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Weekly Report: Influenza Summary Update

Week ending March 5, 2005-Week 9

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The following information may be quoted:

Synopsis:

During week 9 (February 27-March 5, 2005)*, influenza activity continued to decline in the United States. Eight hundred thirty-eight (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) remained above the national baseline but has declined during the last 2 weeks. The proportion of deaths attributed to pneumonia and influenza is above the epidemic threshold for the third consecutive week. There have been 15 influenza-associated pediatric deaths reported to CDC this season. Twenty-four states reported widespread influenza activity and 20 states reported regional influenza activity. Five states, the District of Columbia, and New York City reported local activity and Puerto Rico reported sporadic activity.

Laboratory Surveillance*:

During week 9, WHO and NREVSS laboratories in the United States reported testing 3,985 specimens for influenza viruses, of which 838 (21.0%) were positive. Of these, 73 were influenza A (H3N2) viruses, 541 were influenza A viruses that were not subtyped, and 224 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 99,198 specimens for influenza viruses and 15,642 (15.8%) were positive. Among the 15,642 influenza viruses, 12,855 (82.2%) were influenza A viruses and 2,787 (17.8%) were influenza B viruses. Four thousand one hundred seventy-six (32.5%) of the 12,855 influenza A viruses have been subtyped; 4,163 (99.7%) were influenza A (H3N2) and 13 (0.3%) were influenza A (H1) viruses. The percentage of specimens testing positive for influenza A during the last three weeks has ranged from 9.9% in the Pacific region to 33.1% in the South Atlantic region**. The percentage of influenza B isolates in the United States has increased during the last 3 weeks overall to 24.0% from 15.8% during the preceding 3 weeks (weeks 4-6). The largest numbers of influenza B isolates were reported from the South Atlantic region but the highest percentage positive for influenza B isolates was in the Pacific region (47.8%). Other regions reporting more than 35.0% positive for influenza B viruses include the New England, Mid-Atlantic, and Mountain regions.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 491 influenza viruses collected by U.S. laboratories since October 1, 2004: 4 influenza A (H1) viruses, 308 influenza A (H3N2) viruses, and 179 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-seven (44%) of the 308 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 seventy-one (56%) 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). One hundred twenty-eight 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 seven showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Forty-four 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 9, 8.9% 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 9.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 9, two pediatric deaths were reported to CDC. Fifteen pediatric deaths have been reported to CDC from 11 states (California, Colorado, Georgia, Maine, Massachusetts, Mississippi, New Jersey, New York, Ohio, Pennsylvania, and Vermont) since January; of these 15 deaths, nine have been reported in the last 3 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 9, 4.0%†† of patient visits to U.S. sentinel providers were due to ILI. This percentage remained above the national baseline of 2.5% but has declined since week 8 in the United States overall and in all regions except the Mid-Atlantic and West North Central regions. Regional percentages ranged from 1.2% in the New England region to 5.4% 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 9, 24 states (Alaska, Colorado, Connecticut, Delaware, Georgia, Idaho, Indiana, Kansas, Kentucky, Missouri, Nevada, New Jersey, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, South Dakota, Tennessee, Vermont, Virginia, Washington, West Virginia, and Wyoming) reported widespread activity. Twenty states (Arizona, Arkansas, California, Florida, Hawaii, Illinois, Iowa, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Montana, Nebraska, New York, North Dakota, Rhode Island, Texas, Utah, and Wisconsin) reported regional influenza activity. Five states (Alabama, Maine, New Hampshire, New Mexico, and South Carolina), the District of Columbia, and New York City reported local activity. Puerto Rico reported sporadic influenza activity. Louisiana did not report.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: March 10, 2005

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