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Weekly Report: Influenza Summary Update

Week ending December 30, 2006-Week 52

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During week 52 (December 24 - December 30, 2006)*, influenza activity continued to increase in the United States. Among specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories for influenza, 11,8 % were positive. Four states reported widespread influenza activity; 10 states reported regional influenza activity; 12 states reported local influenza activity; 20 states, the District of Columbia, and New York City reported sporadic influenza activity; one state reported no influenza activity, and three states did not report. The reporting of widespread or regional influenza activity decreased from 16 states for week 51 to 14 states for week 52. On a national level, laboratory and outpatient influenza-like illness (ILI) surveillance data indicated an increase in activity from week 48 to week 52. ILI was above baseline for the third consecutive week this season and for the second consecutive week, more than 10 % of specimens tested were positive for influenza. However, the percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 52, WHO and NREVSS laboratories reported 1 894 specimens tested for influenza viruses, 224 (11,8 %) of which were positive: 70 influenza A (H1) viruses, four influenza A (H3) viruses, 109 influenza A viruses that were not subtyped, and 41 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 42 328 specimens for influenza viruses and 2 022 (4,8 %) were positive. Among the 2 022 influenza viruses, 1 598 (79 %) were influenza A viruses and 424 (21 %) were influenza B viruses. Five hundred seven (31,7 %) of the 1 598 influenza A viruses have been subtyped: 485 (95,7 %) were influenza A (H1) viruses and 22 (4,3 %) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (December 10-30, 2006), on a regional basis, the percent of specimens testing positive for influenza exceeded 10 % in the East North Central (19,9 %), South Atlantic (19,9 %), and East South Central (29,7 %) regions. The percent of specimens tested in the most recent three weeks testing positive for influenza was below 10 % in the remaining six regions and ranged from 2 % to 9.2 %.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 97 influenza viruses [59 influenza A (H1), one influenza A (H3), and 37 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [59]
• Fifty-three (90 %) of the 59 viruses characterized were similar to A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.
• Six (10 %) of the 59 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99.
Influenza A (H3) [1]
• The virus was characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.
Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages) [37]
Victoria lineage [22]
• Twenty-two (59 %) of the 37 influenza B viruses characterized belong to the B/Victoria lineage of viruses.
o Nine (41 %) of these 22 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Thirteen (59 %) of these 22 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.
Yamagata lineage [15]
• Fifteen (41 %) of the 37 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 52, 6,4 % of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7,5 % for week 52.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 52. Since October 1, 2006, CDC has received one report of an influenza-associated pediatric death that occurred during the current 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 52, 3,1 %*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline**** of 2,1 %. On a regional level**, the percentage of visits for ILI ranged from 1,5 % to 7,3 %.

Eight out of nine surveillance regions reported ILI above their region-specific baseline****:


Reported ILI (%)

Region-Specific Baseline (%)

New England



Mid Atlantic



East North Central



West North Central



South Atlantic



East South Central



West South Central






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

During week 52, the following influenza activity†† was reported:

• Widespread activity was reported by four states (Alabama, Florida, Georgia, and Tennessee).
• Regional activity was reported by 10 states (Indiana, Iowa, Kentucky, Louisiana, Maryland, Mississippi, Nevada, North Carolina, Oklahoma, and Texas).
• Local activity was reported by 12 states (Alaska, Arizona, Connecticut, Hawaii, Illinois, Kansas, Massachusetts, Michigan, Missouri, South Dakota, Utah, and Wisconsin).
• Sporadic activity was reported by the District of Columbia, New York City and 20 states (Arkansas, California, Colorado, Delaware, Idaho, Maine, Minnesota, Montana, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Dakota, Ohio, Oregon, Virginia, Washington, West Virginia, and Wyoming, ).
• No influenza activity was reported by one state (Vermont).
• Three states did not report (Pennsylvania, Rhode Island, and South Carolina).

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared January 5, 2007

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