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Weekly Report: Influenza Summary Update

Week ending January 27, 2007-Week 4

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Synopsis:

During week 4 (January 21 - January 27 2007)*, influenza activity increased 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, 17.9% were positive. ILI data was above baseline for week 4. Eight states reported widespread influenza activity; 14 states reported regional influenza activity; 15 states reported local influenza activity; and 13 states, the District of Columbia, and New York City reported sporadic influenza activity. The reporting of widespread or regional influenza activity increased from 18 states for week 3 to 22 states for week 4. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 4, WHO and NREVSS laboratories reported 3,450 specimens tested for influenza viruses, 618 (17.9%) of which were positive: 117 influenza A (H1) viruses, 44 influenza A (H3) virus, 381 influenza A viruses that were not subtyped, and 76 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 75,765 specimens for influenza viruses and 5,109 (6.7%) were positive. Among the 5,109 influenza viruses, 4,162 (81.5%) were influenza A viruses and 947 (18.5%) were influenza B viruses. One thousand three hundred eighty-eight (33.3%) of the 4,162 influenza A viruses have been subtyped: 1,234 (88.9%) were influenza A (H1) viruses and 154 (11.1%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (January 7 January 27, 2007), on a regional basis, the percent of specimens testing positive for influenza exceeded 10% in the East North Central (33.5%), West North Central (17.4%), East South Central (17.7%), West South Central (13.7%), Mountain (13.8%), and the Pacific (12.6%) regions. In the remaining three regions, the percent of specimens testing positive for influenza in the most recent three weeks was below 10% and ranged from 3.6% to 8.8%.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 107 influenza viruses [59 influenza A (H1), two influenza A (H3) viruses, and 46 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) [2]
Both viruses were 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) [46]
Victoria lineage [31]
Thirty-one (67%) of the 46 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Sixteen (52%) of these 31 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Fifteen (48%) of these 31 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [15]
Fifteen (33%) of the 46 influenza B viruses characterized belong to the B/Yamagata lineage of viruses

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

During week 4, 7.1% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.8% for week 4.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported during week 4. Since October 1, 2006, CDC has received seven 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 017 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 4, 2.6%*** 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.

Four out of nine surveillance regions reported ILI above their region-specific baseline and one surveillance region reported ILI at their region-specific baseline****:

Region

Reported ILI (%)

Region-Specific Baseline (%)

New England

1.0

1.2

Mid Atlantic

2.2

2.6

East North Central

2.1

1.9

West North Central

2.0

1.5

South Atlantic

1.5

2.3

East South Central

2.2

2.4

West South Central

5.9

3.0

Mountain

2.3

1.7

Pacific

3.2

3.2

 

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

During week 4, the following influenza activity was reported:

Widespread activity was reported by eight states (Arkansas, Indiana, Iowa, Maryland, Minnesota, Oklahoma, South Carolina, and Texas).

Regional activity was reported by 14 states (California, Connecticut, Georgia, Idaho, Kentucky, Illinois, Massachusetts, Mississippi, Nebraska, Nevada, Oregon, South Dakota, Tennessee, and Virginia).

Local activity was reported by 15 states (Alabama, Arizona, Florida, Kansas, Louisiana, Maine, Michigan, Missouri, Montana, New Jersey, North Carolina, North Dakota, Pennsylvania, West Virginia, and Wisconsin).

Sporadic activity was reported by the District of Columbia, New York City and 13 states (Alaska, Colorado, Delaware, Hawaii, New Hampshire, New Mexico, New York, Ohio, Rhode Island, Utah, Vermont, Washington, and Wyoming).

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared February 2, 2007

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