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Weekly Report: Influenza Summary Update

Week ending November 11, 2006-Week 45

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Synopsis:

During week 45 (November 5 – November 11, 2006)*, a low level of influenza activity was reported in the United States. Eighty-two (4.6%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. Two states reported regional influenza activity; four states reported local influenza activity; 21 states and the District of Columbia reported sporadic influenza activity; and 23 states and New York City reported no influenza activity.

Laboratory Surveillance*:

During week 45, WHO and NREVSS laboratories reported 1,764 specimens tested for influenza viruses, 82 of which were positive: one influenza A (H1) virus, 59 influenza A viruses that were not subtyped, and 22 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 11,858 specimens for influenza viruses and 381 (3.2%) were positive. Among the 381 influenza viruses, 299 (78.5%) were influenza A viruses and 82 (21.5%) were influenza B viruses. Seventy-seven (25.8%) of the 299 influenza A viruses have been subtyped: 70 (90.9%) were influenza A (H1) viruses and 7 (9.1%) were influenza A (H3) viruses. Of the 381 influenza viruses reported this season, 249 (65.4%) have been reported from Florida. Twenty-three states from eight of the nine surveillance regions** have reported laboratory-confirmed influenza this season.

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Antigenic Characterization:

CDC has antigenically characterized 17 influenza viruses [one influenza A (H1) and 16 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1)

• The virus was characterized as A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.
• Eight additional isolates collected from an outbreak in September were also characterized as A/New Caledonia/20/99-like.

Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages)

Victoria lineage [5]
• Five (32.2%) of the 16 influenza viruses characterized belong to the B/Victoria lineage of viruses.

o Two (12.5%) of these 5 viruses that have been characterized were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.
o Three (18.8%) of these 5 viruses characterized showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [11]
• Eleven (68.8%) of the 16 influenza B viruses were identified as belonging to the B/Yamagata lineage.

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

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

During week 45, 6.16% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 6.76% for week 45.

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 45 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^†: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). No influenza-associated pediatric hospitalizations have been reported from the EIP. NVSN estimated rates of hospitalization for influenza will be reported every 2 weeks, beginning December 2006.

Influenza-like Illness Surveillance*:

During week 45, 1.5%*** 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 0.5% to 3.4%. The only region that reported ILI above its region-specific baseline**** was the West South Central region, which reported 3.4% compared to the baseline of 3.0%.

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 45, the following influenza activity was reported:

• Regional activity was reported by two states (Alabama and North Carolina).
• Local activity was reported by four states (Florida, Hawaii, Louisiana, and Mississippi).
• Sporadic activity was reported by the District of Columbia and 21 states (Alaska, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Indiana, Iowa, Kentucky, Maryland, Massachusetts, New York, North Dakota, Pennsylvania, South Dakota, Tennessee, Texas, Utah, Virginia, and Wyoming).
• No influenza activity was reported by New York City and 23 states.

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Foot notes

Report prepared November 17, 2006 Error processing SSI file