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Weekly Report: Influenza Summary Update

Week ending November 18, 2006-Week 46

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During week 46 (November 12 - November 18, 2006)*, a low level of influenza activity was reported in the United States. Sixty-eight (3,2 %) 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. Four states reported regional influenza activity; four states reported local influenza activity; 23 states, the District of Columbia, and New York City reported sporadic influenza activity; and 19 states reported no influenza activity.

Laboratory Surveillance*:

During week 46, WHO and NREVSS laboratories reported 2 135 specimens tested for influenza viruses, 68 of which were positive: six influenza A (H1) viruses, 54 influenza A viruses that were not subtyped, and 8 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 15 707 specimens for influenza viruses and 477 (3 %) were positive. Among the 477 influenza viruses, 382 (80,1 %) were influenza A viruses and 95 (19,9 %) were influenza B viruses. Ninety-seven (25,4 %) of the 382 influenza A viruses have been subtyped: 90 (92,8 %) were influenza A (H1) viruses and 7 (7,2 %) were influenza A (H3) viruses. Of the 477 influenza viruses reported this season, 308 (64,6 %) have been reported from Florida.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 22 influenza viruses [six influenza A (H1) and 16 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) [16]
Victoria lineage [5]
• Five (32,2 %) of the 16 influenza B viruses characterized belong to the B/Victoria lineage of viruses.
o Two of these 5 viruses that have been characterized were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Three of these 5 viruses characterized showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.
Yamagata lineage [11]
• Eleven (68,8 %) of the 16 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.
It is too early in the influenza season to determine which viruses will predominate or how well the vaccine and circulating strains will match.

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

During week 46, 5,79 % of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 6,86 % for week 46.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 46 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 the EIP. NVSN estimated rates of hospitalization for influenza will be reported every 2 weeks, beginning December 2006.

Influenza-like Illness Surveillance*:

During week 46, 1,5 %*** 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 0,4 % to 3,5 %. The only region that reported ILI above its region-specific baseline**** was the West South Central region, which reported 3,5 % compared to the baseline of 3 %.

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

During week 46, the following influenza activity was reported:

• Regional activity was reported by four states (Alabama, Florida, Georgia, and Mississippi).
• Local activity was reported by four states (Hawaii, Louisiana, Massachusetts, and North Carolina).
• Sporadic activity was reported by the District of Columbia, New York City, and 23 states (Arizona, Arkansas, California, Connecticut, Delaware, Idaho, Illinois, Indiana, Iowa, Kentucky, Maryland, Michigan, New York, North Dakota, Ohio, Pennsylvania, South Dakota, Tennessee, Texas, Utah, Virginia, West Virginia, and Wyoming).
• No influenza activity was reported by 19 states.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared November 24, 2006

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