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Weekly Report: Influenza Summary Update

Week ending January 6, 2007-Week 1

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Synopsis:

During week 1 (December 31, 2006 – January 6, 2007)*, laboratory and outpatient influenza-like illness (ILI) surveillance data indicated a decrease in influenza activity in the United States from week 52 to week 1.  Among specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories for influenza, 7.6% were positive.  The proportion of visits for ILI may have been influenced by a reduction in routine healthcare visits during the holiday season as has been seen in past seasons.  However, ILI remained above baseline for the fourth consecutive week this season.  Five states reported widespread influenza activity; 11 states reported regional influenza activity; 15 states reported local influenza activity; 18 states, the District of Columbia, and New York City reported sporadic influenza activity; and one state reported no influenza activity.  The reporting of widespread or regional influenza activity increased from 14 states for week 52 to 16 states for week 1.  The percent of deaths due to pneumonia and influenza remained below baseline level. 

Laboratory Surveillance*:

During week 1, WHO and NREVSS laboratories reported 2,939 specimens tested for influenza viruses, 222 (7.6%) of which were positive: 62 influenza A (H1) viruses, one influenza A (H3) virus, 120 influenza A viruses that were not subtyped, and 39 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 50,666 specimens for influenza viruses and 2,787 (5.5%) were positive. Among the 2,787 influenza viruses, 2,256 (80.9%) were influenza A viruses and 531 (19.1%) were influenza B viruses. Six hundred ninety-five (30.8%) of the 2,256 influenza A viruses have been subtyped: 669 (96.3%) were influenza A (H1) viruses and 26 (3.7%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (December 17, 2006 ?January 6, 2007), on a regional basis, the percent of specimens testing positive for influenza exceeded 10% in the East North Central (21.6%), West North Central (11.0%), South Atlantic (15.6%), and East South Central (25.3%) regions. The percent of specimens tested in the most recent three weeks testing positive for influenza was below 10% in the remaining five regions and ranged from 1.9% to 6.4%.

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Antigenic Characterization:

CDC has antigenically characterized 98 influenza viruses [59 influenza A (H1), two influenza A (H3) viruses, and 37 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) [37]

Victoria lineage [18]
?Twenty-two (59%) of the 37 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Nine (41%) of these 22 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Thirteen (59%) of these 22 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [15]
?Fifteen (41%) of the 37 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 1, 6.9% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.6% for week 1.

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Influenza-Associated Pediatric Mortality*:

One influenza-associated pediatric death was reported for week 1. Since October 1, 2006, CDC has received two reports of influenza-associated pediatric deaths that occurred during the current season.

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Influenza-Associated Pediatric Hospitalizations*:

Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks^{?/sup>: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). During November 5, 2006 ?December 23, 2006 the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 0.2 per 10,000. No influenza-associated pediatric hospitalizations have been reported from the EIP.}

Influenza-like Illness Surveillance*:

During week 1, 2.3%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline**** of 2.1%. On a regional level**, the percentage of visits for ILI ranged from 1.5% to 4.1%.

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

Region	Reported ILI (%)	Region-Specific Baseline (%)
New England	1.5	1.2
Mid Atlantic	1.7	2.6
East North Central	2.6	1.9
West North Centrall	1.9	1.5
South Atlantic	2.3	2.3
East South Central	1.7	2.4
West South Central	4.1	3.0
Mountain	1.9	1.7
Pacific	2.0	3.2

 

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 1, the following influenza activity^†† was reported:

?Widespread activity was reported by five states (Alabama, Georgia, Kentucky, South Carolina, and Tennessee).
?Regional activity was reported by 11 states (Connecticut, Florida, Indiana, Iowa, Maryland, Mississippi, Nevada, North Carolina, Oklahoma, Pennsylvania, and Texas).
?Local activity was reported by 15 states (Arizona, California, Idaho, Illinois, Kansas, Louisiana, Massachusetts, Michigan, Missouri, Nebraska, New Jersey, South Dakota, Utah, Virginia, and Wisconsin).
?Sporadic activity was reported by the District of Columbia, New York City and 18 states (Alaska, Arkansas, Colorado, Delaware, Hawaii, Maine, Minnesota, Montana, New Hampshire, New Mexico, New York, North Dakota, Ohio, Oregon, Rhode Island, Washington, West Virginia, and Wyoming).
?No influenza activity was reported by one state (Vermont).

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Foot notes

Report prepared January 12, 2007

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