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Weekly Report: Influenza Summary Update

Week ending February 2, 2002-Week 5

The following information may be quoted:

Synopsis:

During week 5 (January 27 - February 2, 2002)*, 410 (20.6%) of 1,987 respiratory specimens tested by World Health Organization (WHO) and National Respiratory Virus Surveillance System (NREVSS) laboratories were positive for influenza. The overall proportion of patient visits to sentinel physicians for influenza-like illness (ILI) was 3.1%, which is above the national baseline of 1.9%. The proportion of deaths attributed to pneumonia and influenza was 7.7%, which is below the epidemic threshold of 8.2% for week 5. Nine state and territorial health departments reported widespread influenza activity, 21 reported regional activity, and 20 reported sporadic activity **.

On February 6, 2002, WHO and the Public Health Laboratory Service (PHLS) in the United Kingdom reported the recent identification of a new influenza virus strain, influenza A(H1N2), isolated from humans in England, Israel, and Egypt. In addition to the viruses reported by PHLS, the Centers for Disease Control and Prevention (CDC) and the Wisconsin Division of Public Health have identified an influenza A(H1N2) virus from a patient specimen collected during December 2001 in Wisconsin. Influenza A(H1N2) viruses have been identified in the past. Between December 1988 and March 1989, 19 influenza A(H1N2) viruses were identified in 6 cities in China, but the virus did not spread further.

Influenza type A viruses are divided into subtypes on the basis of two proteins, hemagglutinin (H) and neuraminidase (N), on the surface of the virus. Since 1977, two influenza A virus subtypes, A(H1N1) and A(H3N2), have circulated widely among humans. The new H1N2 strain appears to have resulted from the reassortment of the genes of the currently circulating influenza A(H1N1) and A(H3N2) subtypes. The hemagglutinin protein of the A(H1N2) virus is similar to that of the currently circulating A(H1N1) viruses, and the neuraminidase protein is similar to that of the current A(H3N2) viruses. Because the current influenza vaccine contains strains with both H1 and N2 proteins similar to those in the new strain, the current vaccine should provide good protection against the new A(H1N2) virus. No unusual levels of disease have been associated with this virus and, at this time, it is uncertain if the A(H1N2) virus will persist and circulate widely.

In the United States, as of the end of January, 99% of the subtyped influenza A viruses reported through the U.S. WHO and NREVSS collaborating laboratories have been H3 viruses and 1% have been H1 viruses. CDC has received 6 influenza A H1 viruses (2 collected in September and 4 collected in October) for further antigenic characterization. These isolates include the A(H1N2) virus from Wisconsin. The neuraminidase type of the other H1 viruses has not yet been determined, but testing is underway. International influenza surveillance conducted through WHO and U.S. surveillance conducted by CDC will continue to track the occurrence of A(H1N2) viruses.

Laboratory Surveillance*:

During week 5, WHO and NREVSS laboratories reported 1,987 specimens tested for influenza viruses, of which 410 (20.6%) were positive. Ninety-three influenza A (H3N2) viruses, 312 unsubtyped influenza A viruses, and 5 influenza B viruses were identified. During the past 3 weeks (weeks 3-5), 40% of the specimens tested for influenza in the West South Central region and 30% of the specimens tested for influenza in the Mountain region were positive. Between 15% and 23% of the specimens tested for influenza in the Mid-Atlantic, East North Central, New England, and South Atlantic regions were positive. Less than 10% of the specimens tested for influenza in the Pacific, West North Central, and East South Central regions were positive.

Since September 30, WHO and NREVSS laboratories have tested a total of 36,167 specimens for influenza viruses and 2,967 (8.2%) were positive. Of the 2,967 isolates identified, 2,925 (99%) were influenza A viruses and 42 (1%) were influenza B viruses. Nine hundred and twenty-six (32%) of the 2,925 influenza A viruses identified have been subtyped; 914 (99%) were influenza A (H3N2) viruses and 12 (1%) were influenza A (H1N1) viruses. Forty-eight states have reported laboratory-confirmed influenza.

Influenza Virus Isolated
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Antigenic Characterization:

CDC has antigenically characterized 167 influenza viruses collected from U.S. laboratories since October 1: 165 influenza A (H3N2) viruses and 4 influenza A H1 viruses. All viruses were similar to the vaccine strains A/Panama/2007/99 (H3N2) and A/New Caledonia/ 20/99 (H1N1), respectively.

Pneumonia and Influenza (P&I) Mortality Surveillance:

During week 5, the percentage of all deaths due to pneumonia and influenza as reported by the vital statistics offices of 122 U.S. cities was 7.7%. This percentage is below the epidemic threshold of 8.2% for week 5.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

During week 5, 3.1% of patient visits to U.S. sentinel physicians were due to ILI. This is above the national baseline of 1.9%. On a regional level, the percentage of visits for ILI ranged from 1.7% to 6.2%.*** Due to wide variability in regional level data, it is not possible to calculate region-specific baselines and it is not appropriate to apply the national baseline to regional level data.

Bar Chart for Influenza-like Illness
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Influenza Activity as Assessed by State and Territorial Epidemiologists**:

Influenza activity was reported as widespread in 9 states (Arizona, Colorado, Kansas, New Mexico, New York, Tennessee, Texas, Utah, and Virginia) and New York City, and regional in 21 states (Connecticut, Florida, Georgia, Indiana, Iowa, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Nebraska, New Hampshire, New Jersey, North Carolina, Oklahoma, Pennsylvania, South Dakota, Vermont, Washington, and Wyoming). Twenty states and Washington, DC reported sporadic activity.

U. S. map for Weekly Influenza Activity


* 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.

*** 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 7, 2002

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