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June 22, 2000
Contact: CDC, Division of Media Relations
(404) 639-3286

Flu Season 2000-01
 
U.S. FLU SEASONS 1995-2000
 
199596
Vaccine components: A/Johannesburg/33/94 (H3N2), A/Texas/36/91 (H1N1), and B/Harbin/07/94
Influenza activity peaked during late December 1995 and early January 1996. For the first time since the 1988-89 season, influenza A(H1N1) viruses predominated in the United States overall. Influenza A(H3N2) and influenza B viruses also circulated; influenza A(H3N2) viruses were more commonly isolated than influenza A(H1N1) viruses in 3 of the 9 influenza surveillance regions. At the peak of the season, 26% of respiratory specimens tested for influenza were positive. The number of state epidemiologists reporting regional or widespread influenza activity reached 33. The percentage of patient visits to sentinel physicians for influenza-like illness (ILI) exceeded baseline levels for 5 weeks peaking at 7%. The proportion of pneumonia and influenza (P&I)-related deaths were above the epidemic threshold for 6 consecutive weeks and peaked at 8.2% during mid-January.

199697
Vaccine components: A/Nanchang/933/95 (H3N2), A/Texas/36/91 (H1N1), and B/Harbin/07/94
Influenza activity in the United States peaked during late December 1996 and early January 1997. Although outbreaks were reported among all age groups, most outbreaks reported to CDC occurred among elderly nursing-home residents. Influenza A(H3N2) viruses predominated, but influenza B viruses also circulated. During the last week of December 1996, 34% of respiratory specimens tested were positive for influenza. At the peak of the season, 39 state epidemiologists reported regional or widespread influenza activity. The percentage of patient visits to sentinel physicians for ILI exceeded baseline levels for 5 weeks peaking at 7%. P&I-related deaths were above the epidemic threshold for 10 consecutive weeks and peaked at 9.1%. Noteworthy: A strain of influenza virus previously known to infect only birds was associated with illness and death in humans in Hong Kong. The first known human case of influenza type A(H5N1) avian flu occurred in a 3-year-old child who died following a respiratory illness in May 1997. Seventeen additional cases were identified during November and December 1997 for a total of 18 confirmed cases and 6 deaths associated with this outbreak.

1997-98
Vaccine components: A/Nanchang/933/95 (H3N2), A/Johannesburg/82/96 (H1N1), and B/Harbin/07/94
Influenza activity began to increase in early December 1997 and peaked during late January to early February 1998. The predominant virus was influenza A(H3N2); few influenza type B or influenza A(H1N1) isolates were reported. At the peak of the influenza season, 29% of respiratory specimens tested were positive for influenza and 46 state epidemiologists reported regional or widespread influenza activity. P&I deaths were above the epidemic threshold for 10 consecutive weeks and peaked at 9%. The percentage of patient visits to sentinel physicians for ILI was elevated for 7 consecutive weeks, peaking at 5%. Noteworthy: In July 1998, CDC and Health Canada began investigating reports of respiratory illness with fever and associated pneumonia among persons traveling on land and sea (both independent and tour packages) in Alaska and the Yukon Territory. Laboratory evidence confirmed that influenza A(H3N2) infection was the cause of many of the illnesses. Approximately 40,000 tourists and tourism workers were affected by this outbreak.

1998-99
Vaccine components: A/Sydney/05/97 (H3N2), A/Beijing/262/95 (H1N1), and B/Harbin/07/94
During the 1998-99 influenza season, both influenza A(H3N2) and influenza B viruses circulated worldwide, and influenza A(H3N2) predominated in the United States for the third consecutive year. Influenza activity began to increase in mid-January 1999 and peaked during February. At the peak of the influenza season, 28% of respiratory specimens tested were positive for influenza and 42 state epidemiologists reported regional or widespread influenza activity. P&I deaths were above the epidemic threshold for 12 consecutive weeks and peaked at 8.8%. The percentage of patient visits to sentinel physicians for ILI was elevated for 7 consecutive weeks, peaking at 5%. Noteworthy: 1) This was the third consecutive year that influenza A(H3N2) viruses had predominated in the United States; 2) Influenza A (H9N2) virus, a strain not previously isolated from humans, was isolated from two persons in Hong Kong; and 3) For the second consecutive summer, an influenza outbreak occurred among travelers and tourism workers in Alaska and the Yukon Territory.

1999-2000
Vaccine components: A/Sydney/05/97 (H3N2), A/Beijing/262/95 (H1N1), and B/Yamanashi/166/98
Influenza activity began to increase in November 1999 and peaked during late December 1999 and early January 2000. During the 1999-2000 influenza season, influenza A (H3N2) viruses predominated in the United States and worldwide. This was the fourth consecutive season in which influenza A(H3N2) viruses predominated and the third in which the A/Sydney/05/97 (H3N2) strain predominated. At the end of December, 33% of respiratory specimens tested for influenza were positive. State epidemiologists from 44 states reported regional or widespread influenza activity at the peak of the season. P&I mortality was above epidemic threshold for all but 2 weeks during the season peaking at 11.2% but this data must be interpreted with caution because changes were made in the case reporting definition that may have lead to higher estimates of P&I mortality than those reported in previous seasons. Noteworthy: This was the third consecutive season that influenza A/Sydney/05/97-like (H3N2) viruses were the most frequently isolated influenza viruses in the United States.


Flu Season 2000-01: See also...



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