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Weekly Report: Influenza Summary Update

Week ending February 26, 2005-Week 8

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The following information may be quoted:

Synopsis:

During week 8 (February 20-26, 2005)*, influenza activity has declined in the United States. One thousand twenty-five (21.0%) 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 seven consecutive weeks. The proportion of deaths attributed to pneumonia and influenza is above the epidemic threshold for the second consecutive week. There have been 13 influenza-associated pediatric deaths reported to CDC this season. Thirty states reported widespread influenza activity, and 16 states and New York City reported regional influenza activity. Three states and the District of Columbia reported local activity and Puerto Rico reported sporadic activity.

Laboratory Surveillance*:

During week 8, WHO and NREVSS laboratories in the United States reported testing 4,875 specimens for influenza viruses, of which 1,025 (21.0%) were positive. Of these, 142 were influenza A (H3N2) viruses, 631 were influenza A viruses that were not subtyped, and 252 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 91,968 specimens for influenza viruses and 13,759 (15.0%) were positive. Among the 13,759 influenza viruses, 11,415 (83.0%) were influenza A viruses and 2,344 (17.0%) were influenza B viruses. Three thousand five hundred fifty-four (31.1%) of the 11,415 influenza A viruses have been subtyped; 3,543 (99.7%) were influenza A (H3N2) and 11 (0.3%) were influenza A (H1) viruses. The percentage of specimens testing positive during the last three weeks has ranged from 9.1% in the Pacific region to 43.1% in the East North Central region**. The percentage of virus isolates that are influenza type A is the highest in the East South Central (90%), Mid-Atlantic (93%) and New England (92%) regions. The percentage of influenza A viruses ranged from 81% to 86% in the East North Central, West North Central, South Atlantic, and West South Central regions and is lowest in the Mountain (69%) and Pacific (60%) regions.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 378 influenza viruses collected by U.S. laboratories since October 1, 2004: 4 influenza A (H1) viruses, 284 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 thirty-four (47%) of the 284 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 fifty (53%) 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 5 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 8, 8.8% 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 8.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 8, four pediatric deaths were reported to CDC. Thirteen pediatric deaths have been reported to CDC from ten states (California, Georgia, Maine, Massachusetts, Mississippi, New Jersey, New York, Ohio, Pennsylvania, and Vermont) since January; seven of which have been reported in the last two weeks.

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 19, 2005, the preliminary influenza-associated hospitalization rate for children 0–4 years old reported by NVSN and EIP was 2.9 and 1.18 per 10,000, respectively. EIP also monitors hospitalizations in children aged 5-17 years and the preliminary influenza-associated hospitalization rate for this age group was 0.16 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 0.55 per 10,000. In years 2000–2004, 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 7.8 per 10,000 children 0-4 years and 0.8 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.

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 8, 4.6%†† of patient visits to U.S. sentinel providers were due to ILI. This percentage has been above the national baseline of 2.5% for seven consecutive weeks but declined during week 8 in the U.S. overall and in all regions except the Mountain region. The percentage of visits for ILI ranged from 1.4% in the New England region to 7.3% 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 8, 30 states (Alaska, Colorado, Connecticut, Delaware, Florida, Georgia, Idaho, Iowa, Kansas, Kentucky, Louisiana, Maryland, Michigan, Montana, Nebraska, Nevada, New Jersey, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, Utah, Vermont, Virginia, Washington, and West Virginia) reported widespread activity. Sixteen states (Arizona, Arkansas, California, Hawaii, Illinois, Indiana, Maine, Massachusetts, Minnesota, Mississippi, North Dakota, Rhode Island, South Carolina, South Dakota, Wisconsin, and Wyoming) and New York City reported regional influenza activity. Alabama, New Hampshire, New Mexico, and the District of Columbia reported local activity. Puerto Rico reported sporadic influenza activity. Missouri did not report.

U. S. map for Weekly Influenza Activity
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foot notes

Report prepared: March 3, 2005

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