Influenza Widespread
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03/16/05 - Influenza activity has been on the decrease for the past few weeks in
Boulder County and throughout the state. At the peak of influenza
transmission, Colorado reported 129 influenza-associated
hospitalizations during the week ending February 5, 2005. Figure 1 shows
the number of hospitalizations reported in Colorado since October 3,
2004 (n=777) compared to 7 metro counties, and demonstrates a decrease
in the past three weeks.
Among the 777 Colorado residents hospitalized, 628 (80.8%) were
influenza A viruses, 93 (12.0%) were influenza B viruses, and 56 (7.2%)
were not specified. Comparatively, the Centers for Disease Control and
Prevention (CDC) reports the proportion of influenza A viruses at
greater 92% and 93% respectively for the New England and Mid-Atlantic
regions through the week ending on February 26, 2005. Influenza B
viruses were more prevalent earlier in the season for the state of
Colorado. However, the type has shifted to predominantly type A viruses
as influenza activity increased.
Antigenic characterization by the CDC has been completed for 378
influenza viruses: 4 influenza A (H1) viruses, 284 influenza A (H3N2)
viruses, and 90 influenza B viruses. One hundred thirty-six (47%) of all
influenza A viruses have been characterized as antigenically similar to
the components of the vaccine. The remaining 152 (53%) influenza A
viruses characterized had reduced titers to the vaccine components and
were more closely related to a recent reference strain,
A/California/7/2004 (H3N2). Additionally, 19 (21%) influenza B viruses
characterized by the CDC belong to the B/Victoria lineage, which is not
a component in this season’s vaccine.
Boulder County Public Health has received reports from providers
concerned about potential vaccine failure. Although there are many
reasons why individuals vaccinated may still acquire infection,
including host factors and inadequate length of time between vaccination
and exposure, the antigenic characterizations described above may be
playing a role. In response, the Colorado Department of Public Health
has sent several isolates from previously vaccinated individuals to the
CDC for antigenic characterization.
Fortunately, BCPH has not received abnormal reports of
influenza-associated complications, nor have they been noted nationally,
especially when compared to the 2003-2004 season.
 Note: Figure 1 depicts influenza-associated hospitalizations based on
when the report was received and is not the most accurate measure of
influenza activity because of the lag time between hospitalization and
the time the report is made to public health. However, this does
generalize the overall trend of influenza activity and reflects that
transmission is decreasing.
Antigenic Characterization
CDC has antigenically characterized 239 influenza viruses collected by
U.S. laboratories since October 1, 2004: 2 influenza A (H1) viruses, 157
influenza A (H3N2) viruses, and 80 influenza B viruses. The
hemagglutinin proteins of the influenza A (H1) viruses were similar
antigenically to the hemagglutinin of the vaccine strain A/New
Caledonia/20/99. One hundred twenty-five (80%) of 157 influenza A(H3N2)
isolates were characterized as antigenically similar to
A/Wyoming/3/2003, which is the A/Fujian/411/2002-like (H3N2) component
of the 2004-05 influenza vaccine. Thirty-two (20%) influenza A(H3N2)
isolates had reduced titers to A/Wyoming/3/2003 and are most closely
related to a recent reference strain, A/California/7/2004 (H3N2).
Sixty-four of the influenza B viruses isolated this season belong to the
B/Yamagata lineage and were characterized as B/Shanghai/361/2002-like,
which is the influenza B component recommended for the 2004-05 influenza
vaccine, and 4 showed somewhat reduced titers to ferret antisera
produced against B/Shanghai/361/2002. Sixteen influenza B viruses belong
to the B/Victoria lineage.
Influenza B viruses currently circulating can be divided into two
antigenically and genetically distinct lineages represented by
B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata lineage viruses
circulated widely between 1990 and 2001, during which time
B/Victoria-like viruses were not identified outside of Asia. However,
between March 2001 and October 2003, B/Victoria-like viruses
predominated in many countries, including the United States, and the
vaccine strains were changed accordingly. While both Victoria lineage
and Yamagata lineage viruses have been reported worldwide during the
past year, Yamagata lineage viruses have once again become predominant.
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