Centers for Disease
Control and Prevention
1999-2000 INFLUENZA SEASON SUMMARY*
Synopsis: During the 1999-2000 season, influenza type A(H3N2) was the predominant strain
circulating in the United States. Influenza activity (virus isolation, morbidity and mortality)
peaked between mid-December and mid-January. The percentage of respiratory specimens
testing positive for influenza viruses peaked at 33% during mid to late December. During the
previous 3 influenza seasons (1996-97, 1997-98, and 1998-99), the peak percentages of
respiratory specimens testing positive for influenza viruses ranged from 28% to 34%. The
percentage of overall patient visits for influenza-like illness (ILI) during the 1999-2000 season
peaked at 6% in late December and the proportion of deaths attributed to pneumonia and
influenza (P&I) peaked at 11.2% in mid-January. During the previous 3 influenza seasons, the
peak percentages of patient visits for ILI ranged between 5% and 7% and peak P&I mortality
levels ranged between 8.8% and 9.1%. This season's P&I figures must be interpreted with
caution because important changes have taken place in the case definition that may be
contributing to higher estimates of P&I mortality than in previous years.
Pneumonia and Influenza Mortality: The percentage of P&I deaths exceeded the epidemic
threshold for 31 of 33 weeks during the 1999-2000 influenza season and peaked at 11.2%
during mid-January. Whether the higher-than-expected percentage of P&I deaths was due to
influenza activity, respiratory illness due to some other pathogen, or reporting changes in the
122 Cities Mortality Reporting System is unknown. Because of changes in the reporting case
definition that occurred just prior to the start of the 1999-2000 season, the current increase in
P&I mortality should be interpreted with caution.
U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus
Surveillance System (NREVSS) Collaborating Laboratory Reports:*
From October 1999 through May 2000, WHO and NREVSS laboratories tested 92,403
respiratory specimens for influenza and 13,773 (15%) were positive by viral isolation or antigen
testing. Of the positive results, 13,696 (99.4%) were influenza type A and 77 (0.6%) were
influenza type B. Of the 13,696 influenza A viruses, 3,772 (28%) were subtyped; 3,640 (97%)
were A(H3N2) and 132 (3%) were A(H1N1). Influenza A(H1N1) viruses were reported by
WHO and NREVSS laboratories from all regions. Of the 132 influenza A(H1N1) viruses
reported, 118 (89%) were identified between February and May.
WHO and NREVSS laboratory data suggest that influenza activity peaked during late
December in most of the country (New England, Mid-Atlantic, East North Central, West North
Central, West South Central, Mountain, and Pacific regions). However, in the South Atlantic
and East South Central regions, influenza activity peaked during mid to late January. (See
Appendix I for a listing of the regions and Appendix II for regional graphs.)
|
|
|
Of
Those Type A Viruses
Subtyped
|
| Region |
% A |
% B |
% A(H3N2) |
% A(H1N1) |
| New England |
99.5 |
0.5 |
99.5 |
0.5 |
| Mid-Atlantic |
99.5 |
0.5 |
95.6 |
4.4 |
| East North Central |
99.6 |
0.4 |
97.3 |
2.7 |
| West North Central |
99.7 |
0.3 |
98.9 |
1.1 |
| South Atlantic |
99.6 |
0.4 |
96.3 |
3.7 |
| East South Central |
98.9 |
1.1 |
87.1 |
12.9 |
| West South Central |
99.4 |
0.6 |
98.6 |
1.4 |
| Mountain |
98.8 |
1.2 |
93.5 |
6.5 |
| Pacific |
99.0 |
1.0 |
91.3 |
8.7 |
| All Regions |
99.4 |
0.6 |
96.5 |
3.5 |
Influenza Morbidity Reports from U.S. Sentinel Physicians: Nationally, influenza morbidity
as reported by U.S. sentinel physicians peaked during the end of December, when 6% of the
total patient visits were due to ILI. These data suggest that influenza activity peaked during
late December or early January in the Mid-Atlantic, South Atlantic, East South Central, East
North Central, West North Central, West South Central, Mountain, and Pacific regions.
Influenza activity in the New England region peaked during mid-January. (See Appendix II for
regional graphs.)
Influenza Activity as Assessed by State and Territorial Epidemiologists:** Peak influenza
activity occurred during mid-January when 44 states reported either regional or widespread
influenza activity.
Antigenic Characterization of Viral Isolates: CDC antigenically characterized 617 influenza
viruses received from U.S. laboratories between October 1999 and May 2000. Of the 507
influenza A(H3N2) viruses tested, 492 (97%) were similar to the 1999-2000 vaccine strain
A/Sydney/05/97. All 29 of the influenza B viruses antigenically characterized were similar to
B/Beijing/184/93, which was represented in the 1999-2000 vaccine by B/Yamanashi/166/98.
Of the 81 influenza A(H1N1) viruses antigenically characterized, 1 (1%) was similar to
A/Beijing/262/95, the H1N1 component of the 1999-2000 vaccine, 26 (32%) were similar to
A/Bayern/07/95, and 54 (67%) were more closely related to the antigenic variant A/New
Caledonia/20/99. A/Bayern/07/95-like viruses are antigenically distinct from the
A/Beijing/262/95-like viruses; however, the A/Beijing/262/95 vaccine strain produces high titers
of antibody that cross-react with A/Bayern/07/95-like viruses.
Influenza Strains Contained in the 2000-2001 Vaccine: The trivalent influenza vaccine
prepared for the 2000-2001 season will include A/New Caledonia/20/99-like (H1N1),
A/Moscow/10/99-like (H3N2), and B/Beijing/184/93-like antigens. For the A/Moscow/10/99-like
(H3N2) antigen, U.S. manufacturers will use the antigenically equivalent A/Panama/2007/99
(H3N2) virus and for the B/Beijing/184/93-like antigen, they will use the antigenically equivalent
B/Yamanashi/166/98 virus; 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, so numbers may change as more reports are received.
**Influenza activity is defined as influenza-like illness (fever 100F and a cough or a sore throat) and/or
culture-confirmed influenza.
Appendix I: 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