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Weekly Report: Influenza Summary Update

Week ending March 19, 2005-Week 11

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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 11 (March 13-19, 2005)*. Six hundred thirty-nine (17.0%) 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 4 weeks. The proportion of deaths attributed to pneumonia and influenza has declined and is equal to the epidemic threshold. There have been 21 influenza-associated pediatric deaths reported to CDC this season. Nine states reported widespread influenza activity and 20 states reported regional influenza activity. Seventeen states and New York City reported local activity and four states and the District of Columbia reported sporadic activity.

Laboratory Surveillance*:

During week 11, WHO and NREVSS laboratories in the United States reported testing 3,763 specimens for influenza viruses, of which 639 (17.0%) were positive. Of these, 1 was an influenza A (H1) virus, 33 were influenza A (H3N2) viruses, 359 were influenza A viruses that were not subtyped, and 246 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 115,125 specimens for influenza viruses and 18,983 (16.5%) were positive. Among the 18,983 influenza viruses, 15,168 (80.0%) were influenza A viruses and 3,815 (20.0%) were influenza B viruses. Four thousand eight hundred ninety (32.2%) of the 15,168 influenza A viruses have been subtyped; 4,876 (99.7%) were influenza A (H3N2) and 14 (0.3%) were influenza A (H1) viruses. The percentage of specimens testing positive for influenza during the last three weeks (weeks 9-11) has ranged from 9.5% in the Pacific region to 31.7% 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 9–11, 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 (59.0%). Other regions reporting more than 30.0% of recent isolates as influenza B include the Mid-Atlantic, East North Central, Mountain, and Pacific regions.

INFLUENZA Virus Isolated


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Antigenic Characterization:

CDC has antigenically characterized 618 influenza viruses collected by U.S. laboratories since October 1, 2004: 6 influenza A(H1) viruses, 399 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 (38%) of the 399 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. Two hundred forty-nine (62%) 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 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 11, 8.1% of all deaths reported by the vital statistics offices of 122 U.S. cities were attributed to pneumonia or influenza. This percentage is equal to the epidemic threshold of 8.1% for week 11.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

During week 11, six pediatric deaths were reported to CDC. Twenty-one 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 5, 2005, the preliminary influenza-associated hospitalization rate for children 0–4 years old reported by NVSN and EIP was 4.7 and 1.5 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 0.2 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 0.7 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 11, 3.0%†† 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 each week since week 7. Regional percentages ranged from 1.1% in the New England region to 6.0% in the East 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 11, 9 states (Delaware, Kentucky, Maryland, Massachusetts, Mississippi, North Carolina, South Dakota, Virginia, and West Virginia) reported widespread activity. Twenty states (Alaska, Connecticut, Florida, Georgia, Hawaii, Indiana, Kansas, Minnesota, Montana, Nevada, New Jersey, New York, Ohio, Oregon, Pennsylvania, South Carolina, Tennessee, Texas, Utah, and Washington) reported regional influenza activity. Seventeen states (Alabama, Arizona, California, Colorado, Idaho, Illinois, Iowa, Louisiana, Maine, Michigan, Missouri, Nebraska, North Dakota, Oklahoma, Vermont, Wisconsin, and Wyoming) and New York City reported local activity. Four states (Arkansas, New Hampshire, New Mexico, and Rhode Island) and the District of Columbia reported sporadic influenza activity.

U. S. map for Weekly Influenza Activity
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Foot notes

Report prepared: March 24, 2005

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