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Talbot TR, Stapleton JT, Brady RC, Winokur PL, Bernstein DI, Germanson T, Yoder SM, Rock MT, Crowe, Jr JE, Edwards KM. Vaccination Success Rate and Reaction Profile With Diluted and Undiluted Smallpox VaccineA Randomized Controlled Trial. JAMA. 2004;292(10):1205–1212. doi:10.1001/jama.292.10.1205
Context Additional smallpox vaccine doses are needed to augment current US national
stockpile. Aventis Pasteur smallpox vaccine (APSV), initially manufactured
in the 1950s from the New York Board of Health vaccinia strain in a frozen
preparation, appears as effective as lyophilized vaccine but the effectiveness
of diluted doses of APSV is unclear.
Objective To compare the vaccination success rate and the reaction profile of
various APSV dilutions.
Design, Setting, and Participants A double-blind, randomized controlled trial of 340 healthy vaccinia-naive
adults aged 18 to 32 years from 3 academic medical centers who were vaccinated
with 1 of 3 strengths of APSV dilutions (undiluted, 1:5, and 1:10) between
October 9, 2002, and February 24, 2003. Volunteers were followed up every
3 to 5 days until the vaccination site healed for bandage changes, vaccine
response assessment, and adverse event evaluation, followed by 1- and 2-month
clinic evaluations and 6-month telephone interview.
Main Outcome Measures Successful vaccination, defined by presence of a vesicle or pustule
at the inoculation site 6 to 11 days postvaccination, and local and systemic
reactions to vaccination.
Results A total of 340 volunteers were vaccinated (vaccine dose: undiluted,
n = 113; 1:5 dilution, n = 114; and 1:10 dilution, n = 113). Following vaccination,
99.4% (95% confidence interval [CI], 97.9%-99.9%) of all volunteers had successful
vaccinations. Success rates did not differ between the dilution groups (undiluted,
100.0%; 95% CI, 96.8%-100.0%; 1:5 dilution, 98.2%; 95% CI, 93.8%-99.8%; 1:10
dilution, 100.0% 95% CI, 96.8%-100.0%; P = .33).
Overall, 99.7% of volunteers reported at least 1 local symptom at the vaccination
site, and 61.8% had axillary lymphadenopathy, 15.0% developed satellite lesions,
and 7.6% developed a rash away from the vaccination site. Fever developed
in 21.5%. No differences were noted in local or systemic reactions between
the 3 dilution groups (P>.05 for each comparison).
A total of 25% of volunteers missed scheduled duties due to vaccine-related
Conclusions Even at diluted doses, APSV is an effective smallpox vaccine, allowing
for expansion of the current stockpile. However, reactogenicity was not reduced
with dilution of the vaccine and, as with other smallpox vaccines, may impair
In 2002, smallpox vaccination resumed in the United States, using stockpiled
vaccine prepared in the early 1970s.1 Historically
during the years of routine vaccination, 2 types of vaccinia preparations
were used in the United States: a lyophilized (powdered) form and a frozen
preparation, both derived from the New York Board of Health vaccinia strain.
Approximately 15 million doses of the lyophilized product (Dryvax, Wyeth-Ayerst,
Marietta, Pa) had been maintained within the US national stockpile in the
event that widespread vaccination of the population was needed. Recent studies
have demonstrated that the lyophilized vaccine diluted 10-fold still retained
high vaccination take rates. Although this expanded the vaccine stockpile,2 it still was short of the stated Department of Health
and Human Services goal of having a vaccine dose for every US citizen. Until
newer vaccines are available, the need exists for additional vaccine doses
for the United States and other countries.
In fall 2001, Aventis Pasteur (Swiftwater, Pa) reported that multiple
lots of Aventis Pasteur smallpox vaccine (APSV) had been in frozen storage
for decades. An earlier head-to-head study comparing the lyophilized vaccine
and APSV noted that the frozen vaccine retained potency and had similar vaccination
success as the lyophilized vaccine (K.M.E., unpublished data, 2004). We conducted
this study to further assess the vaccination success, safety, and reactogenicity
of APSV at various dilutions in vaccinia-naive individuals in a large multicenter
This double-blind, randomized controlled trial was conducted at 3 sites:
Vanderbilt University School of Medicine, Nashville, Tenn; Cincinnati Children's
Hospital Medical Center, Cincinnati, Ohio; and University of Iowa, Iowa City.
