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Weekly Report: Influenza Summary Update

Week ending March 3, 2007-Week 9

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 9 (February 25 – March 3, 2007)*, influenza activity decreased slightly in the United States. Data from the U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories indicated a decline in activity for the third consecutive week; 22.5% of specimens tested positive for influenza this week. ILI data was above baseline for the eleventh week this season but is declining. Twenty-five states reported widespread influenza activity; 16 states and New York City reported regional influenza activity; six states and the District of Columbia reported local influenza activity; and three states reported sporadic influenza activity. The reporting of widespread or regional influenza activity decreased from 44 states for week 8 to 41 states for week 9. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 9, WHO and NREVSS laboratories reported 4,445 specimens tested for influenza viruses, 1,002 (22.5%) of which were positive: 95 influenza A (H1) viruses, 24 influenza A (H3) viruses, 618 influenza A viruses that were not subtyped, and 265 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 119,002 specimens for influenza viruses and 14,993 (12.6%) were positive. Among the 14,993 influenza viruses, 12,438 (83.0%) were influenza A viruses and 2,555 (17.0%) were influenza B viruses. Three thousand four hundred seventy-five (27.9%) of the 12,438 influenza A viruses have been subtyped: 2,902 (83.5%) were influenza A (H1) viruses and 573 (16.5%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (February 11 – March 3, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

February 11 – March 3, 2007 (specimens testing positive)

>20% positive

10-20% positive

East North Central (31.2%)

New England (15.1%)

West North Central (28.0%)

Mid Atlantic (17.3%)

East South Central (36.2%)

South Atlantic (17.4%)

West South Central (33.1%)

Pacific (17.3%)

Mountain (21.0%)



INFLUENZA Virus Isolated

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Composition of the 2007-08 Influenza Vaccine: 

WHO has recommended that the 2007-08 trivalent influenza vaccine for the Northern Hemisphere contain A/Solomon Islands/3/2006-like (H1N1), A/Wisconsin/67/2005-like (H3N2), and B/Malaysia/2506/2004-like viruses.  The influenza A (H1N1) component has been changed from the 2006-07 season vaccine components.  A/Solomon Islands/3/2006 is a recent antigenic variant of the current vaccine strain A/New Caledonia/20/99. The influenza A (H3N2) and influenza B components remain the same.  B/Ohio/1/2005 is antigenically equivalent to B/Malaysia/2506/2004.  This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.

Antigenic Characterization:

CDC has antigenically characterized 303 influenza viruses [200 influenza A (H1), 25 influenza A (H3) viruses, and 78 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [200]

• One hundred eighty-nine (95%) of the 200 viruses characterized were similar to A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.

• Eleven (5%) of the 200 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99 and are similar to A/Solomon Islands/3/2006-like.

Influenza A (H3) [25]

• Twelve (48%) of the 25 viruses were characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.

• Thirteen (52%) of the 25 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) [78]
Victoria lineage [52]
• Fifty-two (67%) of the 78 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Twenty-nine (56%) of these 52 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Twenty-three (44%) of these 52 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [26]
• Twenty-six (33%) of the 78 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 9, 6.8% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.9% for week 9.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Five influenza-associated pediatric deaths were reported during week 9. Since October 1, 2006, CDC has received 25 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 17, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 1.51 per 10,000.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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During October 1, 2006 – February 17, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0–17 years old was 0.29 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 0.74 per 10,000 and 0.11 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 9, 2.8%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline**** of 2.1%.

Seven 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 9, the following influenza activity†† was reported:

• Widespread activity was reported by 25 states (Alabama, Alaska, Arkansas, Colorado, Connecticut, Delaware, Georgia, Idaho, Indiana, Iowa, Kansas, Kentucky, Massachusetts, New Jersey, New York, North Carolina, North Dakota, Oklahoma, Pennsylvania, South Dakota, Texas, Virginia, Washington, West Virginia, and Wisconsin).

• Regional activity was reported by New York City and 16 states (Arizona, California, Hawaii, Illinois, Maine, Michigan, Montana, Nebraska, Nevada, New Hampshire, New Mexico, Ohio, Oregon, South Carolina, Tennessee, and Wyoming).

• Local activity was reported by the District of Columbia and six states (Florida, Louisiana, Maryland, Minnesota, Missouri, and Vermont).

• Sporadic activity was reported by three states (Mississippi, Rhode Island, and Utah).

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared March 9, 2007

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