Centers for Disease Control and Prevention
2000-2001 INFLUENZA SEASON SUMMARY*
Synopsis: The 2000-01 influenza season was mild in the United States and was the first season since 1995-96 that influenza A(H3N2) viruses did not predominate. The predominant strain nationwide this season was influenza A(H1N1). In some regions†, however, influenza B viruses were reported more frequently than influenza A viruses. The percentage of respiratory specimens testing positive for influenza viruses peaked at 24% during mid to late January. During the previous 3 influenza seasons (1997-98, 1998-99, and 1999-2000), the peak percentages of respiratory specimens testing positive for influenza viruses ranged from 28% to 32%. The percentage of patient visits for influenza-like illness (ILI) peaked at 4% for 4 consecutive weeks during mid- January and early February. During the previous 3 influenza seasons, the peak percentage of patient visits for ILI ranged between 5% and 7%. On the basis of data from state and territorial epidemiologist reports, influenza activity¥ peaked during late January and early February, when 38 states reported regional or widespread influenza activity. The peak number of states reporting regional or widespread activity during the previous 3 years ranged from 43 to 46. The proportion of deaths attributed to pneumonia and influenza (P&I) did not exceed the epidemic threshold during this season.
U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratory Reports: From October 2000 through May 2001, WHO and NREVSS laboratories tested 88,598 respiratory specimens for influenza viruses and 9,962 (11%) were positive. Of the positive results, 5,337 (54%) were influenza type A and 4,625 (46%) were influenza type B. Of the 5,337 influenza A viruses, 2,127 (40%) were subtyped; 2,061 (97%) were A (H1N1) and 66 (3%) were A (H3N2). Influenza B viruses were isolated more frequently than type A viruses from the week ending February 10 through the week ending May 19. Influenza type A viruses predominated in the East North Central, South Atlantic, West North Central, and West South Central regions. Influenza B viruses predominated in the Mid-Atlantic, Mountain, and Pacific regions. The East South Central and New England regions reported approximately equal numbers of influenza A and B viruses.
WHO and NREVSS laboratory data suggest that influenza activity peaked during mid to late January in most of the country (East North Central, Mid-Atlantic, Mountain, New England, Pacific, South Atlantic, and West North Central). However, influenza activity peaked during late December in the West South Central region and early February in the East South Central region.
Antigenic Characterization of Viral Isolates: CDC has antigenically characterized 678 influenza viruses received from U.S. laboratories since October 1. Of the 354 influenza A (H1N1) isolates that have been characterized, 335 (95%) were similar to A/New Caledonia/20/99, the H1N1 component of the 2000-01 influenza vaccine, and 19 (5%) were similar to A/Bayern/07/95. Although A/Bayern-like viruses are antigenically distinct from the A/New Caledonia-like viruses, the A/New Caledonia/20/99 vaccine strain produces high titers of antibody that cross-react with A/Bayern/07/95-like viruses. Of the 23 influenza A (H3N2) viruses that have been characterized, all were antigenically similar to the vaccine strain A/Panama/2007/99. Of the 301 influenza B viruses characterized, 33 (11%) were similar to the vaccine strain, B/Beijing/184/93, and 268 (89%) were more closely related antigenically to the B/Sichuan/379/99 reference strain. It should be noted that the B/Sichuan virus exhibits cross-reactivity with the vaccine strain.
Pneumonia and Influenza Mortality: The percentage of P&I deaths in the United States did not exceed the epidemic threshold during the 2000-01 influenza season. During the previous 3 seasons, the percentage of deaths attributed to P&I was above the epidemic threshold for 10 consecutive weeks each season.
Influenza Morbidity Reports from U.S. Sentinel Physicians: Nationally, influenza morbidity as reported by U.S. sentinel physicians peaked at 4% for 4 consecutive weeks from the week ending January 20 through the week ending February 10. These data suggest that influenza activity peaked during this time in the East North Central, East South Central, Mid-Atlantic, Mountain, New England, South Atlantic, West North Central, and West South Central regions. However, influenza activity as reported by sentinel physicians in the Pacific region peaked during late December.
Influenza Activity as Assessed by State and Territorial Epidemiologists: On the basis of data from state and territorial epidemiologist reports, influenza activity peaked during late January and early February, when 38 states reported regional or widespread influenza activity. State and territorial epidemiologists reported regional influenza activity during consecutive weeks from the week ending November 18 through the week ending March 31. Widespread activity was reported by one or more states during consecutive weeks for the week ending January 6 through the week ending March 10.
Influenza Strains Contained in the 2001-02 Vaccine: The trivalent influenza vaccine prepared for the 2001-02 season will include A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Sichuan/379/99-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/Sichuan/379/99-like virus, they will use one of the antigenically equivalent viruses B/Johannesburg/5/99, B/Victoria/504/2000, or B/Guangdong/120/2000. These viruses will be used because of their growth properties and because they are representative of currently circulating A (H3N2) and B viruses.
* Reporting is incomplete and numbers may change as more reports are received.
† 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)
¥ Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza.
Report prepared: June 7, 2001
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