were influenza A (H1N1) viruses. Five hundred forty-seven (29.6%) of the 1,851 influenza A isolates have been reported by the Mid-Atlantic region** and 503 (27.2%) were from the New England region. Eighty-one (23.9%) of the 339 influenza B isolates have been reported from the Mountain region, 73 (21.5%) were from the Pacific region, and 64 (18.9%) were from the South Atlantic region.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 112 influenza viruses collected by U.S. laboratories since October 1, 2004: 2 influenza A (H1) viruses, eighty-eight influenza A (H3N2) viruses, and 22 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. 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. Seventeen 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, and 2 showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Three influenza B viruses belong to the B/Victoria lineage.

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.

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

During week 1, 7.6% of all deaths reported by the vital statistics offices of 122 U.S. cities were attributed to pneumonia or influenza. This percentage is below the epidemic threshold of 8.0% for week 1.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

As of week 1, one pediatric death has been reported to CDC by the Bureau of Health in Maine.

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 25, 2004, the preliminary influenza-associated hospitalization rate for children 0-4 years old was 0.42 per 10,000 children based on 2 laboratory-confirmed hospitalizations. Three hospitalizations previously reported as being associated with influenza have since been reclassified as non-influenza hospitalizations after 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 1, 2.5%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is equal to the national baseline of 2.5%. The percentage of visits for ILI ranged from 1.5% in the East South Central and Mountain regions** to 3.8% 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 1, 8 states (Connecticut, Delaware, Kentucky, Maryland, New York, Rhode Island, Texas, and Vermont) and New York City reported widespread activity. Twelve states (Colorado, Idaho, Iowa, Kansas, Maine, Massachusetts, Minnesota, Nevada, New Hampshire, New Jersey, Pennsylvania, and Wisconsin) reported regional influenza activity. Ten states (Alaska, Illinois, Louisiana, Michigan, North Carolina, North Dakota, South Dakota, Tennessee, Utah, and Virginia) reported local activity. Twenty states (Alabama, Arizona, Arkansas, California, Florida, Georgia, Hawaii, Indiana, Mississippi, Missouri, Montana, Nebraska, New Mexico, Ohio, Oklahoma, Oregon, South Carolina, Washington, West Virginia, and Wyoming), District of Columbia, and Puerto Rico reported sporadic influenza 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.

** 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: January 13, 2005

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