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Weekly Report: Influenza Summary Update

Week ending November 25, 2006-Week 47

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Synopsis:

During week 47 (November 19 - November 25, 2006)*, a low level of influenza activity was reported in the United States. Sixty-seven (3,7 %) 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) and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. One state reported widespread influenza activity; three states reported regional influenza activity; four states reported local influenza activity; 24 states, the District of Columbia, and New York City reported sporadic influenza activity; and 18 states reported no influenza activity.

Laboratory Surveillance*:

During week 47, WHO and NREVSS laboratories reported 1 823 specimens tested for influenza viruses, 67 (3,7 %) of which were positive: four influenza A (H1) viruses, 53 influenza A viruses that were not subtyped, and 10 influenza B viruses. Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 18 443 specimens for influenza viruses and 570 (3,1 %) were positive. Among the 570 influenza viruses, 455 (79,8 %) were influenza A viruses and 115 (20,2 %) were influenza B viruses. One hundred and six (23,3 %) of the 455 influenza A viruses have been subtyped: 99 (93,4 %) were influenza A (H1) viruses and 7 (6,6 %) were influenza A (H3) viruses. Of the 570 influenza viruses reported this season, 353 (61,9 %) have been reported from Florida.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 23 influenza viruses [six influenza A (H1) and 17 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1)[6]
• All viruses were characterized as A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.
• Eight additional isolates collected from an outbreak in September were also characterized as A/New Caledonia/20/99-like.
Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages)[17]
Victoria lineage [6]
• Six (35,3 %) of the 17 influenza B viruses characterized belong to the B/Victoria lineage of viruses.
o Three of these 6 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Three of these 6 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.
Yamagata lineage [11]
• Eleven (64,7 %) of the 17 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.
It is too early in the influenza season to determine which influenza viruses will predominate or how well the vaccine and circulating strains will match.

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

During week 47, 6,29 % of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 6,97 % for week 47.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 47 and no deaths have been reported for the 2006-07 influenza season.

Influenza-Associated Pediatric Hospitalizations*:

Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). No influenza-associated pediatric hospitalizations have been reported from either network this season.

Influenza-like Illness Surveillance*:

During week 47, 1,9 %*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline**** of 2,1 %. On a regional level**, the percentage of visits for ILI ranged from 0,8 % to 3,5 %. Two regions reported ILI above their region-specific baseline****: the East South Central region reported 2,5 % compared to its baseline of 2,4 % and the West South Central region reported 3,5 % compared to its baseline of 3 %.

Bar Chart for Influenza-like Illness
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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

Influenza Activity as Assessed by State and Territorial Epidemiologists*: During week 47, the following influenza activity was reported:

• Widespread activity was reported by Alabama.
• Regional activity was reported by three states (Florida, Georgia, and Mississippi).
• Local activity was reported by four states (Hawaii, Louisiana, Massachusetts, South Carolina).
• Sporadic activity was reported by the District of Columbia, New York City, and 24 states (Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Idaho, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Tennessee, Texas, Utah, Virginia, and Wyoming).
• No influenza activity was reported by 18 states.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared December 1, 2006

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