Volunteers were enrolled from October 9, 2002, to February 24, 2003. Approval
for the trial was granted by the institutional review boards of each participating
organization. Written informed consent was obtained from all volunteers. Healthy
adults aged 18 to 32 years with no prior history of smallpox vaccination and
the absence of a vaccination scar were eligible for enrollment. In addition,
volunteers were excluded if they had a history of any of the following conditions:
autoimmune disease, human immunodeficiency virus infection, solid organ or
bone marrow transplantation, malignancy, eczema, prior vaccination with any
vaccinia-vectored or other pox-vectored experimental vaccine, or allergies
to the vaccine components. Volunteers with a history of or current illegal
injection drug use, current exfoliative skin disorders, use of immunosuppressive
medications, medical or psychiatric conditions or occupational responsibilities
that precluded volunteer compliance, and pregnant or lactating women were
also excluded. Those volunteers with household or sexual contacts with a history
of or concurrent eczema, a history of exfoliative skin disorders, a history
of the immunosuppressive conditions noted above, ongoing pregnancy, or children
younger than 1 year were also prohibited from study participation. All volunteers
underwent a comprehensive screening history, physical examination, and laboratory
evaluation, including serologic testing for hepatitis B surface antigen and
antibodies to human immunodeficiency virus and hepatitis C virus. Those volunteers
with abnormal screening test results were excluded. All women were required
to have a negative urine pregnancy test result within 48 hours of vaccination.
The APSV used in this study (lot 2243) was initially derived from the
New York Board of Health vaccinia strain and manufactured between 1956 and
1957. Approximately 85 million doses had been reserved for the Department
of Defense and maintained at –20°C since manufacture. Potency determinations
of the lot revealed chorionic allantoic membrane titer of 107.6 plaque
forming units per milliliter, maintained following 4 freeze-thaw cycles (data
on file at Aventis Pasteur).
Eligible volunteers were randomized using an in-house, custom-built
program with fixed blocks of size 6 to receive 1 of 3 strengths of APSV dilutions
(undiluted, 1:5, and 1:10). Each vial of frozen APSV was thawed to room temperature
immediately before first use. Vaccine was diluted with sterile water diluent
containing 50% glycerin and 0.25% phenol (Chesapeake Biological Laboratories,
Baltimore, Md) and was administered to the deltoid area via scarification
by 15 punctures with a bifurcated needle. The site was covered with 2 occlusive
bandages, as described previously.3 Study staff
and volunteers were blinded to vaccine-dose assignment.
Demographic data and information on race were recorded for each volunteer.
Race was determined by volunteer self-declaration and was collected to assess
for potential racial differences in vaccine success and reactogenicity. Volunteers
were observed every 3 to 5 days for scheduled bandage changes, vaccine response
assessment, and adverse event evaluation. Vaccination success was measured
by the development of a vaccination site take, defined
as the presence of a vesicle or pustule at the inoculation site 6 to 11 days
postvaccination (Figure 1A). At
each follow-up visit, study staff inspected and measured the vaccination lesion,
the surrounding erythema and induration, and any regional lymphadenopathy.
Volunteers were questioned at each follow-up visit for the presence of any
vaccine-related adverse events and instructed to note via symptom diary daily
oral temperatures and the presence and severity of various local (site pain
and pruritus, axillary pain, and swelling) and systemic (malaise, fatigue,
chills, myalgias, nausea, headache, joint pain, anorexia) symptoms for at
least 2 weeks after vaccination until resolution of all symptoms. Fever was
defined as oral temperature of at least 37.8°C. Daily absence from work
or school due to adverse events after vaccination was recorded. As this study
occurred before the identification of cardiac adverse events in the civilian
and military populations,4,5 specific
screening for signs and symptoms of cardiac disease were not actively solicited
at each follow-up visit.
Volunteers were trained on bandage removal and application in the event
an unscheduled bandage change was needed. Sterile gloves and bandages were
provided if the bandage required changing. Careful hand washing was stressed.
There were no restrictions on work activities following vaccination, per Centers
for Disease Control and Prevention recommendations.6 An
assessment of vaccinia shedding from the inoculation site, outside of the
site dressing, and the vaccinee's hands was conducted in volunteers at the
Vanderbilt University School of Medicine site and has been published elsewhere.7 Follow-up visits occurred until the vaccination site
was considered healed by study staff. Additional 1-month and 2-month clinic
visits and a 6-month telephone interview were conducted to evaluate for any
delayed adverse events. In addition, volunteers were encouraged at the 6-month
telephone interview to report to the study staff any concerning symptoms should
they occur in the future; if such symptoms were considered clinically significant,
volunteers would then be evaluated in the clinic. An independent data and
safety monitor at each site promptly reviewed all adverse events to ensure
Specimens to examine the postvaccination serologic responses were collected
from each volunteer at baseline before vaccination and at 28 and 56 days postvaccination.
