1995–96
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.
1996–97
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. |