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Weekly Report: Influenza Summary Update

Week ending December 9, 2006-Week 49

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Synopsis:

During week 49 (December 3 ?December 9, 2006)*, influenza activity remained low overall in the United States, but increased in the Southeast. Among specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories for influenza, 4.9% were positive. One state reported widespread influenza activity; three states reported regional influenza activity; seven states reported local influenza activity; 30 states and New York City reported sporadic influenza activity; and nine states and the District of Columbia reported no influenza activity. On a national level, laboratory, outpatient influenza-like illness (ILI) and mortality surveillance increased slightly from week 48 to week 49, but the ILI and mortality data remain below baseline levels.

Laboratory Surveillance*:

During week 49, WHO and NREVSS laboratories reported 2,470 specimens tested for influenza viruses, 121 (4.9%) of which were positive: 21 influenza A (H1) viruses, 75 influenza A viruses that were not subtyped, and 25 influenza B viruses. No influenza A (H3) viruses were reported.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 27,474 specimens for influenza viruses and 884 (3.2%) were positive. Among the 884 influenza viruses, 689 (78%) were influenza A viruses and 195 (22%) were influenza B viruses. One hundred seventy-one (25%) of the 689 influenza A viruses have been subtyped: 162 (95%) were influenza A (H1) viruses and nine (5.3%) were influenza A (H3) viruses. Thirty-seven states have reported positive laboratory influenza tests. Of the 884 influenza positive tests reported this season, 441 (50%) have been reported from Florida.

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Antigenic Characterization:

CDC has antigenically characterized 28 influenza viruses [10 influenza A (H1), one influenza A (H3), and 17 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [10]

?Eight of the 10 viruses were characterized as A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.
?Two of the 10 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99.

Influenza A (H3)

?The virus was 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) [17]

Victoria lineage [6]
?Six (35.3%) of the 17 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Three of these six viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Three of these six viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [11]
?Eleven (64.7%) of the 17 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

It is too early in the influenza season to determine which influenza viruses will predominate or how well the vaccine and circulating strains will match.

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

During week 49, 6.3% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.2% for week 49.

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 49, and no deaths have been reported for the 2006-07 influenza season.

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). No influenza-associated pediatric hospitalizations have been reported from either network this season.}

Influenza-like Illness Surveillance*:

During week 49, 1.9%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline**** of 2.1%. On a regional level**, the percentage of visits for ILI ranged from 0.9% to 3.2%. Two regions reported ILI above their region-specific baseline****: the East South Central region reported 2.6% compared to its baseline of 2.4% and the West South Central region reported 3.2% compared to its baseline of 3.0%.

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 49, the following influenza activity^†† was reported:

?Widespread activity was reported by Florida.
?Regional activity was reported by three states (Alabama, Georgia, and South Carolina).
?Local activity was reported by seven states (Connecticut, Hawaii, Louisiana, Massachusetts, Mississippi, Oklahoma, and Tennessee).
?Sporadic activity was reported by New York City and 30 states (Alaska, Arizona, Arkansas, California, Colorado, Delaware, Kentucky, Idaho, Illinois, Indiana, Iowa, Maryland, Michigan, Minnesota, Montana, New Hampshire, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Rhode Island, Texas, Utah, Virginia, Washington, West Virginia, Wisconsin, and Wyoming).
?No influenza activity was reported by the District of Columbia and nine states (Kansas, Maine, Missouri, Nebraska, Nevada, New Jersey, New Mexico, South Dakota, and Vermont).

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Foot notes

Report prepared December 14, 2006

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