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Weekly Report: Influenza Summary Update

Week ending November 12, 2005-Week 45

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Synopsis:

During week 45 (November 6 – November 12, 2005)*, influenza activity occurred at a low level in the United States. Nine (0.7%) 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-one states, New York City, the District of Columbia, and Puerto Rico reported sporadic influenza activity, and 29 states reported no influenza activity.

Laboratory Surveillance*:

During week 45, WHO and NREVSS laboratories reported 1,243 specimens tested for influenza viruses and 9 (0.7%) were positive. Of these, 2 were influenza A (H3N2) viruses, 6 were influenza A viruses that were not subtyped, and 1 was an influenza B virus.

Since October 2, WHO and NREVSS laboratories have tested a total of 10,328 specimens for influenza viruses and 58 (0.6%) were positive. Among the 58 influenza viruses, 49 (84.5%) were influenza A viruses and 9 (15.5%) were influenza B viruses. Twenty-six (53.1%) of the 49 influenza A viruses have been subtyped and all were influenza A (H3N2) viruses. Seventeen 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 45, 6.1% 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 7.2% for week 45.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 45.

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 – October 29, 2005 there were no reported cases of laboratory-confirmed influenza-associated pediatric hospitalizations from the EIP. NVSN’s estimated rates of hospitalization for influenza will be reported every 2 weeks beginning November 25, 2005.

In years 2000–2005, 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 45, 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 0.5% to 2.3%. 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 45, 21 states (Arizona, California, Colorado, Connecticut, Florida, Hawaii, Idaho, Indiana, Massachusetts, Michigan, Minnesota, Montana, Nevada, New York, North Carolina, North Dakota, Pennsylvania, South Dakota, Texas, Utah, and West Virginia), New York City, the District of Columbia, and Puerto Rico reported sporadic influenza activity. Twenty-nine states reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared November 18, 2005

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