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Weekly Report: Influenza Summary Update

Week ending November 26, 2005-Week 47

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Synopsis:

During week 47 (November 20 November 26, 2005)*, influenza activity occurred at a low level in the United States. Twenty-two (1.8%) 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. Twenty-three states, New York City, the District of Columbia, and Puerto Rico reported sporadic influenza activity, and 27 states reported no influenza activity.

Laboratory Surveillance*:

During week 47, WHO and NREVSS laboratories reported 1,204 specimens tested for influenza viruses and 22 (1.8%) were positive. Of these, 4 were influenza A (H3N2) viruses, 15 were influenza A viruses that were not subtyped, and 3 were influenza B viruses.

Since October 2, WHO and NREVSS laboratories have tested a total of 16,568 specimens for influenza viruses and 138 (0.8%) were positive. Among the 138 influenza viruses, 119 (86.2%) were influenza A viruses and 19 (13.8%) were influenza B viruses. Sixty (50.4%) of the 119 influenza A viruses have been subtyped; 59 (98.3%) were influenza A (H3N2) viruses and 1 (1.7%) was an influenza A (H1N1) virus. Fifty-eight (42.0%) of the 138 isolates have been reported by the Pacific region and 33 (23.9%) were from the Mountain region**.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 3 influenza viruses collected by U.S. laboratories since October 1, 2005: 1 influenza A (H3N2) virus and 2 influenza B viruses. 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. 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. One of the influenza B viruses isolated belonged to the B/Yamagata lineage and was characterized as B/Florida/07/2004-like. This is a minor antigenic variant of B/Shanghai/361/2002, the recommended influenza B component for the 2005-06 influenza vaccine. The other influenza B virus was identified as belonging to the B/Victoria lineage.

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

During week 47, 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 7.4% for week 47.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

No influenza-associated pediatric deaths were reported for week 47. One influenza-associated pediatric death has been reported to CDC since October 2, 2005.

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.

Influenza-like Illness Surveillance*:

During week 47, 1.7%*** 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 2.7%. 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 47, 23 states (Alaska, Arizona, California, Colorado, Connecticut, Florida, Hawaii, Idaho, Indiana, Michigan, Minnesota, Montana, Nebraska, New York, North Dakota, Ohio, Pennsylvania, Rhode Island, South Dakota, Texas, Utah, Washington, and West Virginia), New York City, the District of Columbia, and Puerto Rico reported sporadic influenza activity. Twenty-seven states reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared December 2, 2005

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