Due to the high volume of samples collected from this trial and from several
other concurrently conducted smallpox vaccine studies, serum antibody data
for all volunteers in this trial are not yet available. Serum neutralizing
antibody data, however, were assessed on a subset of volunteers from the Vanderbilt
University School of Medicine site who were invited to participate in a site-specific
substudy, investigating various detailed aspects of the immune response, including
cell-mediated immunity and cytokine responses postvaccination.8 Serum
antibody responses were measured by plaque reduction neutralization assay
as described previously.9 Serial 4-fold dilutions
of serum collected at baseline and 1 month following vaccination were incubated
at 37°C for 15 hours with vaccinia virus diluted to contain 50 to 70 plaque
forming units of virus. Triplicate virus-serum mixtures at each dilution were
then plated onto 90% confluent BSC-40 (African green monkey kidney) cell monolayer
cultures, incubated at room temperature for 1 hour, and overlaid with media
containing 2% fetal bovine serum. Following a 48-hour incubation, the monolayer
was fixed with 10% formalin and stained with crystal violet. Results were
expressed as reciprocal 60% neutralization titers using a standard linear
regression curve. Seroconversion was defined as a 5-fold or more increase
in titers from baseline. Negative and both low-titered and high-titered serum
positive controls were run with each assay to determine assay acceptance.
The study investigators decided that providing the humoral immunity data from
these volunteers at this time, although they represent a subset of all study
participants, would provide useful information on reactogenicity and vaccine
Based on an estimated 90% vaccine take frequency in each dilution group,
a take frequency 95% confidence interval (CI) half-width of 5%, and a potential
5% loss to follow-up, the study sample size was calculated at 148 volunteers
per study group (N = 444). On February 24, 2003, the US Food and Drug Administration
temporarily suspended all vaccinia trials due to adverse events noted in the
ongoing civilian and military vaccination campaigns; therefore, the study
was terminated.4,10 At that time,
340 volunteers had been vaccinated and evaluated for clinical take status.
At the time of suspension, only 2 volunteers had not developed a take
and no volunteers were lost to follow-up, providing a minimum possible take
rate in any of the dilution groups of 98%, surpassing the confidence interval
half-width criterion of 5%. Thus, with the higher-than-expected take rates
and lower-than-expected percentage of volunteers lost to follow-up, the sample
size of 340 provided power similar to the original calculations. Because the
objectives of the trial had been met, the study investigators and the US National
Institute of Allergy and Infectious Diseases project leaders chose to terminate
Vaccination success rates and safety measurements were compared between
dilution groups. The uncorrected χ2 test was used to compare
reactogenicity rates. Symptom severity and extent of induration and erythema
were compared using the analysis of variance. Take rates were evaluated using
Fisher exact test and exact methods were used to determine 95% CIs. Comparisons
of the mean neutralizing antibody titers at 1 month after vaccination between
the dilution groups were performed using the Kruskal-Wallis test and, when
a significant difference was detected by this test, Mann-Whitney U test was performed between the individual dilution groups. SAS statistical
software version 8.2 (SAS Institute Inc, Cary, NC) was used for all analyses; P<.05 was considered statistically significant.
A total of 340 volunteers were vaccinated before study closure (n =
148 at Vanderbilt University School of Medicine, n = 150 at University of
Iowa, and n = 42 at Cincinnati Children's Hospital Medical Center) (Figure 2). Mean (SD) age of the cohort was
24 (3) years, 42.2% were men, and the cohort was 94.4% white. Significant
differences in volunteer age, sex, and racial characteristics were not detected
between the 3 dilution groups (data not shown). The specific dilution of vaccine
received was evenly distributed among volunteers (undiluted, n = 113; 1:5
dilution, n = 114; and 1:10 dilution, n = 113). Volunteers had a mean (range)
of 7.6 (6-11) follow-up visits with a mean (range) follow-up time of 55 (44-102)
days. All volunteers completed follow-up.
Following vaccination, overall 99.4% (95% CI, 97.9%-99.9%) of volunteers
developed a clinical take (Table 1).
Success rates did not differ between the different dilution groups (P = .33). Mean vaccination lesion size was 16 mm and median
(range) peak size was 15 (5-30) mm; lesion size peaked at 11 (3-15) days postvaccination.
