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Weekly Report: Influenza Summary Update

Week ending April 2, 2005-Week 13

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The following information may be quoted:

Synopsis:

Influenza activity in the United States peaked in early February and continued to decline during week 13 (March 27-April 2, 2005)*. Two hundred thirty-nine (11.2%) 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) was below the national baseline for the first time in 12 weeks. The proportion of deaths attributed to pneumonia and influenza has been above the epidemic threshold for 7 consecutive weeks. There have been 26 influenza-associated pediatric deaths reported to CDC this season. One state reported widespread influenza activity and 10 states reported regional influenza activity. Twenty-two states reported local activity and 15 states, New York City, and the District of Columbia, and Puerto Rico reported sporadic activity. Alabama reported no influenza activity.

Laboratory Surveillance*:

During week 13, WHO and NREVSS laboratories in the United States reported testing 2,143 specimens for influenza viruses, of which 239 (11.2%) were positive. Of these, 12 were influenza A(H3N2) viruses, 114 were influenza A viruses that were not subtyped, and 113 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 126,119 specimens for influenza viruses and 20,816 (16.5%) were positive. Among the 20,816 influenza viruses, 16,350 (78.5%) were influenza A viruses and 4,466 (21.5%) were influenza B viruses. Five thousand two hundred thirty-three (32.0%) of the 16,350 influenza A viruses have been subtyped: 5,220 (99.8%) were influenza A(H3N2) and 13 (0.2%) were influenza A(H1) viruses. The percentage of specimens testing positive for influenza during the last 3 weeks (weeks 11-13) has ranged from 6.9% in the Pacific region to 27.6% in the South Atlantic region**. During weeks 11-13, the number of influenza B isolates reported has declined, but the proportion of influenza isolates identified as influenza type B viruses continued to increase. The proportions of isolates being reported as influenza type B ranged from 77.9% in the New England region to 21.2% in the East South Central region.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 721 influenza viruses collected by U.S. laboratories since October 1, 2004: 6 influenza A(H1) viruses, 493 influenza A(H3N2) viruses, and 222 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. Two hundred four (41%) of the 493 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, and 289 (59%) were characterized as A/California/7/2004 (H3N2)-like. One hundred forty-five 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 24 showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Fifty-three 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 13, 8.7% 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 8.0% for week 13.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 13, one pediatric death was reported to CDC. Twenty-six pediatric deaths have been reported to CDC from 13 states (California, Colorado, Florida, Georgia, Iowa, Maine, Massachusetts, Mississippi, New Jersey, New York, Ohio, Pennsylvania, and Vermont); all deaths were reported during January-March.

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–March 19, 2005, the preliminary influenza-associated hospitalization rate for children 0–4 years old reported by NVSN and EIP was 5.2 and 1.9 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 0.3 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 0.9 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 13, 2.1%†† of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline of 2.5% for the first time in 12 weeks. Regional percentages ranged from 1.0% in the New England region to 2.8% 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 13, Kentucky reported widespread activity. Ten states (Connecticut, Massachusetts, Nevada, New York, North Carolina, Pennsylvania, Tennessee, Utah, Virginia, and West Virginia) reported regional influenza activity. Twenty-two states (Alaska, Arizona, California, Colorado, Florida, Georgia, Hawaii, Idaho, Indiana, Louisiana, Maryland, Michigan, Minnesota, Montana, New Jersey, North Dakota, Ohio, South Carolina, South Dakota, Washington, Wisconsin, and Wyoming) reported local activity. Fifteen states (Arkansas, Illinois, Iowa, Kansas, Maine, Mississippi, Missouri, Nebraska, New Hampshire, New Mexico, Oklahoma, Oregon, Rhode Island, Texas, and Vermont), New York City, the District of Columbia, and Puerto Rico reported sporadic activity. Alabama reported no influenza activity. Delaware did not report.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: April 6, 2005

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