Synopsis:

During week 13 (March 25 ? March 31, 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 seventh consecutive week; 10.3% of specimens tested positive for influenza this week. ILI data remained at similar levels nationally and fewer regions were above baseline levels this week compared to last week (two versus four, respectively). Ten states reported widespread influenza activity; nine states reported regional influenza activity; 13 states, and New York City reported local influenza activity; the District of Columbia and 17 states reported sporadic influenza activity; and one state reported no influenza activity. The number of jurisdictions reporting widespread or regional influenza activity decreased from 29 for week 12 to 19 for week 13. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 13, WHO and NREVSS laboratories reported 2,524 specimens tested for influenza viruses, 260 (10.3%) of which were positive: 17 influenza A (H1) viruses, 19 influenza A (H3) viruses, 145 influenza A viruses that were not subtyped, and 79 influenza B viruses. This represents a substantial drop in the percentage of specimens that have tested positive for influenza compared to the two prior weeks of 19.7% and 15.3% positive.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 147,866 specimens for influenza viruses and 20,006 (13.5%) were positive. Among the 20,006 influenza viruses, 15,986 (79.9%) were influenza A viruses and 4,020 (20.1%) were influenza B viruses. Four thousand seven hundred forty-five (29.7%) of the 15,986 influenza A viruses have been subtyped: 3,550 (74.8%) were influenza A (H1) viruses and 1,195 (25.2%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (March 11 ? March 31, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

Although influenza A (H1) viruses have predominated overall in the U.S. this season, the predominant virus has varied by region during the past three weeks.  Influenza A has predominated in all except the East South Central region, where 90.2% of viruses reported from March 11 – March 31, 2007 were influenza B.  Within the eight regions where influenza A predominated, A (H3) viruses accounted for more than 50% of the subtyped influenza A viruses in four regions (New England, East North Central, Mid-Atlantic, and Pacific regions) while A (H1) viruses predominated in the other four regions (Mountain, West North Central, West South Central, and South Atlantic regions)

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Composition of the 2006-07 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 454 influenza viruses [296 influenza A (H1), 40 influenza A (H3) viruses, and 118 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [296]

? Two hundred seventy-six (93%) of the 296 viruses characterized were similar to A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.? Twenty (7%) of the 296 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99 and are similar to A/Solomon Islands/3/2006-like. A/Solomon Islands/3/2006 is a recent antigenic variant of A/New Caledonia/20/99.

Influenza A (H3) [40]

? Sixteen (40%) of the 40 viruses were characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.

? Twenty-four (60%) of the 40 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) [118]

Victoria lineage [87]
? Eighty-seven (74%) of the 118 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Fifty-two (60%) of these 87 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Thirty-five (40%) of these 87 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [31]
? Thirty-one (26%) of the 118 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 13, 7.4% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.7% for week 12.

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Influenza-Associated Pediatric Mortality*:

One influenza-associated pediatric death was reported during week 13. Since October 1, 2006, CDC has received 40 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 17, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years old in the NVSN was 2.31 per 10,000.

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During October 1, 2006 ? March 17, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0?17 years old was 0.61 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 1.58 per 10,000 and 0.22 per 10,000, respectively.

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Influenza-like Illness Surveillance*:

During week 13, 2.2%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is slightly above the national baseline**** of 2.1%.

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

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 13, the following influenza activity^?? was reported:

? Widespread activity was reported by 10 states (Alaska, Connecticut, Delaware, Hawaii, Kentucky, Montana, New Hampshire, New Jersey, New York, and Virginia).

? Regional activity was reported by 9 states (Colorado, Idaho, Maryland, Massachusetts, Michigan, North Carolina, Pennsylvania, Texas, and Wyoming).

? Local activity was reported by New York City and 13 states (Arizona, Georgia, Illinois, Iowa, Maine, Nevada, New Mexico, North Dakota, Oklahoma, South Carolina, South Dakota, Tennessee, and Vermont).

? Sporadic activity was reported by the District of Columbia and 17 states (Alabama, Arkansas, California, Florida, Indiana, Kansas, Louisiana, Minnesota, Mississippi, Missouri, Nebraska, Oregon, Rhode Island, Utah, Washington, West Virginia, and Wisconsin).

? Ohio reported no influenza activity.

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