Ranges of lesion sizes are listed in Table
2. Measurements did not differ across all dilution groups (P = .30 and P = .47, respectively).
The 2 volunteers without a clinical take each developed a detectable papule
at the vaccination site that did not progress to a vesicle or pustule.
Local induration and erythema of the vaccination site occurred in all
volunteers, including the 2 who did not develop a clinical take, and reached
their peaks on postvaccination day 10 (erythema: range, 4-15 days; induration:
range, 5-15 days). Women had significantly less local erythema (mean, 46 vs
58 mm; P = .002) and induration (mean, 38 vs 44 mm; P = .02) than men did. Axillary lymphadenopathy was detected
in 61.8% of volunteers after vaccination. Differences were not detected in
the maximum erythema, maximum induration, time to maximum erythema, time to
maximum induration, and frequency of axillary lymphadenopathy between the
3 dilution groups (P>.05 for each comparison).
Satellite lesions surrounding the vaccination site were noted in 51
volunteers (15.0%) and did not differ between dilution groups (Table 2 and Figure 1B).
At the Vanderbilt University School of Medicine site, suspected cases of satellite
lesions were cultured for vaccinia and 8 of 16 lesions tested positive. Rashes
away from the vaccination site were noted in 26 volunteers (7.6%), including
postvaccinia folliculitis, as described previously,3 in
15 volunteers. The incidence of these rashes did not differ between dilution
groups. One volunteer noted migratory arthralgias, fever, and nodular skin
lesions 2 months after vaccination; a complete rheumatologic evaluation and
serologic panel were negative. This condition resolved with anti-inflammatory
therapy. No episodes of autoinoculation were detected.
Volunteer-reported symptoms during the 2 weeks after vaccination are
shown in Table 3. Overall, 99.7%
of volunteers reported at least 1 symptom localized to the vaccination site;
92.1% reported local site pain, 96.8% described localized pruritus, and 87.1%
noted axillary pain. No significant differences in these local symptoms between
the various dilution groups were noted. In the 2 weeks postvaccination, volunteers
also noted a variety of systemic symptoms, including fever (21.5%; mean peak,
38.5°C) and malaise (79.1%). In general, symptoms occurred with greatest
frequency between 6 to 11 days after vaccination. Due to vaccine-related symptoms,
25% of volunteers missed scheduled duties at work or school. A difference
in systemic symptoms between the various dilution groups was not detected;
however, a significantly greater proportion of volunteers vaccinated with
the 1:5 dilution missed scheduled activities (33.3% vs 21.2% in 1:10 dilution
group and 20.4% in undiluted group, P = .04).
Although symptoms of cardiac disease were not actively elicited in volunteers
in this study, a post hoc analysis of volunteer symptom reports revealed 4
volunteers who reported chest pain or tightness and 1 additional volunteer
who reported exercise-associated dyspnea and tachycardia in the 2 weeks after
The first volunteer with chest pain noted 2 episodes of exercise-associated
stabbing chest pain 6 days after vaccination that lasted for approximately
20 minutes. These episodes resolved without specific therapy and were not
reported by the volunteer until the following day. Examination of the volunteer
at that time revealed mild tachycardia (pulse of 100) but no evidence of a
cardiac rub or gallop, an enlarged axillary lymph node, as well as a normal
pulmonary, extremity, and vascular examination.
A second volunteer presented with anterior chest and axillary pain 10
days after vaccination. This was not associated with dyspnea, diaphoresis,
or radiation and lasted a few hours. This volunteer had also noted multiple
systemic symptoms compatible with a vigorous vaccine response, including headache,
fatigue, site pruritus, and axillary pain. Full physical examination, including
auscultation of the heart and lungs and extremity assessment, revealed only
an enlarged axillary lymph node. These symptoms resolved with rest.
A third volunteer noted chest pain starting 5 days after vaccination
along with other signs of systemic reactogenicity, including headache, fatigue,
and axillary pain. Physical examination was without evidence of a pericardial
rub and lung fields were clear. The volunteer took ibuprofen as needed for
all systemic symptoms, which resolved 6 days later.
A fourth volunteer noted periodic episodes of chest tightness beginning
on the fifth postvaccination day that lasted for several minutes at a time.
This volunteer also had numerous symptoms of systemic inflammatory response
to the vaccine, such as headache, fatigue, axillary pain, and nausea. Physical
examination did not reveal any signs of pericarditis or ischemic heart disease.
