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Weekly Report: Influenza Summary Update

Week ending February 19, 2005-Week 7

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The following information may be quoted:

Synopsis:

During week 7 (February 13-19, 2005)*, influenza activity continued to increase in the United States. Nine hundred ten (20.4%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza viruses. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) has been above the national baseline for 6 consecutive weeks. The proportion of deaths attributed to pneumonia and influenza is above the epidemic threshold. There have been nine influenza-associated pediatric deaths reported to CDC this season. Thirty-three states reported widespread influenza activity, 15 states reported regional influenza activity, and two states, New York City, and the District of Columbia reported local activity.

Laboratory Surveillance*:

During week 7, WHO and NREVSS laboratories in the United States reported testing 4,452 specimens for influenza viruses, of which 910 (20.4%) were positive. Of these, one was an influenza A (H1N1) virus, 128 were influenza A (H3N2) viruses, 594 were influenza A viruses that were not subtyped, and 187 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 83,753 specimens for influenza viruses and 11,547 (13.8%) were positive. Among the 11,547 influenza viruses, 9,773 (84.6%) were influenza A viruses and 1,774 (15.4%) were influenza B viruses. Three thousand one (30.7%) of the 9,773 influenza A viruses have been subtyped; 2,990 (99.6%) were influenza A (H3N2) and 11 (0.4%) were influenza A (H1N1) viruses. The percentage of specimens testing positive during the last 3 weeks has ranged from 8.9% in the Pacific region to 42.7% in the East North Central region**. The percentage of total isolates that are influenza type A virus is the highest in the Mid-Atlantic (95%) and New England (94%) regions. Influenza A viruses represent a smaller proportion of positive results in the other seven regions, ranging from 88% in the East South Central region to 60% in the Pacific region.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 320 influenza viruses collected by U.S. laboratories since October 1, 2004: 2 influenza A (H1) viruses, 228 influenza A (H3N2) viruses, and 90 influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. One hundred twenty-five (55%) of the 228 influenza A(H3N2) isolates were characterized as antigenically similar to A/Wyoming/3/2003, which is the A/Fujian/411/2002-like (H3N2) component of the 2004-05 influenza vaccine. One hundred three (45%) influenza A(H3N2) isolates had reduced titers to A/Wyoming/3/2003 and are most closely related to a recent reference strain, A/California/7/2004 (H3N2). Sixty-six of the influenza B viruses isolated this season belong to the B/Yamagata lineage and were characterized as B/Shanghai/361/2002-like, which is the influenza B component recommended for the 2004-05 influenza vaccine, and five showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Nineteen influenza B viruses belong to the B/Victoria lineage.

Influenza B viruses currently circulating can be divided into two antigenically and genetically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata lineage viruses circulated widely between 1990 and 2001, during which time B/Victoria-like viruses were not identified outside of Asia. However, between March 2001 and October 2003, B/Victoria-like viruses predominated in many countries, including the United States, and the vaccine strains were changed accordingly. While both Victoria lineage and Yamagata lineage viruses have been reported worldwide during the past year, Yamagata lineage viruses have once again become predominant.

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

During week 7, 8.5% of all deaths reported by the vital statistics offices of 122 U.S. cities were attributed to pneumonia or influenza. This percentage is above the epidemic threshold of 8.2% for week 7.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 7, three pediatric deaths were reported to CDC. Nine pediatric deaths have been reported to CDC from nine states (California, Georgia, Maine, Massachusetts, Mississippi, New Jersey, Ohio, Pennsylvania, and Vermont) during January and February 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 3, 2004–February 5, 2005, the preliminary influenza-associated hospitalization rate for children 0–4 years old reported by the NVSN and EIP was 2.0 and 0.81 per 10,000, respectively. The EIP also monitors hospitalizations in children 5-17 years, and the preliminary influenza-associated hospitalization rate for this age group was 0.11 per 10,000. The overall hospitalization rate reported by the EIP for children aged 0-17 years was 0.35 per 10,000. In years 2000–2004, the end-of-season hospitalization rate for the NVSN ranged from 3.7(2002-2003) to 12 (2003-2004) per 10,000 children. The 2003-04 end of season hospitalization rate for the EIP was 7.8 per 10,000 children 0-4 years and 0.8 per 10,000 for children 5-17 years. The difference in rates between the 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.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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EIP Influenza Laboratory-Confirmed Cumulative Hospitalization Rates for Children 0-4 years and 5-17 years, 2004-05 and 2003-04
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Influenza-like Illness Surveillance*:

During week 7, 5.7%†† of patient visits to U.S. sentinel providers were due to ILI. This percentage has been above the national baseline of 2.5% for 6 consecutive weeks. The percentage of visits for ILI ranged from 2.0% in the New England region to 9.3% 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 7, 33 states (Alaska, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia, Iowa, Kansas, Kentucky, Louisiana, Maryland, Michigan, Mississippi, Missouri, Montana, Nebraska, Nevada, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, Tennessee, Texas, Utah, Vermont, Virginia, Washington, and Wisconsin) reported widespread activity. Fifteen states (Arizona, California, Hawaii, Idaho, Illinois, Indiana, Maine, Minnesota, New Hampshire, New Mexico, North Dakota, South Carolina, South Dakota, West Virginia, and Wyoming) reported regional influenza activity. Alabama, Massachusetts, New York City, and the District of Columbia reported local activity.

U. S. map for Weekly Influenza Activity
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foot notes

Report prepared: February 24, 2005

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