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Weekly Report: Influenza Summary Update

Week ending February 17, 2007-Week 7

NOTE: This document is provided for historical purposes only and may not reflect the most accurate and up-to-date information on this subject. For current flu information, please visit the CDC Flu Homepage.


During week 7 (February 11 - February 17, 2007)*, 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, 24.8% were positive. ILI data was above baseline for the ninth week this season. Twenty-four states reported widespread influenza activity; 14 states and New York City reported regional influenza activity; 10 states and the District of Columbia reported local influenza activity; and two states reported sporadic influenza activity. The reporting of widespread or regional influenza activity increased from 33 states for week 6 to 38 states for week 7. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 7, WHO and NREVSS laboratories reported 4,798 specimens tested for influenza viruses, 1,189 (24.8%) of which were positive: 129 influenza A (H1) viruses, 31 influenza A (H3) viruses, 794 influenza A viruses that were not subtyped, and 235 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 101,388 specimens for influenza viruses and 10,458 (10.3%) were positive. Among the 10,458 influenza viruses, 8,748 (83.6%) were influenza A viruses and 1,710 (16.4%) were influenza B viruses. Two thousand four hundred thirty-nine (27.9%) of the 8,748 influenza A viruses have been subtyped: 2,117 (86.8%) were influenza A (H1) viruses and 322 (13.2%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (January 28 – February 17, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

January 28 – February 17, 2007 (specimens testing positive)

>20% positive

10-20% positive

East North Central (45.3%)

New England (11.6%)

West North Central (23.7%)

Mid Atlantic (13.6%)

South Atlantic (20.6%)

Mountain (16.0%)

East South Central (35.4%)

Pacific (12.0%)

West South Central (32.9%)



INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 216 influenza viruses [141 influenza A (H1), 20 influenza A (H3) viruses, and 55 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [141]

• One hundred thirty-four (95%) of the 141 viruses characterized were similar to A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.

• Seven (5%) of the 141 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99.

Influenza A (H3) [20]

• Eleven (55%) of the 20 viruses were characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.

• Nine (45%) of the 20 viruses showed somewhat reduced titers with antisera produced against A/Wisconsin/67/2005.

Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages) [55]
Victoria lineage [37]
• Thirty-seven (67%) of the 55 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Eighteen (49%) of these 37 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Nineteen (51%) of these 37 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [18]
• Eighteen (33%) of the 55 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 7, 6.9% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.9% for week 7.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Three influenza-associated pediatric deaths were reported during week 7. Since October 1, 2006, CDC has received 15 reports of influenza-associated pediatric deaths 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).

During November 5, 2006 – February 3, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 0.96 per 10,000.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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During October 1, 2006 – February 3, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0–17 years old was 0.18 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 0.47 per 10,000 and 0.06 per 10,000, respectively.

EIP Influenza Laboratory-Confirmed Cumulative Hospitalization Rates for Children Aged 0-4 and 5-17 years, 2005-06 and Previous 2 Seasons
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Influenza-like Illness Surveillance*:

During week 7, 3.4%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline**** of 2.1%. The proportion of visits for influenza-like illness may have been influenced by changes in health-care seeking patterns during the holiday season as has been seen in past seasons.

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 7, the following influenza activity†† was reported:

• Widespread activity was reported by 24 states (Alaska, Arkansas, Connecticut, Delaware, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Massachusetts, Nebraska, New York, North Dakota, Oklahoma, Oregon, South Dakota, Tennessee, Texas, Utah, Virginia, Washington, West Virginia, and Wisconsin).

• Regional activity was reported by New York City and 14 states (Alabama, Arizona, California, Colorado, Hawaii, Illinois, Maine, Minnesota, Montana, Nevada, New Jersey, North Carolina, Pennsylvania, and South Carolina).

• Local activity was reported by the District of Columbia and 10 states (Florida, Louisiana, Maryland, Michigan, Mississippi, Missouri, New Hampshire, New Mexico, Ohio, and Wyoming).

• Sporadic activity was reported by two states (Rhode Island and Vermont).

U. S. map for Weekly Influenza Activity
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