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Weekly Report: Influenza Summary Update

Week ending February 15, 2003-Week 7

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The following information may be quoted:

Synopsis:

During week 7 (February 9-15, 2003)*, 451 (17.8%) 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 3.0%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.3%. Eleven state and territorial health departments reported widespread influenza activity, 23 reported regional activity, and 13 reported sporadic influenza activity**.

Laboratory Surveillance*:

During week 7, U.S. WHO and NREVSS laboratories reported 2,527 specimens tested for influenza viruses, of which 451 (17.8%) were positive. Forty-four influenza A (H1)† viruses, eight influenza A (H3N2) viruses, 155 unsubtyped influenza A viruses, and 244 influenza B viruses were identified. During the past 3 weeks (weeks 5-7), 36.8% of specimens tested for influenza in the New England region*** and between 27.4% and 31.3% of the specimens tested for influenza in the South Atlantic, West South Central, and East North Central regions were positive. Between 16.2% and 17.2% of specimens tested for influenza during the past 3 weeks in the East South Central, Mountain, Mid-Atlantic, and West North Central regions were positive for influenza and in the Pacific region, 5.2% of specimens tested for influenza during the past 3 weeks were positive.

Since September 29, WHO and NREVSS laboratories have tested a total of 48,384 specimens for influenza viruses and 3,830 (7.9%) were positive. Of the 3,830 viruses identified, 1,442 (38%) were influenza A viruses and 2,388 (62%) were influenza B viruses. Four hundred and eighty-two (33%) of the 1,442 influenza A viruses have been subtyped; 409 (85%) were influenza A (H1)† viruses and 73 (15%) were influenza A (H3N2) viruses. One thousand two hundred and forty-seven (52%) of the 2,388 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 60% - 88%) in the East North Central, Mountain, Mid-Atlantic, and Pacific regions, and influenza B viruses were reported more frequently than influenza A viruses (range 66% - 90%) in the South Atlantic, West South Central, West North Central, and East South Central regions. However, during the past 3 weeks (weeks 5-7), 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 (55%) than influenza B viruses. The New England region has reported approximately equal numbers of influenza A and influenza B viruses this season.

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Antigenic Characterization of Viral Isolates:

CDC has antigenically characterized 179 influenza viruses submitted by U.S. laboratories since September 29: twenty-eight influenza A (H3N2) viruses, 65 influenza A (H1)† viruses, and 86 influenza B viruses. Forty-five of the influenza A (H1) viruses had the N1 neuraminidase, 19 had the N2 neuraminidase, and the neuraminidase typing for one H1 virus is pending. The A (H3N2) viruses, and the hemagglutinin proteins of the A (H1) viruses were similar antigenically to those of the corresponding vaccine strains A/Panama/2007/99 (H3N2), and A/New Caledonia/20/99 (H1N1), respectively. Eighty-five of the influenza B viruses belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/01. One influenza B virus 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 7, 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 8.3% for week 7.

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Influenza-like Illness Surveillance *:

During week 7, 3.0% 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 1.6% to 5.7%. 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 11 states (Colorado, Indiana, Iowa, Missouri, North Carolina, Rhode Island, Tennessee, Texas, Utah, Virginia, and Wisconsin). Twenty-three states and New York City reported regional activity, 13 states and Washington, D.C. reported sporadic influenza activity, and 3 states 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 20, 2003

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