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Weekly Report: Influenza Summary Update

Week ending February 25, 2006-Week 8

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Synopsis:

During week 8 (February 19 February 25, 2006)*, influenza activity increased in the United States. Four hundred thirty-nine specimens (21.2%) 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) was above the national baseline. The proportion of deaths attributed to pneumonia and influenza was below the baseline level. Twenty-one states and New York City reported widespread influenza activity; 14 states reported regional influenza activity; 10 states and the District of Columbia reported local influenza activity; and 5 states and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 8, WHO and NREVSS laboratories reported 2,066 specimens tested for influenza viruses and 439 (21.2%) were positive. Of these, 134 were influenza A (H3N2) viruses, 12 were influenza A (H1N1) viruses, 231 were influenza A viruses that were not subtyped, and 62 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 79,336 specimens for influenza viruses and 7,256 (9.1%) were positive. Among the 7,256 influenza viruses, 6,853 (94.4%) were influenza A viruses and 403 (5.6%) were influenza B viruses. Three thousand one hundred and five (45.3%) of the 6,853 influenza A viruses have been subtyped: 3,046 (98.1%) were influenza A (H3N2) viruses and 59 (1.9%) were influenza A (H1N1) viruses. All states have reported laboratory-confirmed influenza this season. During the past 3 weeks (weeks 6 8), the percentage of specimens testing positive for influenza has ranged from 39.7% in the East North Central region to 7.5% in the Pacific region**.

INFLUENZA Virus Isolated


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Composition of the 2006-07 Influenza Vaccine:

WHO has recommended that the 2006-07 trivalent influenza vaccine for the Northern Hemisphere contain A/New Caledonia/20/99-like (H1N1), A/Wisconsin/67/2005-like (H3N2), and B/Malaysia/2506/2004-like viruses. The influenza A (H3N2) and the influenza B components have been changed from the 2005-06 season vaccine components. A/Wisconsin/67/2005 is an antigenic variant of the current vaccine strain A/California/07/2004. Influenza B viruses currently circulating can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. The updating of the influenza B component to B/Ohio/1/2005 (which is antigenically equivalent to B/Malaysia/2506/2004) represents a change to the B/Victoria lineage. 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 295 influenza viruses [250 influenza A (H3N2), 14 influenza A (H1), and 31 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 250 influenza A (H3N2) viruses, 201 were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 49 viruses showed reduced titers with antisera produced against A/California/07/2004. Of the 49 low-reacting viruses, 23 were tested with antisera produced against A/Wisconsin/67/2005 (the H3N2 component selected for the 2006-07 vaccine), and 19 are A/Wisconsin-like. The hemagglutinin proteins of 12 influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99, and 2 showed reduced titers with antisera produced against A/New Caledonia/20/99. Nineteen of the influenza B viruses that have been characterized belong to the B/Yamagata lineage. Two were similar to B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine, and 17 were characterized as B/Florida/07/2004-like. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. Twelve influenza B viruses were identified as belonging to the B/Victoria lineage and all were similar to B/Ohio/1/2005, the influenza B component selected for the 2006-07 vaccine.

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

During week 8, 7.0% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia or influenza. This percentage is below the epidemic threshold of 8.3% for week 8.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

One influenza-associated pediatric death was reported during week 8. Since October 2, 2005, CDC has received reports of 15 influenza-associated pediatric deaths, 13 of which occurred during the current 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). During October 1, 2005 February 18, 2006, the preliminary influenza-associated hospitalization rate reported by the EIP for children aged 0-17 years was 0.48 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 1.11 per 10,000 and 0.15 per 10,000, respectively. During October 30, 2005 February 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years in the NVSN was 0.48 per 10,000. EIP and NVSN hospitalization rate estimates are preliminary and may change as data continue to be collected.

EIP Influenza Laboratory-Confirmed Cumulative Hospitalization Rates for Children Aged 0-4 and 5-17 years, 2005-06 and Previous 2 Seasons
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NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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Influenza-like Illness Surveillance*:

During week 8, 3.4%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 2.2%. The percentage of visits for ILI ranged from 1.8% in the Pacific region to 6.9% in the West South Central region. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

Bar Chart for Influenza-like Illness
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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

During week 8, 21 states (Alabama, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia, Indiana, Iowa, Kansas, Kentucky, Massachusetts, New York, North Carolina, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Virginia, and Wyoming) and New York City reported widespread influenza activity. Fourteen states (Alaska, Illinois, Louisiana, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, New Hampshire, New Jersey, Ohio, South Dakota, and Wisconsin) reported regional influenza activity. Ten states (California, Hawaii, Idaho, Maine, Maryland, Nevada, North Dakota, Oklahoma, Oregon, and Vermont) and the District of Columbia reported local influenza activity. Five states (Arizona, New Mexico, Utah, Washington, and West Virginia) and Puerto Rico reported sporadic influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared March 3, 2006

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