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Weekly Report: Influenza Summary Update

Week ending January 28, 2006-Week 4

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Synopsis:

During week 4 (January 22 January 28, 2006)*, influenza activity continued approximately at the same level as recent weeks in the United States. Three hundred forty-three specimens (12.0%) 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. Five states reported widespread influenza activity; 21 states and New York City reported regional influenza activity; 13 states reported local influenza activity; and 11 states, the District of Columbia, and Puerto Rico reported sporadic influenza activity.

Laboratory Surveillance*:

During week 4, WHO and NREVSS laboratories reported 2,854 specimens tested for influenza viruses and 343 (12.0%) were positive. Of these, 117 were influenza A (H3N2) viruses, 2 were influenza A (H1N1) viruses, 212 were influenza A viruses that were not subtyped, and 12 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested a total of 56,596 specimens for influenza viruses and 3,771 (6.7%) were positive. Among the 3,771 influenza viruses, 3,654 (96.9%) were influenza A viruses and 117 (3.1%) were influenza B viruses. One thousand eight hundred and two (49.3%) of the 3,654 influenza A viruses have been subtyped: 1,787 (99.2%) were influenza A (H3N2) viruses and 15 (0.8%) were influenza A (H1N1) viruses. Forty-nine states from all surveillance regions** have reported laboratory-confirmed influenza this season. During the past 3 weeks (weeks 2 4), the largest number of isolates have been reported from the Mountain and West South Central regions. During this time, the percentage of specimens testing positive for influenza has ranged from 20.9% and 20.6% in the East North Central and West South Central regions, respectively, to 4.9% in the East South Central region.

INFLUENZA Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 166 influenza viruses [149 influenza A (H3N2), 6 influenza A (H1), and 11 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 149 influenza A (H3N2) viruses, 123 were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 26 showed reduced titers with antisera produced against A/California/07/2004. The hemagglutinin proteins of 4 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. 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 characterized 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 4, 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 8.2% for week 4.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

Two influenza-associated pediatric deaths were reported during week 4. Since October 2, 2005, CDC has received reports of 13 influenza-associated pediatric deaths, 11 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 21, 2006, the preliminary influenza-associated hospitalization rate reported by the EIP for children aged 0-17 years was 0.24 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 0.66 per 10,000 and 0.04 per 10,000, respectively. During October 30, 2005 January 21, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0-4 years in the NVSN was 0.21 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 4, 2.4%*** 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 1.4% in the New England region to 5.9% 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 4, 5 states (Colorado, Connecticut, Kansas, Texas, and Wyoming) reported widespread influenza activity. Twenty-one states (Alaska, Arizona, Arkansas, California, Georgia, Florida, Iowa, Minnesota, Mississippi, Nebraska, New Mexico, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia, and Wisconsin) and New York City reported regional influenza activity. Thirteen states (Idaho, Illinois, Indiana, Maine, Massachusetts, Michigan, Missouri, Montana, Nevada, North Dakota, Ohio, South Dakota, and Washington) reported local influenza activity. Eleven states (Alabama, Delaware, Hawaii, Kentucky, Louisiana, Maryland, New Hampshire, New Jersey, Utah, Vermont, and West Virginia), the District of Columbia, and Puerto Rico reported sporadic influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared February 3, 2006

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