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Weekly Report: Influenza Summary Update

Week ending April 1, 2006-Week 13

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Synopsis:

During week 13 (March 26 – April 1, 2006)*, influenza activity decreased in the United States. Four hundred sixty-four specimens (16.6%) 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. Thirteen states reported widespread influenza activity; 14 states reported regional influenza activity; 12 states, New York City, and the District of Columbia reported local influenza activity; 10 states and Puerto Rico reported sporadic influenza activity; and one state reported no activity.

Laboratory Surveillance*:

During week 13, WHO and NREVSS laboratories reported 2,790 specimens tested for influenza viruses and 464 (16.6%) were positive. Of these, 47 were influenza A (H3N2) viruses, 3 were influenza A (H1N1) viruses, 198 were influenza A viruses that were not subtyped, and 216 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 114,891 specimens for influenza viruses and 14,377 (12.5%) were positive. Among the 14,377 influenza viruses, 12,500 (86.9%) were influenza A viruses and 1,877 (13.1%) were influenza B viruses. Five thousand ninety-six (40.8%) of the 12,500 influenza A viruses have been subtyped: 4,837 (94.9%) were influenza A (H3N2) viruses and 259 (5.1%) were influenza A (H1N1) viruses. During the past 3 weeks (weeks 11–13), the percentage of specimens testing positive for influenza has ranged from 26.6% and 25.7% in the East South Central and South Atlantic regions, respectively, to 12.7% in the Pacific region**. During this period, the proportion of isolates identified as influenza type B virus continued to increase. During weeks 11-13, the Mountain region reported the highest proportion of isolates as influenza B (62.8%). Other regions reporting more than 30.0% of recent isolates as influenza B include the East North Central, West North Central, West South Central, and Pacific regions.

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 458 influenza viruses [379 influenza A (H3N2), 27 influenza A (H1), and 52 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 379 influenza A (H3N2) viruses, 301 (79.4%) were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 78 (20.6%) viruses showed reduced titers with antisera produced against A/California/07/2004. Of the 78 low-reacting viruses, 52 were tested with antisera produced against A/Wisconsin/67/2005 (the H3N2 component selected for the 2006-07 vaccine), and 44 are A/Wisconsin-like. The hemagglutinin proteins of 25 (92.6%) influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99, and 2 (7.4%) showed reduced titers with antisera produced against A/New Caledonia/20/99. Twenty-four (46.2%) of the influenza B viruses that have been characterized belong to the B/Yamagata lineage. Four were similar to B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine, and 20 were characterized as B/Florida/07/2004-like. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. Twenty-eight (53.8%) 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 13, 7.6% 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 8.1% for week 13.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During October 2, 2005 – April 1, 2006, CDC has received reports of 21 influenza-associated pediatric deaths, 18 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†: Emerging Infections Program (EIP) and New Vaccine Surveillance Network (NVSN). During October 1, 2005 – March 18, 2006, the preliminary influenza-associated hospitalization rate reported by EIP for children aged 0-17 years was 0.79 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 1.88 per 10,000 and 0.22 per 10,000, respectively. During October 30, 2005 – March 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years in NVSN was 3.0 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 13, 2.6%*** 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 1.4% in the East South Central region to 3.3% 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 13, 13 states (Connecticut, Delaware, Indiana, Kentucky, Maine, Massachusetts, New York, Rhode Island, South Carolina, Vermont, Virginia, West Virginia, and Wisconsin) reported widespread influenza activity. Fourteen states (Iowa, Maryland, Michigan, Minnesota, Montana, New Hampshire, New Jersey, North Carolina, North Dakota, Ohio, Pennsylvania, South Dakota, Tennessee, and Washington) reported regional influenza activity. Twelve states (Alaska, California, Colorado, Florida, Georgia, Hawaii, Illinois, Nebraska, Nevada, Oregon, Texas, and Wyoming), New York City, and the District of Columbia reported local influenza activity. Ten states (Alabama, Arizona, Arkansas, Idaho, Kansas, Louisiana, Missouri, New Mexico, Oklahoma, and Utah) and Puerto Rico reported sporadic influenza activity. Mississippi reported no activity. One state did not report.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared April 7, 2006

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