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Weekly Report: Influenza Summary Update

Week ending December 4, 2004-Week 48

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The following information may be quoted:

Synopsis:

During week 48 (November 28-December 4, 2004)*, influenza activity overall was low in the United States. Forty-nine (2.7%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) and the proportion of deaths attributed to pneumonia and influenza were below national baseline values. Two states and New York City reported regional influenza activity and 4 states reported local activity. Thirty-six states, the District of Columbia, and Puerto Rico reported sporadic influenza activity and 8 states reported no influenza activity.

Laboratory Surveillance*:

During week 48, WHO and NREVSS laboratories reported testing 1,845 specimens for influenza viruses, of which 49 (2.7%) were positive. Of these, 16 were influenza A (H3N2) viruses, 22 were influenza A viruses that were not subtyped, and 11 were influenza B viruses.

Since October 3, WHO and NREVSS laboratories have tested a total of 19,825 specimens for influenza viruses and 295 (1.5%) were positive. Among the 295 influenza viruses, 200 (67.8%) were influenza A viruses and 95 (32.2%) were influenza B viruses. Ninety-three (46.5%) of the 200 influenza A viruses have been subtyped and all were influenza A (H3N2) viruses. Thirty-two states from all surveillance regions** have reported laboratory-confirmed influenza this season.

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Antigenic Characterization:

CDC has antigenically characterized 36 influenza viruses collected by U.S. laboratories since October 1, 2004: twenty-six influenza A (H3N2) viruses and 10 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. All influenza B viruses characterized were B/Shanghai/361/2002-like, which is the influenza B component recommended for the 2004-05 influenza vaccine.

Pneumonia and Influenza (P&I) Mortality Surveillance*:

During week 48, 5.9% of all deaths reported by the vital statistics offices of 122 U.S. cities were reported as due to pneumonia or influenza. This percentage is below the epidemic threshold of 7.5% for week 48.

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Influenza-Associated Pediatric Mortality*:

As of week 48, no influenza-associated pediatric deaths 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-November 27, 2004, the preliminary influenza hospitalization rate for children 0-4 years old was 0.4 per 10,000 children based on 3 laboratory confirmed inpatient cases. In years 2000-2003, the end-of-season hospitalization rate ranged from 3.7 to 12 per 10,000 children. Hospitalization rate estimates are preliminary and may change as data continue to be collected.

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The Emerging Infections Program (EIP) is collecting data in 9 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 48, 1.7%*** of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 2.5%. On a regional level**, the percentage of visits for ILI ranged from 0.7% in the New England region to 3.0% 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*:

During week 48, Alaska, New York, and New York City reported regional influenza activity. Four states (Kentucky, Minnesota, South Carolina, and Virginia) reported local activity. Thirty-six states (Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Illinois, Indiana, Kansas, Louisiana, Maine, Maryland, Massachusetts, Michigan, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, South Dakota, Tennessee, Texas, Utah, Vermont, Washington, West Virginia, and Wisconsin), the District of Columbia, and Puerto Rico reported sporadic influenza activity. Eight states reported no 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 9, 2004

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