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Weekly Report: Influenza Summary Update

Week ending April 14, 2007-Week 15

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Synopsis:

During week 15 (April 8 ?14, 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 an increase in the percentage of specimens testing positive for influenza; however reporting from one state was responsible for this increase. Other surveillance systems indicated a decrease in influenza activity. The percentage of visits for ILI to sentinel providers decreased during week 15 and was below the national baseline for the third consecutive week. Four states reported widespread influenza activity; seven states reported regional influenza activity; 13 states and New York City reported local influenza activity; the District of Columbia and 23 states reported sporadic influenza activity; and three states reported no influenza activity. The number of jurisdictions reporting widespread or regional influenza activity decreased from 13 for week 14 to 11 for week 15. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 15, WHO and NREVSS laboratories reported 2,106 specimens tested for influenza viruses, 291 (13.8%) of which were positive: 15 influenza A (H1) viruses, 110 influenza A (H3) viruses, 97 influenza A viruses that were not subtyped, and 69 influenza B viruses. Of the 110 influenza A (H3) viruses reported for week 15, 83 (75.5%) were reported from one state.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 157,049 specimens for influenza viruses and 21,336 (13.5%) were positive. Among the 21,336 influenza viruses, 16,958 (79.5%) were influenza A viruses and 4,378 (20.5%) were influenza B viruses. Five thousand one hundred nineteen (30.2%) of the 16,958 influenza A viruses have been subtyped: 3,638 (71.1%) were influenza A (H1) viruses and 1,481 (28.9%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (March 25 ?April 14, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

March 25 – April 14, 2007 (specimens testing positive)
< 10% positive	10-20% positive	> 20% positive
West North Central (4.7%)	New England (12.4%)	East North Central (35.3%)
West South Central (4.9%)	Mid Atlantic (12.8%)	South Atlantic (37.4%)
Mountain (7.3%)		East South Central (38.5%)
		Pacific (21.8%)

Although influenza A (H1) viruses have predominated overall in the United States this season, the predominant virus has varied by region during the past three weeks.  Influenza A has predominated in all regions except the East South Central region, where 88.1% of all viruses reported from March 25 – April 14, 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 six regions (New England, East North Central, Mid-Atlantic, Pacific, South Atlantic, and West North Central regions) while A (H1) viruses predominated in the two regions (Mountain and West South Central 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 599 influenza viruses [324 influenza A (H1), 123 influenza A (H3) viruses, and 152 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [324]

?Three hundred and one (93%) of the 324 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-three (7%) of the 324 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) [123]

?Thirty-five (28%) of the 123 viruses were characterized as A/Wisconsin/67/2005-like, which is the influenza A (H3) component of the 2006-07 influenza vaccine.

?Eighty-eight (72%) of the 123 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) [152]

Victoria lineage [113]
?One hundred and thirteen (74%) of the 152 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Seventy-two (64%) of these 113 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Forty-one (36%) of these 113 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [39]
?Thirty-nine (26%) of the 152 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 15, 6.6% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.6% for week 15.

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Influenza-Associated Pediatric Mortality*:

Two influenza-associated pediatric deaths were reported during week 15. Since October 1, 2006, CDC has received 43 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^{?/sup>: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN).}

During November 5, 2006 to March 31, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children 0-4 years old in the NVSN was 2.69 per 10,000.

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During October 1, 2006 to March 31, 2007, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0?7 years old was 0.61 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 1.54 per 10,000 and 0.21 per 10,000, respectively.

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Influenza-like Illness Surveillance*:

During week 15, 1.3%** of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline*** of 2.1%.

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

Region	Reported ILI (%)	Region-Specific Baseline (%)
New England	1.6	1.2
Mid Atlantic	1.4	2.6
East North Central	1.4	1.9
West North Central	0.4	1.5
South Atlantic	1.0	2.3
East South Central	0.9	2.4
West South Central	2.7	3.0
Mountain	1.1	1.7
Pacific	0.8	3.2

 

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 15, the following influenza activity^†† was reported:

?Widespread activity was reported by four states (Delaware, Montana, New Hampshire, and New York).

?Regional activity was reported by seven states (Alaska, Connecticut, Hawaii, Kentucky, Maine, New Jersey, and Virginia).

?Local activity was reported by New York City and 13 states (Alabama, Arizona, Colorado, Idaho, Maryland, Massachusetts, Michigan, Nevada, Pennsylvania, South Carolina, Utah, Vermont, and Wyoming).

?Sporadic activity was reported by the District of Columbia and 23 states (Arkansas, California, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Louisiana, Minnesota, Missouri, Nebraska, New Mexico, North Carolina, North Dakota, Oklahoma, Rhode Island, South Dakota, Tennessee, Texas, Washington, West Virginia, and Wisconsin).

?Mississippi, Ohio, and Oregon reported no influenza activity

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Foot notes

Report prepared April 20, 2007 Error processing SSI file