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The 2001-02 influenza season was mild to moderate in the United States and influenza A(H3N2) viruses predominated. Influenza A(H3N2) viruses were the most frequently isolated influenza viruses in the United States during 5 of the last 6 influenza seasons. Although influenza A (H3N2) viruses predominated during the 2001-02 season, influenza B viruses were reported more frequently than influenza A viruses toward the end of the season. On the basis of data reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratories, U.S. Influenza Sentinel Physicians, U.S. State and Territorial Epidemiologists, and the122 Cities Mortality Reporting System, influenza morbidity peaked during mid-to-late February, 2002 and pneumonia and influenza mortality peaked during early-to-mid March, 2002.

Pneumonia And Influenza Mortality

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Laboratory Surveillance*:

U.S. World Health Organization (WHO) and National Respiratory And Enteric Virus Surveillance System(NREVSS) Collaborating Laboratory Reports *: From September 30, 2001 through May 18, 2002, WHO and NREVSS laboratories tested 100,815 respiratory specimens for influenza viruses and 15,671 (15.5%) were positive. Of the positive results, 13,706 (87.5%) were influenza type A viruses and 1,965 (12.5%) were influenza type B viruses. Of the 4,507 influenza A viruses subtyped, 4,420 (98.1%) were H3 viruses and 87 (1.9%) were H1 viruses. Influenza B viruses were reported more frequently than influenza A viruses each week since the end of March.

Nationwide, the percentage of respiratory specimens testing positive for influenza peaked at 26% during the week ending February 23, 2002. During the previous 3 influenza seasons (1998-99, 1999-2000, and 2000-01), the peak percentage of respiratory specimens testing positive for influenza viruses ranged from 23% to 31%. During the 2001-02 season, influenza activity as reported by WHO/NREVSS laboratories in the East North Central, Mid-Atlantic, New England, South Atlantic, and West North Central regions peaked during early to mid-February, while activity in the Pacific, Mountain, and West South Central regions peaked during late December, early January, and late January, respectively. Activity in the East South Central region peaked during early March.

 Influenza Virus Isolated

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Antigenic Characterization:

CDC has antigenically characterized 690 influenza viruses collected since October 1,2001 from U.S. laboratories: 393 influenza A(H3N2) viruses, 30 influenza A H1 viruses, and 267 influenza B viruses. All 393 influenza A (H3N2) viruses were similar to the vaccine strain A/Panama/2007/99 (H3N2) and all 30 influenza A H1 viruses had an H1 protein similar antigenically to the vaccine strain A/New Caledonia/20/99 (H1N1). Sixteen of the H1 viruses were H1N2 viruses collected in Hawaii, Massachusetts, New York, Pennsylvania, and Wisconsin. Two additional H1N2 viruses were identified from patient specimens collected during July and September, 2001, in Texas and Nevada, respectively. The influenza A(H1N2) strain appears to have resulted from the reassortment of the genes of the currently circulating influenza A(H1N1) and A(H3N2) subtypes. 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. International influenza surveillance conducted through WHO and U.S. surveillance conducted by CDC will continue to track the occurrence of A(H1N2) viruses.

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

Influenza B viruses currently circulating worldwide can be divided into 2 antigenically distinct lineages, B/Yamagata/16/88 and B/Victoria/2/87. Viruses of the B/Yamagata lineage have circulated widely since 1990. The B component of the current influenza vaccine belongs to the B/Yamagata lineage. Viruses of the B/Victoria lineage had not been identified outside of Asia between 1991 and March 2001. Since March 2001, B/Victoria lineage viruses have been identified in Africa, Asia, Europe, and North America. CDC has antigenically characterized 267 influenza B viruses received from U.S. laboratories and collected since October 1, 2001; 61 belonged to the B/Yamagata lineage and 206 belonged to the B/Victoria lineage. Of the 61 B/Yamagata lineage viruses, 13 were similar to the vaccine strain, B/Sichuan/379/99, and 48 showed somewhat reduced titers to ferret antisera produced against B/Sichuan/379/99. Most of the viruses that showed somewhat reduced titers to ferret antisera produced against B/Sichuan/379/99 are closely related to B/Shizuoka/15/2001, a minor antigenic variant of B/Sichuan/379/99. The majority of B/Yamagata lineage viruses were collected during the fall and early winter, and the proportion of influenza B isolates belonging to the B/Victoria lineage increased throughout the season. The influenza B component of the influenza vaccine for the 2002-03 season will be an influenza B/Victoria lineage virus.

Influenza Strains Contained in the 2002-03 Vaccine:

The trivalent influenza vaccine for the 2002-03 season will include A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Hong Kong/330/2001-like viruses. For the A/Moscow/10/99-like (H3N2) virus, U.S. manufacturers will use the antigenically equivalent A/Panama/2007/99 (H3N2) virus, and for the B/Hong Kong/330/2001-like virus, they will use either B/Hong Kong/330/2001 or the antigenically equivalent virus B/Hong Kong/1434/2002. These viruses will be used because of their growth properties and because they are representative of currently circulating A (H3N2) and B viruses.

Pneumonia and Influenza (P&I) Mortality Surveillance:

The percentage of P&I deaths in the United States was above the epidemic threshold for 5 consecutive weeks (week ending March 2 through week ending March 30) and peaked at 9.1% during the week ending March 16. During the 1998-99 and 1999-2000 seasons, the percentage of deaths attributed to P&I was above the epidemic threshold for 12 and 13 consecutive weeks, respectively, but did not exceed the epidemic threshold during the 2000-01 season.

Pneumonia And Influenza Mortality

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Influenza-like Illness Surveillance *:

Nationally, influenza morbidity as reported by U.S. sentinel physicians peaked at 3.2% during the week ending February 16. During the previous 3 influenza seasons, the peak percentage of patient visits for influenza-like illness ranged between 4% and 6%. The data for the 2001-02 season suggest that influenza activity peaked during mid- to-late February in the East North Central, East South Central, Mountain, New England, Pacific, West North Central, and West South Central regions. However, influenza activity as reported by sentinel physicians in the Mid-Atlantic and South Atlantic regions peaked during mid-to-late January. Appendix 1 contains region-specific data.

Bar Chart for Influenza-like Illness

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Influenza Activity as Assessed by State and Territorial Epidemiologists**:

On the basis of data from state and territorial epidemiologist reports, influenza activity¥ peaked during mid-February, when 40 states reported regional or widespread influenza activity. The peak number of states reporting regional or widespread activity during the previous 3 years ranged from 38 to 44. During the 2001-02 season, state and territorial epidemiologists reported regional influenza activity during 25 consecutive weeks from the week ending November 17 through the week ending May 4. Widespread activity was reported by one or more states during 18 consecutive weeks from the week ending December 1 through the week ending March 30.

Usmap 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)

Reported Prepared: June 10, 2002.

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