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Weekly Report: Influenza Summary Update

Week ending March 24, 2007-Week 12

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Synopsis:

During week 12 (March 18 ?March 24, 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 sixth consecutive week; 15.1% of specimens tested positive for influenza this week. ILI data decreased during week 12 and was at the national baseline for the first time since early January. Twelve states reported widespread influenza activity; 17 states reported regional influenza activity; 11 states, and New York City reported local influenza activity; the District of Columbia and nine states reported sporadic influenza activity; and one state did not report. The reporting of widespread or regional influenza activity decreased from 37 for week 11 to 29 for week 12. The percent of deaths due to pneumonia and influenza remained below baseline level.

Laboratory Surveillance*:

During week 12, WHO and NREVSS laboratories reported 3,569 specimens tested for influenza viruses, 540 (15.1%) of which were positive: 26 influenza A (H1) viruses, 51 influenza A (H3) viruses, 299 influenza A viruses that were not subtyped, and 164 influenza B viruses.

Since October 1, 2006, WHO and NREVSS laboratories have tested a total of 143,251 specimens for influenza viruses and 19,239 (13.4%) were positive. Among the 19,239 influenza viruses, 15,494 (80.5%) were influenza A viruses and 3,745 (19.5%) were influenza B viruses. Four thousand four hundred ninety (29.0%) of the 15,494 influenza A viruses have been subtyped: 3,427 (76.3%) were influenza A (H1) viruses and 1,063 (23.7%) were influenza A (H3) viruses. Among specimens tested for influenza during the most recent three weeks (March 4 ?March 24, 2007), on a regional basis, the percent of specimens testing positive for influenza were as follows:

February 25 – March 17, 2007 (specimens testing positive)
>20% positive	10-20% positive
East North Central (29.3%)	New England (14.0%)
East South Central (37.3%)	Mid Atlantic (16.8%)
West South Central (20.4%)	West North Central (17.3%)
	South Atlantic (15.4%)
	Mountain (18.2%)
	Pacific (13.3%)

Although influenza A (H1) viruses have predominated in the US this season, the predominant virus has varied by region during the past three weeks.  Influenza A has predominated in all but the East South Central region, where 83% of viruses reported from March 4 – March 24, 2007 were influenza B.  However, within the eight regions where influenza A continues to predominate, A (H3) viruses accounted for more than 50% of the subtyped influenza A viruses in the Mid Atlantic, East North Central, West North Central, and Pacific 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 402 influenza viruses [262 influenza A (H1), 40 influenza A (H3) viruses, and 100 influenza B viruses] collected by U.S. laboratories since October 1, 2006.

Influenza A (H1) [262]

?Two hundred forty-five (94%) of the 262 viruses characterized were similar to A/New Caledonia/20/99-like, which is the influenza A (H1) component of the 2006-07 influenza vaccine.

?Seventeen (6%) of the 262 viruses showed somewhat reduced titers with antisera produced against A/New Caledonia/20/99 and are similar to A/Solomon Islands/3/2006-like.

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) [100]

Victoria lineage [71]
?Seventy-one (71%) of the 100 influenza B viruses characterized belong to the B/Victoria lineage of viruses.

o Forty-two (59%) of these 71 viruses were similar to B/Ohio/01/2005, the B component of the 2006-07 influenza vaccine.

o Twenty-nine (41%) of these 71 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005.

Yamagata lineage [29]
?Twenty-nine (29%) of the 100 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.

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

During week 12, 7.5% of all deaths were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.8% for week 12.

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Influenza-Associated Pediatric Mortality*:

Four influenza-associated pediatric deaths were reported during week 12. Since October 1, 2006, CDC has received 39 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 ?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?7 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 12, 2.1%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is at the national baseline**** of 2.1%.

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

Region	Reported ILI (%)	Region-Specific Baseline (%)
New England	2.3	1.2
Mid Atlantic	1.8	2.6
East North Central	2.8	1.9
West North Central	0.6	1.5
South Atlantic	1.8	2.3
East South Central	1.6	2.4
West South Central	3.4	3.0
Mountain	2.0	1.7
Pacific	1.9	3.2

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 12, the following influenza activity^†† was reported:

?Widespread activity was reported by 12 states (Alaska, Colorado, Connecticut, Delaware, Hawaii, Kentucky, Montana, New Hampshire, New York, Ohio, Vermont, and Virginia).

?Regional activity was reported by 17 states (Arkansas, Georgia, Idaho, Maine, Maryland, Massachusetts, Michigan, New Jersey, New Mexico, North Carolina, Pennsylvania, South Carolina, Tennessee, Texas, Utah, Washington, and Wyoming).

?Local activity was reported by New York City and 11 states (Alabama, Arizona, California, Florida, Illinois, Nebraska, Nevada, North Dakota, Oklahoma, South Dakota, and Wisconsin).

?Sporadic activity was reported by the District of Columbia and nine states (Indiana, Kansas, Louisiana, Minnesota, Mississippi, Missouri, Oregon, Rhode Island, and West Virginia).

?No report was received from Iowa.

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Foot notes

Report prepared March 30, 2007 Error processing SSI file