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Weekly Report: Influenza Summary Update

Week ending November 19, 2005-Week 46

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Synopsis:

During week 46 (November 13 November 19, 2005)*, influenza activity occurred at a low level in the United States. Eighteen (1.1%) 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) and the proportion of deaths attributed to pneumonia and influenza were below baseline levels. Twenty states, New York City, and the District of Columbia reported sporadic influenza activity, and 30 states reported no influenza activity.

Laboratory Surveillance*:

During week 46, WHO and NREVSS laboratories reported 1,708 specimens tested for influenza viruses and 18 (1.1%) were positive. Of these, 5 were influenza A (H3N2) viruses, 11 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 13,965 specimens for influenza viruses and 82 (0.6%) were positive. Among the 82 influenza viruses, 68 (82.9%) were influenza A viruses and 14 (17.1%) were influenza B viruses. Thirty-one (45.6%) of the 68 influenza A viruses have been subtyped and all were influenza A (H3N2) viruses. Twenty-four states from 7 of the 9 surveillance regions** have reported laboratory-confirmed influenza this season.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 2 influenza viruses collected by U.S. laboratories since October 1, 2005: 1 influenza A (H3N2) virus and 1 influenza B virus. The influenza A (H3N2) virus was characterized as A/California/07/2004-like, which is the influenza A (H3N2) component recommended for the 2005-06 influenza vaccine. The influenza B virus was identified as belonging to the B/Victoria/2/87 lineage. 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 recommended B component for the 2005-06 influenza vaccine is from the B/Yamagata lineage.

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

During week 46, 6.6% 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 7.3% for week 46.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

One influenza-associated pediatric death was reported for week 46.

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 November 12, 2005 there were no reported cases of laboratory-confirmed influenza-associated pediatric hospitalizations from the EIP. During October 30 November 12, 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.

In years 20002005, the end-of-season hospitalization rate for NVSN ranged from 3.7 (2002-03) to 12 (2003-04) per 10,000 children. The 2003-04 end-of-season hospitalization rate for EIP was 8.9 per 10,000 children aged 0-4 years and 0.8 per 10,000 for children aged 5-17 years. The 2004-05 NVSN end-of-season hospitalization rate for children aged 0-4 years was 7 per 10,000. The preliminary 2004-05 end-of-season hospitalization rate for EIP was 3.3 per 10,000 children aged 0-4 years and 0.6 per 10,000 for children aged 5-17 years. The difference in rates between NVSN and EIP may be due to different case-finding methods and the different populations monitored. For a summary of the methods used in each system, please refer to the surveillance methods in the Flu Activity section of the CDC influenza website.

Influenza-like Illness Surveillance*:

During week 46, 1.5%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 2.2%. On a regional level**, the percentage of visits for ILI ranged from 0.4% to 2.2%. 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 46, 20 states (Arizona, California, Colorado, Connecticut, Florida, Hawaii, Indiana, Maryland, Michigan, Minnesota, Montana, Nevada, New York, Ohio, Pennsylvania, South Dakota, Texas, Utah, Washington, and Wyoming), New York City, and the District of Columbia reported sporadic influenza activity. Thirty states reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: November 25, 2005

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