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Weekly Report: Influenza Summary Update

Week ending December 31, 2005-Week 52

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Synopsis:

During week 52 (December 25 - December 31, 2005)*, influenza activity continued to increase in the United States. One hundred sixty-nine (10.1%) 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) was above the national baseline. The proportion of deaths attributed to pneumonia and influenza was below the baseline level. Seven states reported widespread influenza activity; 3 states reported regional influenza activity; 9 states and the District of Columbia reported local influenza activity; 27 states, and New York City reported sporadic influenza activity; and 2 states reported no influenza activity.

Laboratory Surveillance*:

During week 52, WHO and NREVSS laboratories reported 1,677 specimens tested for influenza viruses and 169 (10.1%) were positive. Of these, 117 were influenza A (H3N2) viruses, 2 were influenza A (H1N1) viruses, 48 were influenza A viruses that were not subtyped, and 2 were influenza B viruses.

Since October 2, WHO and NREVSS laboratories have tested a total of 35,006 specimens for influenza viruses and 1,203 (3.4%) were positive. Among the 1,203 influenza viruses, 1,153 (95.8%) were influenza A viruses and 50 (4.2%) were influenza B viruses. Six hundred eight (52.7%) of the 1,153 influenza A viruses have been subtyped: 602 (99.0%) were influenza A (H3N2) viruses and 6 (1.0%) were influenza A (H1N1) viruses. Forty-three states from all surveillance regions** have reported laboratory-confirmed influenza this season. Four hundred thirty-two (35.9%) of the 1,203 isolates have been reported by the Mountain region and 302 (25.1%) have been reported by the Pacific region.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 31 influenza viruses [23 influenza A (H3N2), 1 influenza A (H1), and 7 influenza B viruses] collected by U.S. laboratories since October 1, 2005. Of the 23 influenza A (H3N2) viruses, 21 were characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine, and 2 showed reduced titers with antisera produced against A/Califonia/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. Five 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 4 were characterized as B/Florida/07/2004-like. B/Florida/07/2004 is a minor antigenic variant of B/Shanghai/361/2002. Two influenza B viruses were identified as belonging to the B/Victoria lineage.

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

During week 52, 6.8% 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 7.9% for week 52.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 52. Since October 2, 2005, CDC has received reports of five influenza-associated pediatric deaths, three 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 - December 24, 2005, the preliminary influenza-associated hospitalization rate for children 0-4 years old reported by the EIP was 0.17 per 10,000. The EIP also monitors hospitalizations in children 5-17 years old. The preliminary influenza-associated hospitalization rate for this age group reported by EIP was 0.01 per 10,000. The overall hospitalization rate reported by the EIP for children aged 0-17 years was 0.06 per 10,000. During October 30 - December 24, 2005, 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 52, 3.3%*** 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 1.6% in the New England region to 6.7% 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 52, 7 states (Arizona, California, Colorado, Nevada, New Mexico, Texas, and Utah) reported widespread influenza activity. Idaho, Kansas, and Oregon reported regional influenza activity, and 9 states (Connecticut, Delaware, Mississippi, Montana, Nebraska, Ohio, Oklahoma, Pennsylvania, and Washington) and the District of Columbia reported local influenza activity. Twenty-seven states (Alabama, Alaska, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kentucky, Maine, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Rhode Island, South Carolina, South Dakota, Tennessee, Virginia, West Virginia, Wisconsin, and Wyoming), and New York City, reported sporadic influenza activity. Two states reported no influenza activity and 2 states did not report.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared January 6, 2005

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