Acetaminophen was taken as needed and these symptoms resolved after 8 days.
Finally, 8 days after vaccination, 1 volunteer noted a transient episode
of dyspnea and tachycardia during exercise that lasted for several minutes
and resolved with rest. Examination after symptom resolution revealed mild
tachycardia (pulse of 101), axillary lymphadenopathy, and normal cardiac,
pulmonary, and vascular examination results.
Because of the low index of suspicion for myopericarditis at the time,
further cardiac evaluation, including electrocardiogram and echocardiogram,
was not conducted. Each of the preceding volunteers was contacted at the 6-month
telephone follow-up. All symptoms resolved without residual sequelae and had
not recurred by 6 months postvaccination.
One study volunteer required hospitalization 1 week after vaccination
for nausea and dehydration that resolved completely within 24 hours after
intravenous hydration. A second volunteer presented with orthostatic dizziness
10 days after vaccination that rapidly cleared with oral hydration. Right
facial nerve palsy developed in 1 volunteer 37 days after vaccination, which
fully resolved with corticosteroid treatment. Another volunteer with a history
of ulcerative colitis diagnosed 10 years before vaccination presented with
a colitis flare 2 months after vaccination. Reinstitution of immunosuppressive
therapy with mesalamine was associated with resolution of symptoms.
Despite a negative urine pregnancy test on the day of vaccination, verbal
confirmation of a normal menstrual period the month before vaccination, and
continued counseling by study staff to avoid pregnancy, 1 volunteer had a
positive pregnancy test noted 26 days after vaccination. Her estimated date
of conception was 16 days before vaccination. The volunteer was counseled
on the risks of fetal vaccinia and opted to continue with the pregnancy. The
volunteer moved from the area before delivery and was lost to follow-up, despite
numerous attempts to locate her. Two other volunteers also became pregnant
at the end of the 2-month postvaccination follow-up (estimated dates of conception,
postvaccination days 38 and 56, respectively). One person delivered a healthy
neonate at term, while the second volunteer terminated her pregnancy during
the first trimester.
Of the 148 volunteers enrolled at Vanderbilt University School of Medicine
site, informed consent and successful serum collection at each time was obtained
from 109 volunteers for the immune response substudy measuring serum neutralizing
responses (undiluted, 34; 1:5 dilution, 36; 1:10 dilution, 39), all of whom
developed evidence of a clinical take (Figure
3). There were no significant differences between mean age, race,
ethnicity, or sex among those who consented for the substudy and those who
did not consent. Before vaccination, all volunteers had baseline reciprocal
antibody titers of less than 40; 1 month after vaccination all, except 1 volunteer
who received 1:10 dilution of vaccine, developed a neutralizing response.
At 1 month after vaccination, volunteers receiving the 1:5 dilution of vaccine
had significantly higher neutralizing titers than volunteers administered
the 1:10 dilution (P = .007). However, significant
differences in neutralizing antibody responses between the undiluted and 1:5
or 1:10 dilution groups were not detected.
Although manufactured nearly 50 years ago, APSV is associated with high
vaccination success rates, even at a 1:10 dilution. Therefore, the existing
supply of approximately 85 million doses of APSV can be expanded, leaving
an ample stockpile of smallpox vaccine to protect the entire US population
in the event widespread vaccination is imminently needed. With adequate supplies
of vaccine for the population of the United States, the potential exists for
sharing additional supplies with other countries as well. Reactogenicity to
the vaccine was also similar between the different dilutions of vaccine.
The ability to dilute vaccinia vaccines without a resultant decline
in success rates has also been reported with other vaccine preparations. Investigators
recently discovered that various dilutions of lyophilized vaccine had similar
vaccination success rates as undiluted doses, with more than 97% of vaccinees,
who received a 1:10 vaccine dilution, successfully developing a clinical take
after initial vaccination.2 In contrast with
the current study, a previous study with lyophilized vaccine reported that
local reactogenicity was significantly reduced with 1:5 and 1:10 dilutions.2 Those individuals vaccinated with undiluted lyophilized
vaccine had significantly larger areas of surrounding erythema and induration
and higher incidence of regional lymphadenopathy when compared with those
receiving 1:5 and 1:10 vaccine dilutions. Interestingly, those volunteers
vaccinated with the diluted lyophilized vaccine doses had a higher incidence
of satellite lesions, a finding not found in our study.
The clinical success rate of APSV in our study was mirrored by the humoral
immune response to vaccination observed in a subsample of the study. All volunteers
except 1 exhibited a 5-fold or more increase in plaque reduction neutralization
titers 1 month after vaccination, regardless of dilution of vaccine received.
