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Weekly Report: Influenza Summary Update

Week ending March 26, 2005-Week 12

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The following information may be quoted:

Synopsis:

Influenza activity in the United States appears to have peaked during February and continued to decline during week 12 (March 20-26, 2005)*. Two hundred thirty-three (13.1%) 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 slightly above the national baseline but has declined during the last 5 weeks. The proportion of deaths attributed to pneumonia and influenza is above the epidemic threshold. There have been 24 influenza-associated pediatric deaths reported to CDC this season. Four states reported widespread influenza activity and 15 states reported regional influenza activity. Twenty states, the District of Columbia, and New York City reported local activity and 10 states reported sporadic activity.

Laboratory Surveillance*:

During week 12, WHO and NREVSS laboratories in the United States reported testing 1,774 specimens for influenza viruses, of which 233 (13.1%) were positive. Of these, 30 were influenza A(H3N2) viruses, 56 were influenza A viruses that were not subtyped, and 147 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 121,373 specimens for influenza viruses and 20,135 (16.6%) were positive. Among the 20,135 influenza viruses, 15,932 (79.1%) were influenza A viruses and 4,203 (20.9%) were influenza B viruses. Five thousand eighty-three (31.9%) of the 15,932 influenza A viruses have been subtyped: 5,070 (99.7%) were influenza A(H3N2) and 13 (0.3%) were influenza A(H1) viruses. The percentage of specimens testing positive for influenza during the last three weeks (weeks 10-12) has ranged from 7.5% in the Pacific region to 28.5% in the South Atlantic region**. The proportion of influenza isolates identified as influenza type B viruses continued to increase in the United States. During weeks 10-12, the largest number of influenza B isolates was reported from the South Atlantic region, but the region with the highest proportion of isolates being reported as influenza type B was the New England region (72.3%). More than 60.0% of recent isolates were influenza B in the Mid-Atlantic, East North Central, and Pacific regions.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 638 influenza viruses collected by U.S. laboratories since October 1, 2004: 6 influenza A(H1) viruses, 419 influenza A(H3N2) viruses, and 213 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-one (36%) of the 419 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 268 (64%) were characterized as A/California/7/2004 (H3N2)-like. One hundred thirty-nine 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 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 12, 8.6% 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 8.1% for week 12.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 12, two pediatric deaths were reported to CDC. Twenty-four pediatric deaths have been reported to CDC from 12 states (California, Colorado, Georgia, Iowa, Maine, Massachusetts, Mississippi, New Jersey, New York, Ohio, Pennsylvania, and Vermont) since January.

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 12, 2.6%†† of patient visits to U.S. sentinel providers were due to ILI. This percentage is slightly above the national baseline of 2.5% and has declined each week since week 7. Regional percentages ranged from 0.8% in the New England region to 3.1% in the Mid-Atlantic and Pacific regions. 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 12, 4 states (Delaware, Kentucky, Massachusetts, and Virginia) reported widespread activity. Fifteen states (Alaska, Connecticut, Indiana, Maryland, New Jersey, New York, North Carolina, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Utah, Washington, and West Virginia) reported regional influenza activity. Twenty states (Alabama, Arizona, California, Colorado, Florida, Georgia, Hawaii, Idaho, Kansas, Louisiana, Maine, Michigan, Minnesota, Nevada, North Dakota, Ohio, Oklahoma, Texas, Wisconsin, and Wyoming), New York City, and the District of Columbia reported local activity. Ten states (Arkansas, Illinois, Iowa, Mississippi, Missouri, Nebraska, New Hampshire, New Mexico, Rhode Island, and Vermont) and Puerto Rico reported sporadic activity. Montana did not report.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: March 31, 2005

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