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Weekly Report: Influenza Summary Update

Week ending May 21, 2005-Week 20

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This is the final report of the 2004-05 season.

Synopsis:

Influenza activity in the United States peaked in early February and continued to decline during week 20 (May 15-21, 2005)*. Twenty-five (3.9%) 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 eighth consecutive week. The proportion of deaths attributed to pneumonia and influenza was below the epidemic threshold for the fourth consecutive week. There have been 34 influenza-associated pediatric deaths reported to CDC this season. For week 20, there were no reports of widespread, regional, or local influenza activity. Sixteen states reported sporadic influenza activity and 34 states, New York City and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 20, WHO and NREVSS laboratories in the United States reported testing 645 specimens for influenza viruses, of which 25 (3.9%) were positive. Of these, 17 were influenza A(H3N2) viruses, 2 were influenza A viruses that were not subtyped, and 6 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 149,131 specimens for influenza viruses and 23,023 (15.4%) were positive. Among the 23,023 influenza viruses, 17,432 (75.7%) were influenza A viruses and 5,591 (24.3%) were influenza B viruses. Five thousand seven hundred ninety-six (33.2%) of the 17,432 influenza A viruses have been subtyped: 5,780 (99.7%) were influenza A(H3N2) and 16 (0.3%) were influenza A(H1) viruses.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 920 influenza viruses collected by U.S. laboratories since October 1, 2004: 9 influenza A(H1) viruses, 646 influenza A(H3N2) viruses, and 265 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 fifty-six (24%) of the 646 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 490 (76%) were characterized as A/California/7/2004-like. One hundred seventy-four 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 33 showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Fifty-eight 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.

Composition of the 2005-06 Influenza Vaccine:

The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee has recommended that the 2005-06 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/California/7/2004-like (H3N2), and B/Shanghai/361/2002-like viruses. The influenza A (H3N2) component has been changed from the 2004-05 season vaccine component. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.

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

During week 20, 6.5% of all deaths reported by the vital statistics offices of 122 U.S. cities were attributed to pneumonia or influenza. This percentage is below the epidemic threshold of 7.4% for week 20.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

There was one pediatric death reported to CDC during week 20. For the entire season, 34 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) and New York City; all deaths were reported during January-May.

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-April 30, 2005, the preliminary influenza-associated hospitalization rate for children 0-4 years old reported by NVSN and EIP was 7.0 and 2.8 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 0.5 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 1.2 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 8.9 per 10,000 children aged 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 20, 1.0%¶¶ of patient visits to U.S. sentinel providers were due to ILI. This percentage is below the national baseline of 2.5%. Regional percentages ranged from 0.3% in the West North Central region to 1.6% in the Pacific 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*:

There were no reports of widespread, regional, or local influenza activity during week 20. Sixteen states (Alaska, Arizona, California, Delaware, Florida, Hawaii, Indiana, Maryland, Minnesota, Nevada, New Mexico, New York, Pennsylvania, Utah, Washington, and Wyoming) reported sporadic activity. Thirty-four states (Alabama, Arkansas, Colorado, Connecticut, Georgia, Idaho, Illinois, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Mississippi, Missouri, Montana, Nebraska, New Hampshire, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, West Virginia, and Wisconsin), New York City, and the District of Columbia reported no influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: May 26, 2005

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