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Weekly Report: Influenza Summary Update

Week ending January 25, 2003-Week 4

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The following information may be quoted:

Synopsis:

During week 4 (January 19-25, 2003)*, 148 (10.0%) 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 2.6%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.3%. Three state and territorial health departments reported widespread influenza activity, 14 reported regional activity, 32 reported sporadic activity, and 1 reported no influenza activity**.

Laboratory Surveillance*:

During week 4, WHO and NREVSS laboratories reported 1,475 specimens tested for influenza viruses, of which 148 (10.0%) were positive. Twenty-one influenza A (H1)† viruses, three influenza A (H3N2) viruses, 59 unsubtyped influenza A viruses, and 65 influenza B viruses were identified. During the past 3 weeks (weeks 1-3), 28.4% of the specimens tested for influenza in the New England region*** were positive. Between 12.3% and 18.1% of specimens tested for influenza during the past 3 weeks in the West North Central, East North Central, and West South Central regions were positive, and between 1.9% and 6.8% of the specimens tested for influenza during the past 3 weeks in the Pacific, East South Central, Mountain, South Atlantic, and Mid-Atlantic regions were positive.

Since September 29, WHO and NREVSS laboratories have tested a total of 33,901 specimens for influenza viruses and 1,195 (3.5%) were positive. Of the 1,195 viruses identified, 375 (31%) were influenza A viruses and 820 (69%) were influenza B viruses. One hundred and thirty-six (36%) of the 375 influenza A viruses have been subtyped; 116 (85%) were influenza A (H1)† viruses and 20 (15%) were influenza A (H3N2) viruses. Five hundred and seventy-six (70%) of the 820 influenza B viruses were identified in Texas and Missouri. Forty-four states have reported laboratory-confirmed influenza. Influenza A and influenza B viruses have been identified in 29 states (AL, AZ, CA, CO, DE, FL, IA, IL, IN, LA, MA, MN, MO, MT, NC, NE, NH, NM, NY, OK, PA, SC, SD, TX, VA, VT, WA, WI, and WV). Ten states (CT, HI, ID, MI, NJ, OH, OR, TN, UT, and WY) have reported only influenza A viruses and 5 states (AK, AR, KS, KY, and NV) have reported only influenza B viruses. Influenza A viruses were reported more frequently than influenza B viruses (range 53% - 90%) in the South Atlantic, Mountain, New England, East North Central, Pacific, and Mid-Atlantic regions, and influenza B viruses were reported more frequently than influenza A viruses (range 75% - 92%) in the East South Central, West South Central, and West North Central regions.

 

Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 77 influenza viruses submitted by U.S. laboratories since September 29: forty-seven influenza B viruses, 18 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 4, 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 8.2% for week 4.

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Influenza-like Illness Surveillance *:

During week 4, 2.6% of patient visits to U.S. sentinel providers were due to ILI. This percentage is above the national baseline of 1.9%. On a regional level***, the percentage of visits for ILI ranged from 1.3% to 6.6%. 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 widespread in Missouri, Texas, and Virginia, and regional in 14 states. Thirty-two states, New York City, and Washington, D.C. reported sporadic activity, and 1 state reported no influenza activity.

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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 30, 2003

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