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Weekly Report: Influenza Summary Update

Week ending March 25, 2006-Week 12

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Synopsis:

During week 12 (March 19 March 25, 2006)*, influenza activity continued approximately at the same level as recent weeks in the United States. Six hundred eighty-one specimens (21.6%) 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. Sixteen states reported widespread influenza activity; 16 states reported regional influenza activity; 13 states, New York City, and the District of Columbia reported local influenza activity; and 5 states and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 12, WHO and NREVSS laboratories reported 3,158 specimens tested for influenza viruses and 681 (21.6%) were positive. Of these, 77 were influenza A (H3N2) viruses, 70 were influenza A (H1N1) viruses, 347 were influenza A viruses that were not subtyped, and 187 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 110,066 specimens for influenza viruses and 13,532 (12.3%) were positive. Among the 13,532 influenza viruses, 11,989 (88.6%) were influenza A viruses and 1,543 (11.4%) were influenza B viruses. Four thousand eight hundred seventy-five (40.7%) of the 11,989 influenza A viruses have been subtyped: 4,629 (95.0%) were influenza A (H3N2) viruses and 246 (5.0%) were influenza A (H1N1) viruses. During the past 3 weeks (weeks 1012), the percentage of specimens testing positive for influenza has ranged from 30.6% and 28.6% in the South Atlantic and East South Central regions, respectively, to 9.7% in the Pacific region**. During this time period the proportion of isolates identified as influenza type B virus continued to increase. During weeks 10-12, the Mountain region reported the highest proportion of isolates as influenza B (60.3%). 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 454 influenza viruses [379 influenza A (H3N2), 23 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 were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 78 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 21 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. Twenty-four 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 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 12, 7.7% 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 8.2% for week 12.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

One influenza-associated pediatric death was reported during week 12. Since October 2, 2005, CDC has received reports of 19 influenza-associated pediatric deaths, 16 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 March 18, 2006, the preliminary influenza-associated hospitalization rate reported by the 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 the 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 12, 2.5%*** 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 3.5% 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 12, 16 states (Arkansas, Connecticut, Delaware, Indiana, Kentucky, Maine, Maryland, Massachusetts, New York, North Dakota, Ohio, Rhode Island, South Carolina, Vermont, Virginia, and West Virigina) reported widespread influenza activity. Sixteen states (Georgia, Hawaii, Illinois, Iowa, Michigan, Minnesota, Montana, Nebraska, New Hampshire, New Jersey, North Carolina, Pennsylvania, South Dakota, Tennessee, Texas, and Wisconsin) reported regional influenza activity. Thirteen states (Alabama, Alaska, California, Colorado, Florida, Idaho, Kansas, Missouri, Nevada, Oklahoma, Oregon, Washington, and Wyoming), New York City, and the District of Columbia reported local influenza activity. Five states (Arizona, Louisiana, Mississippi, New Mexico, and Utah) and Puerto Rico reported sporadic influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared March 31, 2006

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