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Weekly Report: Influenza Summary Update

Week ending February 3, 2007-Week 5

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 5 (January 28 - February 3, 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, 19.6% were positive. ILI data was above baseline for week 5. Nine states reported widespread influenza activity; 19 states reported regional influenza activity; 14 states and New York City reported local influenza activity; seven states and the District of Columbia reported sporadic influenza activity; and one state did not report. The reporting of widespread or regional influenza activity increased from 22 states for week 4 to 28 states for week 5. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 5, WHO and NREVSS laboratories reported 4,180 specimens tested for influenza viruses, 820 (19.6%) of which were positive: 128 influenza A (H1) viruses, 21 influenza A (H3) viruses, 578 influenza A viruses that were not subtyped, and 93 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 83,332 specimens for influenza viruses and 6,244 (7.5%) were positive. Among the 6,244 influenza viruses, 5,161 (82.6%) were influenza A viruses and 1,083 (17.4%) were influenza B viruses. One thousand six hundred ninety-six (32.9%) of the 5,161 influenza A viruses have been subtyped: 1,507 (88.9%) were influenza A (H1) viruses and 189 (11.1%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (January 14 – February 3, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

January 14 – February 3, 2007 (specimens testing positive)

>20% positive

10-20% positive

<10% positive

East North Central (34.4%)

South Atlantic (13.6%)

New England (5.4%)

West North Central (21.6%)

West South Central (18.6%)

Mid Atlantic (6.9%)

East South Central (20.4%)

Mountain (13.0%)



Pacific (10.6%)


INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 161 influenza viruses [99 influenza A (H1), seven influenza A (H3) viruses, and 55 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [99]
• Ninety-three (94%) of the 99 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 (6%) of the 99 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99.
Influenza A (H3) [7]
• Four (57%) of the seven viruses were characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.
• Three (43%) of the seven viruses showed somewhat reduced titers with antiserra 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 5, 7.4% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.9% for week 5.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Two influenza-associated pediatric deaths were reported during week 5. Since October 1, 2006, CDC has received nine 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 – January 20, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 0.63 per 10,000.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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During October 1, 2006 – January 20, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0–17 years old was 0.13 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 0.34 per 10,000 and 0.05 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 5, 3.0%*** 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 healthcare seeking patterns during the holiday season as has been seen in past seasons.

Six out of nine surveillance regions reported ILI above their region-specific baseline and two surveillance regions reported ILI at 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 5, the following influenza activity†† was reported:

• Widespread activity was reported by nine states (Arkansas, Delaware, Indiana, Iowa, Minnesota, Oklahoma, South Carolina, Tennessee, and Texas).
• Regional activity was reported by 19 states (Alabama, Alaska, California, Connecticut, Georgia, Idaho, Illinois, Kansas, Kentucky, Maryland, Massachusetts, Nevada, New Jersey, New York, Oregon, Pennsylvania, South Dakota, Virginia, and Wisconsin).
• Local activity was reported by New York City and 14 states (Arizona, Florida, Hawaii, Louisiana, Maine, Michigan, Mississippi, Missouri, Montana, North Carolina, North Dakota, Utah, Washington, and West Virginia).
• Sporadic activity was reported by the District of Columbia and 7 states (Colorado, New Hampshire, New Mexico, Ohio, Rhode Island, Vermont, and Wyoming).
• No report was received from one state (Nebraska).

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared February 9, 2007

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