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Weekly Report: Influenza Summary Update

Week ending March 10, 2007-Week 10

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Synopsis:

During week 10 (March 4 – March 10, 2007)*, influenza activity continued to decrease 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 fourth consecutive week; 18.8% of specimens tested positive for influenza this week.  ILI data was above baseline for the twelfth week this season but is declining.  Nineteen states reported widespread influenza activity; 23 states reported regional influenza activity; four states, New York City, and the District of Columbia reported local influenza activity; three states reported sporadic influenza activity; and one state did not report.  The reporting of widespread or regional influenza activity increased from 41 for week 9 to 42 for week 10.  The percent of deaths due to pneumonia and influenza remained below baseline level. 

Laboratory Surveillance*:

During week 10, WHO and NREVSS laboratories reported 4,417 specimens tested for influenza viruses, 830 (18.8%) of which were positive: 86 influenza A (H1) viruses, 29 influenza A (H3) viruses, 448 influenza A viruses that were not subtyped, and 267 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 128,223 specimens for influenza viruses and 16,602 (12.9%) were positive. Among the 16,602 influenza viruses, 13,583 (81.8%) were influenza A viruses and 3,019 (18.2%) were influenza B viruses. Three thousand eight hundred and five (28.0%) of the 13,583 influenza A viruses have been subtyped: 3,104 (81.6%) were influenza A (H1) viruses and 701 (18.4%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (February 18 March 10, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

February 18 – March 10, 2007 (specimens testing positive)

>20% positive

10-20% positive

East North Central (34.7%)

New England (12.8%)

West North Central (22.7%)

Mid Atlantic (18.8%)

East South Central (31.0%)

South Atlantic (16.8%)

West South Central (29.5%)

Mountain (19.7%)

 

Pacific (14.1%)

 

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 325 influenza viruses [200 influenza A (H1), 25 influenza A (H3) viruses, and 100 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) [100]
Victoria lineage [71]
Seventy-one (71%) of the 100 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Forty-two (59%) of these 71 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine. 

o Twenty-nine (41%) of these 71 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [29]
Twenty-nine (29%) of the 100 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 10, 7.2% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.9% for week 10.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Seven influenza-associated pediatric deaths were reported during week 10. Since October 1, 2006, CDC has received 32 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 March 3, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 1.62 per 10,000.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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During October 1, 2006 March 3, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 017 years old was 0.49 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 1.27 per 10,000 and 0.18 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 10, 2.7%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline**** of 2.1%.

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

Region

Reported ILI (%)

Region-Specific Baseline (%)

New England

1.5

1.2

Mid Atlantic

2.4

2.6

East North Central

2.8

1.9

West North Central

1.2

1.5

South Atlantic

2.6

2.3

East South Central

1.9

2.4

West South Central

5.6

3.0

Mountain

1.9

1.7

Pacific

2.9

3.2

 

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

During week 10, the following influenza activity was reported:

Widespread activity was reported by 19 states (Alabama, Alaska, Colorado, Connecticut, Delaware, Georgia, Indiana, Kentucky, Montana, New Jersey, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, Texas, Vermont, Virginia, and Washington).

Regional activity was reported by 23 states (Arizona, Arkansas, California, Hawaii, Idaho, Illinois, Iowa, Kansas, Maine, Massachusetts, Michigan, Minnesota, Nevada, New Hampshire, New Mexico, North Dakota, Oregon, South Carolina, South Dakota, Tennessee, Utah, Wisconsin, and Wyoming).

Local activity was reported by New York City, the District of Columbia, and four states (Florida, Louisiana, Maryland, and Mississippi).

Sporadic activity was reported by three states (Missouri, Rhode Island, and West Virginia).

No report was received from Nebraska.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared March 16, 2007

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