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Weekly Report: Influenza Summary Update

Week ending May 20, 2006-Week 20

This is the final report of the 2005-06 season.

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Synopsis:

Influenza activity in the United States peaked in early March and continued to decrease during week 20 (May 14 – May 20, 2006)*. Fifty-one specimens (6.3%) 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 below the national baseline. The proportion of deaths attributed to pneumonia and influenza was below the threshold level. Twenty-five states, the District of Columbia, New York City, and Puerto Rico reported sporadic influenza activity; and 25 states reported no activity.

Laboratory Surveillance*:

During week 20, WHO and NREVSS laboratories reported 811 specimens tested for influenza viruses and 51 (6.3%) were positive. Of these, 1 was an influenza A (H3N2) virus, 1 was an influenza A (H1N1) virus, 8 were influenza A viruses that were not subtyped, and 41 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 135,973 specimens for influenza viruses and 17,068 (12.6%) were positive. Among the 17,068 influenza viruses, 13,857 (81.2%) were influenza A viruses and 3,211 (18.8%) were influenza B viruses. Five thousand six hundred forty-eight (40.8%) of the 13,857 influenza A viruses have been subtyped: 5,228 (92.6%) were influenza A (H3N2) viruses and 420 (7.4%) were influenza A (H1N1) viruses. During the past three weeks (weeks 18–20), the percentage of specimens testing positive for influenza has ranged from 16.4% in the West South Central region to 1.7% in the Mid-Atlantic region**. During this period, 66.4% of all isolates identified were influenza B viruses.

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 828 influenza viruses [503 influenza A (H3N2), 88 influenza A (H1), and 237 influenza B viruses] collected by U.S. laboratories since October 1, 2005.

Of the 503 influenza A (H3N2) viruses, 381 (75.7%) were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 122 (24.3%) viruses showed reduced titers with antisera produced against A/California/07/2004. Of the 122 low-reacting viruses, 96 were tested with antisera produced against A/Wisconsin/67/2005 (the H3N2 component selected for the 2006-07 vaccine), and 70 are A/Wisconsin-like.

The hemagglutinin proteins of 85 (96.6%) of the 88 characterized influenza A (H1) viruses were antigenically similar to the hemagglutinin of the vaccine strain A/New Caledonia/20/99, and the other 3 (3.4%) showed reduced titers with antisera produced against A/New Caledonia/20/99.

Fifty-two (21.9%) of the 237 influenza B viruses that have been characterized belong to the B/Yamagata lineage: 8 were similar to B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine, 43 were characterized as B/Florida/07/2004-like, and 1 showed reduced titers with antisera produced against both B/Shanghai/361/2002 and B/Florida/07/2004. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. One hundred eighty-five (78.1%) of the 237 influenza B viruses were identified as belonging to the B/Victoria lineage: 184 were similar to B/Ohio/1/2005, the influenza B component selected for the 2006-07 vaccine, and 1 showed reduced titers with antisera produced against B/Ohio/1/2005.

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

During week 20, 6.3% 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 7.4% for week 20.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During October 2, 2005 – May 20, 2006, CDC received reports of 35 influenza-associated pediatric deaths, 33 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 – April 30, 2006, the preliminary influenza-associated hospitalization rate reported by EIP for children aged 0-17 years was 1.21 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 2.76 per 10,000 and 0.38 per 10,000, respectively. During October 30, 2005 – April 29, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years in NVSN was 5.4 per 10,000. EIP and NVSN hospitalization data collection have ended as of April 30, 2006. Rate estimates are preliminary and may continue to change as data are finalized.

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 20, 0.9%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline of 2.2%. The percentage of visits for ILI ranged from 0.1% in the West North Central region to 2.7% 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 20, 25 states (Alaska, Arizona, California, Connecticut, Florida, Hawaii, Indiana, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Minnesota, Missouri, Nevada, New Jersey, New York, Oklahoma, Oregon, Pennsylvania, South Carolina, Tennessee, Utah, West Virginia, and Wisconsin), the District of Columbia, New York City, and Puerto Rico reported sporadic influenza activity. 25 states (Alabama, Arkansas, Colorado, Delaware, Georgia, Idaho, Illinois, Iowa, Kansas, Michigan, Mississippi, Montana, Nebraska, New Hampshire, New Mexico, North Carolina, North Dakota, Ohio, Rhode Island, South Dakota, Texas, Vermont, Virginia, Washington, and Wyoming) reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

 Report prepared Report prepared: May 25, 2006

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