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Weekly Report: Influenza Summary Update

Week ending January 11, 2003-Week 2

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The following information may be quoted:

Synopsis:

During week 2 (January 5-11, 2003)*, 67 (5.5%) of the specimens tested by World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories were positive for influenza. The proportion of patient visits to sentinel providers for influenza-like illness (ILI) overall was 1.6%, which is less than the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.3%. Nine state and territorial health departments reported regional influenza activity, 33 reported sporadic activity, and 7 reported no influenza activity**.

Laboratory Surveillance*:

During week 2, WHO and NREVSS laboratories reported 1,213 specimens tested for influenza viruses, of which 67 (5.5%) were positive. One influenza A (H1)† virus, one influenza A (H3N2) virus, 23 unsubtyped influenza A viruses, and 42 influenza B viruses were identified. During the past 3 weeks (weeks 52-2), 23.0% of the specimens tested for influenza in the West South Central region*** were positive. Between 5.0% and 11.8% of specimens tested for influenza during the past 3 weeks in the West North Central and New England regions were positive. Less than 5% of the specimens tested for influenza during the past 3 weeks were positive in the Pacific, South Atlantic, East South Central, Mountain, East North Central, and Mid-Atlantic regions.

Since September 29, WHO and NREVSS laboratories have tested a total of 27,861 specimens for influenza viruses and 614 (2.2%) were positive. Of the 614 viruses identified, 164 (27%) were influenza A viruses and 450 (73%) were influenza B viruses. Fifty-three (32%) of the 164 influenza A viruses have been subtyped; 42 (80%) were influenza A (H1)† viruses and 11 (21%) were influenza A (H3N2) viruses. Two hundred and ninety-three (65%) of the 450 influenza B viruses were identified in Texas. Thirty-eight states have reported laboratory-confirmed influenza. Influenza A and influenza B viruses have been identified in 14 states (AZ, CA, CO, IN, LA, MO, NC, NE, NY, OK, SC, TX, VA, WI). Sixteen states (AL, CT, DE, FL, HI, IL, MA, MI, MN, NJ, OR, SD, TN, UT, VT, and WA) have reported only influenza A viruses and 8 states (AK, AR, KS, KY, MT, NH, NV, and WV) have reported only influenza B viruses. Influenza A viruses were reported more frequently than influenza B viruses (range 56% - 86%) in the East North Central, Mid-Atlantic, New England, Pacific, and South Atlantic regions, and influenza B viruses were reported more frequently than influenza A viruses (range 58% - 87%) in the East South Central, Mountain, West North Central, and West South Central regions.

Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 48 influenza viruses submitted by U.S. laboratories since September 29: thirty-two influenza B viruses, four influenza A (H3N2) viruses, and 12 influenza A (H1)† viruses. Eleven of the influenza A (H1) viruses had the N1 neuraminidase and one had the N2 neuraminidase. The influenza B viruses, the A (H3N2) viruses, and the hemagglutinin proteins of the A (H1N1) and A (H1N2) viruses were similar antigenically to those of the corresponding vaccine strains B/Hong Kong/330/01, A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively.

Click here for more information about influenza A (H1N2) viruses

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 2, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.3%. This percentage is below the epidemic threshold of 8.1% for week 2.

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Influenza-like Illness Surveillance *:

During week 2, 1.6% of patient visits to U.S. sentinel providers were due to ILI. This percentage is less than the national baseline of 1.9%. On a regional level***, the percentage of visits for ILI ranged from 0.5% to 3.3%. 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**:

Influenza activity was reported as regional in 9 states (Arkansas, Kansas, Missouri, Nebraska, New York, Oklahoma, Tennessee, Texas, and Virginia) and New York City. Thirty-three states reported sporadic activity, 7 states reported no influenza activity, and 1 state did not report.

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

** Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.

†Includes both the A (H1N1) and A (H1N2) influenza virus subtypes. The influenza A (H1N2) strain appears to have resulted from the reassortment of the genes of currently circulating influenza A (H1N1) and A (H3N2) subtypes. Because the hemagglutinin proteins of the A (H1N2) viruses are similar to those of the currently circulating A (H1N1) viruses and the neuraminidase proteins are similar to those of the currently circulating A (H3N2) viruses, the 2002-03 influenza vaccine should provide protection against A (H1N2) viruses.

*** 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 January 16, 2003

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