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Weekly Report: Influenza Summary Update

Week ending January 14, 2006-Week 2

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Synopsis:

During week 2 (January 8 – January 14, 2006)*, influenza activity continued approximately at the same level as recent weeks in the United States. Two hundred thirty-eight (11.8%) specimens 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) and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. Eight states reported widespread influenza activity; 14 states and New York City reported regional influenza activity; 11 states reported local influenza activity; 16 states, the District of Columbia, and Puerto Rico reported sporadic influenza activity; and 1 state reported no influenza activity.

Laboratory Surveillance*:

During week 2, WHO and NREVSS laboratories reported 2,016 specimens tested for influenza viruses and 238 (11.8%) were positive. Of these, 105 were influenza A (H3N2) viruses, 2 were influenza A(H1N1) viruses, 125 were influenza A viruses that were not subtyped, and 6 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 43,434 specimens for influenza viruses and 2,092 (4.8%) were positive. Among the 2,092 influenza viruses, 2,026 (96.8%) were influenza A viruses and 66 (3.2%) were influenza B viruses. One thousand eighty-two (53.4%) of the 2,026 influenza A viruses have been subtyped: 1,075 (99.4%) were influenza A (H3N2) viruses and 7 (0.6%) were influenza A (H1N1) viruses. Forty-six states from all surveillance regions** have reported laboratory-confirmed influenza this season. Seven hundred fifty-four (36.0%) of the 2,092 isolates have been reported by the Mountain region and 479 (22.9%) have been reported by the Pacific region.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 77 influenza viruses [65 influenza A (H3N2), 1 influenza A (H1), and 11 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 65 influenza A (H3N2) viruses, 54 were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 11 showed reduced titers with antisera produced against A/California/07/2004. The hemagglutinin protein of the influenza A (H1) virus was similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. 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. Eight of the influenza B viruses isolated belong to the B/Yamagata lineage. One was similar to B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine, and 7 were characterized as B/Florida/07/2004-like. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. Three influenza B viruses were identified as belonging to the B/Victoria lineage.

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

During week 2, 7.8% of all deaths reported by the vital statistics offices of 122 U.S. cities were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 8.1% for week 2.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Four influenza-associated pediatric deaths were reported during week 2. Since October 2, 2005, CDC has received reports of 10 influenza-associated pediatric deaths, 8 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 – January 7, 2006, the preliminary influenza-associated hospitalization rate reported by the EIP for children aged 0-17 years was 0.18 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 0.48 per 10,000 and 0.02 per 10,000, respectively. During October 30, 2005 – January 7, 2006, there were no laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old in the NVSN. EIP and NVSN hospitalization rate estimates are preliminary and may change as data continue to be collected.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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Influenza-like Illness Surveillance*:

During week 2, 2.1%*** 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 1.1% in the West North Central region to 4.6% 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 2, 8 states (Arizona, Colorado, Kansas, Nevada, New Mexico, New York, Texas, and Wyoming) reported widespread influenza activity. Fourteen states (Alaska, California, Connecticut, Georgia, Idaho, Iowa, Kentucky, Mississippi, Oregon, Pennsylvania, Rhode Island, Tennessee, Utah, and Virginia) and New York City reported regional influenza activity. Eleven states (Florida, Indiana, Massachusetts, Minnesota, Montana, Nebraska, North Dakota, Ohio, Oklahoma, Washington, and Wisconsin) reported local influenza activity. Sixteen states (Alabama, Arkansas, Delaware, Hawaii, Illinois, Louisiana, Maine, Maryland, Michigan, Missouri, New Hampshire, New Jersey, North Carolina, South Dakota, Vermont, and West Virginia), the District of Columbia, and Puerto Rico reported sporadic influenza activity. One state reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared January 20, 2006

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