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Weekly Report: Influenza Summary Update
Week ending December 25, 2004-Week 51
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Synopsis:
During week 51 (December 19-25, 2004)*, influenza activity continued to increase mostly in the eastern United States. One hundred thirty-four (10.0%) 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. Fifty-six percent of all isolates this season have been reported from the Mid-Atlantic, New England, and Pacific surveillance regions**. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) is slightly below baseline. The proportion of deaths attributed to pneumonia and influenza is below the national baseline. Delaware, New York State, and New York City reported widespread influenza activity, 6 states reported regional influenza activity, and 16 states reported local activity. Twenty-six states, the District of Columbia, and Puerto Rico reported sporadic influenza activity.
Laboratory Surveillance*:
During week 51, WHO and NREVSS laboratories in the United States reported testing 1,340 specimens for influenza viruses, of which 134 (10.0%) were positive. Of these, one was an influenza A (H1N1), 27 were influenza A (H3N2) viruses, 85 were influenza A viruses that were not subtyped, and 21 were influenza B viruses.
Since October 3, WHO and NREVSS laboratories in the United States have tested a total of 29,134 specimens for influenza viruses and 773 (2.7%) were positive. Among the 773 influenza viruses, 589 (76.2%) were influenza A viruses and 184 (23.8%) were influenza B viruses. Two hundred fifty (42.4%) of the 589 influenza A viruses have been subtyped; 248 (99.2%) were influenza A (H3N2) and 2 (0.8%) were influenza A (H1N1) viruses. One hundred sixty-six (21.5%) of the 773 isolates have been reported by the Mid-Atlantic region**, 149 (19.3%) were from the New England region, and 115 (14.9%) were from the Pacific region.
Antigenic Characterization:
CDC has antigenically characterized 60 influenza viruses collected by U.S. laboratories since October 1, 2004: forty-two influenza A (H3N2) viruses and 18 influenza B viruses. All of the influenza A (H3N2) isolates were characterized as A/Fujian/411/2002-like (H3N2), which is the influenza A (H3N2) component recommended for the 2004-05 influenza vaccine.
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. Sixteen 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. Two influenza B viruses belong to the B/Victoria lineage and were characterized as B/Hong Kong/330/2001-like.
Pneumonia and Influenza (P&I) Mortality Surveillance*:
During week 51, 7.0% 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 7.8% for week 51.
Influenza-Associated Pediatric Mortality*:
As of week 51, no pediatric deaths associated with laboratory-confirmed influenza have been reported to CDC.
Influenza-Associated Pediatric Hospitalizations:
The New Vaccine Surveillance Network consists of three sites in Cincinnati OH, Nashville TN, and Rochester NY. The sites conduct population-based surveillance for laboratory-confirmed influenza among children 0-4 years of age who are admitted to the hospital with fever or acute respiratory illnesses. During October 3-December 11, 2004, there were no laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old. Three hospitalizations previously reported as being associated with influenza have since been reclassified as non-influenza hospitalizations after laboratory results were shown to be false-positives. Hospitalization rate estimates are preliminary and may change as data continue to be collected. In years 2000-2003, the end-of-season hospitalization rate ranged from 3.7 to 12 per 10,000 children.
The Emerging Infections Program (EIP) is collecting data in 10 sites on hospitalizations of children <18 years of age with illnesses associated with laboratory-confirmed influenza. Hospitalization data from the EIP sites will be included as they become available.
Influenza-like Illness Surveillance*:
During week 51, 2.4%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is slightly below the national baseline of 2.5%. On a regional level**, the percentage of visits for ILI ranged from 1.3% in the Mountain region to 4.0% 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.
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Influenza Activity as Assessed by State and Territorial Epidemiologists*:
During week 51, Delaware, New York State, and New York City reported widespread activity. Connecticut, Florida, Kentucky, Maryland, Massachusetts, and Minnesota reported regional influenza activity. Sixteen states (Alaska, Colorado, Iowa, Kansas, Maine, Michigan, New Hampshire, New Jersey, North Carolina, North Dakota, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, and Virginia) reported local activity. Twenty-six states (Alabama, Arizona, Arkansas, California, Georgia, Hawaii, Idaho, Illinois, Indiana, Louisiana, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, Ohio, Oklahoma, Oregon, South Dakota, Tennessee, Utah, Washington, West Virginia, Wisconsin, and Wyoming), the District of Columbia, and Puerto Rico reported sporadic influenza activity.
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* Reporting is incomplete for this week. Numbers may change as more reports are received.
** Surveillance Regions: New England (Connecticut, Maine, Massachusetts, New Hampshire, Vermont, Rhode Island); Mid-Atlantic (New Jersey, New York City, Pennsylvania, Upstate New York); East North Central (Illinois, Indiana, Michigan, Ohio, Wisconsin); West North Central (Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota); South Atlantic (Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, Washington, D.C., West Virginia); East South Central (Alabama, Kentucky, Mississippi, Tennessee); West South Central (Arkansas, Louisiana, Oklahoma, Texas); Mountain (Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, Wyoming); Pacific (Alaska, California, Hawaii, Oregon, Washington)
*** The national and regional percentage of patient visits for ILI is weighted on the basis of state population.
Report prepared: December 29, 2004
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