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Weekly Report: Influenza Summary Update

Week ending February 22, 2003-Week 8

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The following information may be quoted:

Synopsis:

During week 8 (February 16-22, 2003)*, 361 (17.0%) of the 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) overall was 2.8%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.4%. Thirteen state and territorial health departments reported widespread influenza activity, 20 reported regional activity, and 16 reported sporadic influenza activity**.

Laboratory Surveillance*:

During week 8, U.S. WHO and NREVSS laboratories reported 2,119 specimens tested for influenza viruses, of which 361 (17.0%) were positive. Fifty-three influenza A (H1)† viruses, 40 influenza A (H3N2) viruses, 114 unsubtyped influenza A viruses, and 154 influenza B viruses were identified. During the past 3 weeks (weeks 6-8), between 34.7% and 38.0% of the specimens tested for influenza in the New England and East North Central regions*** were positive. Between 22.7% and 29.5% of specimens tested for influenza during the past 3 weeks in the Mountain, West South Central, and South Atlantic regions were positive, and between 14.4% and 17.7% of the specimens tested in the West North Central, Mid-Atlantic, and East South Central regions were positive for influenza. In the Pacific region, 5.7% of specimens tested for influenza during the past 3 weeks were positive.

Since September 29, WHO and NREVSS laboratories have tested a total of 53,480 specimens for influenza viruses and 4,875 (9.1%) were positive. Of the 4,875 viruses identified, 2,020 (41%) were influenza A viruses and 2,855 (59%) were influenza B viruses. Seven hundred and seventy-eight (39%) of the 2,020 influenza A viruses have been subtyped; 622 (80%) were influenza A (H1)† viruses and 156 (20%) were influenza A (H3N2) viruses. One thousand three hundred and thirteen (46%) of the 2,855 influenza B viruses were identified in Texas and Missouri. Laboratory-confirmed influenza has been reported in all 50 states. Influenza A viruses were reported more frequently than influenza B viruses (range 62% - 89%) in the East North Central, Mountain, Pacific, and Mid-Atlantic regions, and influenza B viruses were reported more frequently than influenza A viruses (range 66% - 89%) in the South Atlantic, West South Central, West North Central, and East South Central regions. However, during the past 3 weeks (weeks 6-8), influenza A activity in the West South Central region has increased to the point that during those weeks, influenza A viruses were reported more frequently (64%) than influenza B viruses. The New England region has reported approximately equal numbers of influenza A and influenza B viruses this season but during the past 3 weeks, influenza B viruses have predominated.

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Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 185 influenza viruses submitted by U.S. laboratories since September 29: sixty-five influenza A (H1)† viruses, 34 influenza A (H3N2) viruses, and 86 influenza B viruses. Forty-five of the influenza A (H1) viruses had the N1 neuraminidase and 20 had the N2 neuraminidase. The hemagglutinin proteins of all 65 influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99 (H1N1). Of the 34 influenza A (H3N2) isolates that have been characterized, 27 (79%) were similar to A/Panama/2007/99, the H3N2 component of the 2002-03 influenza vaccine, and 7 (21%) showed reduced titers to ferret antisera produced against A/Panama/2007/99. Of the 86 influenza B viruses that have been characterized, 85 belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/01 and one belonged to the B/Yamagata lineage and was similar to B/Shizuoka/15/01.

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

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 8, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.4%. This percentage is below the epidemic threshold of 8.3% for week 8.

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Influenza-like Illness Surveillance *:

During week 8, 2.8% 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 1.1% to 4.6%. 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**:

Influenza activity was reported as widespread in 13 states (Colorado, Indiana, Iowa, Minnesota, North Carolina, Oregon, Rhode Island, Tennessee, Texas, Utah, Vermont, Virginia, and Wisconsin). Twenty states reported regional activity, and 16 states and New York City reported sporadic influenza activity. One state and Washington, D.C. 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 February 27, 2003

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