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Weekly Report: Influenza Summary Update

Week ending April 23, 2005-Week 16

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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 16 (April 17-23, 2005)*. Forty-eight (3.6%) 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. The proportion of deaths attributed to pneumonia and influenza was slightly above the epidemic threshold. There have been 30 influenza-associated pediatric deaths reported to CDC this season. There were no reports of widespread influenza activity. Two states reported regional influenza activity, 6 states reported local activity, and 35 states, New York City, and Puerto Rico reported sporadic activity. Six states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*:

During week 16, WHO and NREVSS laboratories in the United States reported testing 1,320 specimens for influenza viruses, of which 48 (3.6%) were positive. Of these, 8 were influenza A(H3N2) viruses, 12 were influenza A viruses that were not subtyped, and 28 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 137,317 specimens for influenza viruses and 21,926 (16.0%) were positive. Among the 21,926 influenza viruses, 16,837 (76.8%) were influenza A viruses and 5,089 (23.2%) were influenza B viruses. Five thousand five hundred sixteen (32.8%) of the 16,837 influenza A viruses have been subtyped: 5,500 (99.7%) were influenza A(H3N2) and 16 (0.3%) were influenza A(H1) viruses. The percentage of specimens testing positive for influenza during the last 3 weeks (weeks 14-16) has ranged from 3.1% in the East North Central region to 10.2% in the South Atlantic region**. Nationally, influenza B viruses became more frequently reported than influenza A viruses during the week ending March 26 and have predominated each week since then.

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Antigenic Characterization:

CDC has antigenically characterized 772 influenza viruses collected by U.S. laboratories since October 1, 2004: 6 influenza A(H1) viruses, 510 influenza A(H3N2) viruses, and 256 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-four (30%) of the 510 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 356 (70%) 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 28 showed somewhat reduced titers to ferret antisera produced against B/Shanghai/361/2002. Fifty-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 16, 7.9% 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 7.8% for week 16.

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Influenza-Associated Pediatric Mortality*:

During week 16, one pediatric death was reported to CDC. Thirty 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-April.

Influenza-Associated Pediatric Hospitalizations:

Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks^{?/sup>: the Emerging Infections Program (EIP) and the New Vaccine Surveillance Network (NVSN). During October 3, 2004–April 16, 2005, the preliminary influenza-associated hospitalization rate for children 0? years old reported by NVSN and EIP was 7.1 and 2.5 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 0.4 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 1.1 per 10,000. In years 2000?004, 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 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.}

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Influenza-like Illness Surveillance*:

During week 16, 1.1%^†† 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 0.6% in the New England and West North Central regions to 2.4% 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.

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Influenza Activity as Assessed by State and Territorial Epidemiologists*:

There were no reports of widespread influenza activity during week 16. Kentucky and New York reported regional influenza activity. Six states (Arizona, Connecticut, Hawaii, New Jersey, Utah, and West Virginia) reported local activity. Thirty-five states (Alaska, California, Colorado, Delaware, Florida, Idaho, Iowa, Kansas, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, Washington, Wisconsin, and Wyoming), New York City, and Puerto Rico reported sporadic activity. Six states (Alabama, Arkansas, Georgia, Illinois, New Hampshire, and North Carolina) and the District of Columbia reported no influenza activity. Indiana did not report.

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Foot notes

Report prepared: April 28, 2005

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