Epidemiologic Notes and Reports
Influenza -- United States
Update on Activity
During the 1987-88 influenza season in the United States, peak
activity occurred in late February and early March. During the
4-week
period February 14-March 12, 1988, widespread or regional
outbreaks of
influenza-like illness were reported from 28 to 30 states each
week.
For the week ending February 20, 30 states reported outbreaks, the
highest number for any single week this season. Surveillance
conducted
throughout the country also showed a peak in the percentage of
patients seen with influenza-like illness during the week ending
February 20: an average of 8.1% of patients seen that week had an
influenza-like illness, compared with the overall seasonal average
of
4.8%. Correlating with these indicators, the proportion of deaths
attributed to pneumonia and influenza (P&I) first exceeded the
epidemic threshold* on the week ending February 20, peaked during
the
week ending March 5, and remained above the threshold on April 2
(Figure 1).
Influenza type A(H3N2) has been the predominant influenza
strain
this season, representing 85% of all influenza virus isolates
reported
in the United States by the World Health Organization
Collaborating
Laboratories as of March 26. Influenza A(H3N2) isolates have been
confirmed in the District of Columbia and in all states except New
Hampshire and Rhode Island. Many states have reported outbreaks of
influenza-like illness in nursing homes, often with isolation of
influenza A(H3N2) virus from specimens collected either from the
nursing-home residents or from residents of nearby communities.
During the latter part of the 1987-88 season, influenza types
A(H1N1) and B have been isolated more frequently, but as of March
26,
these viruses still represented only 6% and 9% of isolates,
respectively. Influenza A(H1N1) has been isolated in 16 states,**
and
influenza B, in 26 states.*** Although these isolates have
primarily
been associated with sporadically occurring cases, three
culture-confirmed outbreaks of influenza B have been reported with
onset dates during late February or early March. Two occurred in
nursing homes in Connecticut, and the third in a pediatric
long-term
care wing of a New York hospital.
Characterization of Antigenic Variants of Influenza A(H3N2)
Viruses
Earlier, CDC reported circulation of two type A(H3N2) viruses,
A/Sichuan/2/87 and A/Victoria/7/87, that were antigenically
distinct
from viruses circulating from 1985 through the spring of 1987,
such as
A/Leningrad/360/86 and A/Mississippi/1/85 (2). Viruses with
reaction
patterns that were intermediate between A/Sichuan/2/87 and
A/Leningrad/360/86 were also described. These intermediate viruses
have now been characterized and found to resemble two reference
strains: A/Sydney/1/87 and A/Shanghai/11/87 (Table 1). These
viruses
are inhibited at higher titers than is A/Sichuan/2/87 virus with
antiserum prepared against A/Leningrad/360/86 and at lower titers
with
A/Sichuan/2/87 antiserum. The A/Shanghai/11/87 virus is distinct
from
the A/Sydney/1/87 virus as evidenced by the lower
hemagglutination-inhibition titers with antisera to A/Caen/1/84,
A/Mississippi/1/85, and A/Sydney/1/87. Since the fall of 1987, 158
viruses collected in the United States have been characterized;
seven
(5%) are A/Sichuan/2/87-like, 19 (12%) are A/Victoria/7/87-like,
24
(15%) are A/Sydney/1/87-like, and 108 (68%) are
A/Shanghai/11/87-like.
Reported by: Participating State and Territorial Epidemiologists
and
State Laboratory Directors. Participating Physicians of the
American
Academy of Family Physicians. WHO Collaborating Laboratories. WHO
Collaborating Center for Influenza, Influenza Br, Div of Viral
Diseases, Center for Infectious Diseases, CDC.
Editorial Note: Cocirculation of two or more closely related
antigenic variants in a single season is not uncommon. This
season,
four variants of influenza type A(H3N2) are circulating as well as
types A(H1N1) and B.
Of the influenza viruses currently in circulation, influenza
A(H3N2), which emerged in 1968, has been associated with the
greatest
excess P&I deaths and total excess mortality. Excess mortality has
occurred during each of 11 influenza A(H3N2) epidemics. Thus, the
elevation of the percentage of P&I deaths observed this season is
consistent with observations during other influenza A(H3N2)
epidemics.
Weekly reports of deaths in 121 cities in the United States
are
used to determine preliminary estimates of influenza-related
mortality
during the influenza season. The percentage of deaths attributed
to
P&I is calculated each week and compared with a ratio of P&I
deaths to
total deaths that would be expected in the absence of an influenza
epidemic (1). Data from the National Center for Health Statistics
(NCHS) on all deaths in the United States are used to determine
final
estimates of excess P&I deaths and total excess mortality; these
statistics are not available until approximately 2 years after the
epidemic period. The P&I ratio from 121 cities offers a useful
method
for evaluating the impact of influenza during epidemics, and death
rates calculated with use of the 121-city data and the final NCHS
data
show similar trends. However, the 121-city data cannot be used to
project accurately total influenza-related deaths for this season
or
to make other than general comparisons to past influenza seasons.
References
Lui K-J, Kendal AP. Impact of influenza epidemics on mortality
in
the United States from October 1972 to May 1985. Am J Public
Health 1987;77:712-6.
Centers for Disease Control. Antigenic variation of recent
influenza A(H3N2) viruses. MMWR 1988;37:38-40,46-47.
*The epidemic threshold for the 1987-88 influenza season was
estimated
at 1.645 standard deviations above the values projected on the
basis
of a periodic regression model applied to observed P&I deaths for
the
previous 5-year period, but excluding the observations during
influenza outbreaks (1).
**Alabama, Arkansas, Connecticut, Georgia, Illinois, Louisiana,
Maine,
Massachusetts, Nebraska, New Jersey, New York, North Carolina,
South
Carolina, Texas, Vermont, and Virginia.
***Alabama, Alaska, Arizona, Arkansas, California, Connecticut,
Delaware, Hawaii, Illinois, Iowa, Maine, Massachusetts, Montana,
Nebraska, Nevada, New Mexico, New York, Ohio, Pennsylvania,
Tennessee,
Texas, Utah, Virginia, Washington, West Virginia, and Wisconsin.
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