WEEKLY INFLUENZA SURVEILLANCE REPORT

WEEKLY INFLUENZA SURVEILLANCE REPORT

Week ending March 6, 2004—Week 9

Synopsis: Influenza activity remained low during the week of February 29-March 6, 2004. The percentage of patient visits for influenza-like illness (ILI) remained below the national baseline (2.5%). During week 9, mortality due to pneumonia and influenza (P&I) remained below the epidemic threshold (8.3%). Five (0.7%) of 713 specimens collected from throughout the United States and tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. There were no reports of widespread or regional influenza activity from state and territorial epidemiologists during week 9. Two states reported local activity, 31 states, New York City, Guam, and Puerto Rico reported sporadic activity, and 17 states and the District of Columbia reported no influenza activity.

Laboratory Surveillance*: During week 9, WHO and NREVSS laboratories reported 713 specimens tested for influenza viruses, and 5 (0.7%) were positive. Of these, 2 were influenza A (H3N2) viruses, 1 was an influenza A virus that was not subtyped, and 2 were influenza B viruses.

Since September 28, 2003, WHO and NREVSS laboratories have tested 107,687 specimens for influenza viruses, of which 23,644 (22.0%) were positive. Of these, 23,481 (99.3%) were influenza A viruses, and 163 (0.7%) were influenza B viruses. Of the 23,481 influenza A viruses, 6,462 (27.5%) have been subtyped; 6,460 (99.9%) were influenza A (H3N2) viruses and 2 (0.1%) were influenza A (H1) viruses.

Antigenic Characterization: CDC has antigenically characterized 668 influenza viruses collected by U.S. laboratories since October 1, 2003: two influenza A (H1) viruses, 648 influenza A (H3N2) viruses, and 18 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. Of the 648 influenza A (H3N2) isolates that have been characterized, 106 (16.4%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 542 (83.6%) were similar to the drift variant, A/Fujian/411/2002 (H3N2). Sixteen influenza B viruses characterized were similar antigenically to B/Sichuan/379/99 and 2 were similar antigenically to B/Hong Kong/330/2001.

Pneumonia and Influenza Mortality Surveillance: During week 9, 7.2% of all deaths reported by the vital statistics offices of 122 U.S. cities were due to pneumonia and influenza. This percentage is below the epidemic threshold of 8.3% for week 9.

Influenza-like Illness Surveillance*: The percentage of patient visits to approximately 1,000 U.S. sentinel providers nationwide for ILI was 1.1%** during week 9, which is below the national baseline of 2.5%. The percentage of patient visits for ILI in each of the nine surveillance regions*** was below 2.0%. Due to wide variability in regional level data, it is not appropriate to apply the national baseline to regional level data.

Influenza Activity as Assessed by State and Territorial Epidemiologists*: There were no reports of widespread or regional activity for week 9. Local activity was reported in Mississippi and West Virginia. Sporadic activity was reported in 31 states (Alaska, Arizona, California, Colorado, Connecticut, Florida, Hawaii, Idaho, Kansas, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Tennessee, Texas, Utah, Vermont, Virginia, and Wisconsin), New York City, Guam, and Puerto Rico. No influenza activity was reported in 17 states (Alabama, Arkansas, Delaware, Georgia, Illinois, Indiana, Iowa, Kentucky, New Hampshire, Oklahoma, Oregon,

Pennsylvania, Rhode Island, South Carolina, South Dakota, Washington, Wyoming), and the District of Columbia.

* Reporting is incomplete for this week. Numbers may change as more reports are received.

** The national and regional percentage of patient visits for ILI is weighted on the basis of state population.

*** 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).

Report prepared: March 11, 2004