These results are similar to immune responses reported after dilutions of
lyophilized vaccine.11 However, in contrast
with our study in which antibody titers were significantly lower in those
volunteers who received the 1:10 dilution compared with those vaccinated with
the 1:5 dilution, Belshe et al11 noted significantly
higher antibody titers between volunteers receiving a 1:10 dilution of vaccine
and volunteers receiving undiluted vaccine. The clinical significance of the
difference in titers between the 1:10 and the 1:5 dilution groups is unclear
as all volunteers in both groups developed a clinical take.
Clinical symptoms were common after APSV vaccination, as with other
vaccinia preparations, with all but 1 volunteer exhibiting local symptoms.
Systemic symptoms after vaccination were also quite common, as a majority
of volunteers reported malaise, headache, and fatigue. Seventy-three volunteers
(21.5%) developed fevers. This degree of reactogenicity, observed with other
vaccinia preparations, is not unexpected, as all smallpox vaccines are live
viral vaccines. Vaccine inoculation introduces live vaccinia virus intradermally
and leads to a localized infection.12 Many
of the postvaccination symptoms result from the vigorous inflammatory and
immune response after vaccination.
However, reactogenicity to APSV appears greater than what was observed
in a recent trial of vaccinia-naive volunteers vaccinated with the lyophilized
vaccine. Mean lesion size was larger in those volunteers vaccinated with APSV
(16 mm) than those vaccinated with lyophilized vaccine (12.4 mm).2 More volunteers in this study also developed fever
(21.5% vs 8.9%) and had detectable lymphadenopathy and satellite lesions than
volunteers vaccinated with the lyophilized vaccine.2 Nonetheless,
although both vaccines are derived from similar vaccinia stock, such comparisons
must be performed with caution, as our study did not directly compare APSV
with other vaccinia vaccines.
In our study, reactogenicity to the vaccine was not reduced with decreasing
strengths of vaccine. This may be explained by the fact that once inoculation
succeeds in creating a nidus of vaccinia infection and subsequent host immune
response, the resultant local and systemic reactions occur regardless of the
initial quantity of virus inoculated. The impact of the postvaccination symptoms
are not trivial, as 25% of vaccinees missed regularly scheduled activities
due to vaccine-related symptoms. Fortunately, these symptoms were generally
short-lived, with most volunteers returning to full function within 1 to 2
days. Although symptoms of cardiac disease were not actively sought from volunteers
at follow-up,4,10 5 volunteers
reported cardiac symptoms 5 to 14 days postvaccination that resolved without
sequelae. Retrospective analysis of these symptoms and physical examination
conducted at the time showed no evidence of moderate or severe myopericarditis
or cardiac ischemia.
The finding that local reactogenicity was significantly lower in women
is also unexpected. Although improved immune responses have been observed
in women following some vaccinations,13 studies
of other vaccines, such as the adsorbed anthrax vaccine, have noted increased
reactogenicity in women compared with men.14 Thus,
potential sex differences in vaccine response require further investigation.
Our investigation has a few limitations. As the study occurred before
the increased awareness of postsmallpox vaccination cardiac complications,
we did not prospectively elucidate the occurrence of such adverse events following
vaccination, and all descriptions of symptoms potentially cardiac were analyzed
retrospectively. Nonetheless, volunteers in our study were observed in person
every 3 to 5 days for the first month after vaccination. At each visit, volunteers
were actively assessed for systemic reactions to the vaccine and were reminded
at each study visit to report any visits to other clinicians. Thus, it is
unlikely that volunteers with clinically moderate to severe myopericarditis
or cardiac ischemia would have been missed during the postvaccination assessments.
Another limitation of our study is that, due to the early study closure, the
smaller sample size may have limited our ability to detect subtle differences
in reactogenicity between the dilution groups. However, given the excellent
compliance of the study participants, comprehensive evaluation of the study
population was achieved.
The future of smallpox vaccination in the United States and other countries
is unclear. From experiences in the 1960s and 1970s, smallpox vaccination
is an effective tool to prevent smallpox in persons before as well as after
exposure. Through its use and the concerted efforts of many individuals, wild
smallpox virus was eradicated from the globe.12 More
recently, the US military vaccination campaigns in 2003 successfully vaccinated
more than half a million persons,15 but the
unique adverse events and expected inflammatory reactions to the vaccine resulted
in concern about the use of smallpox vaccine in its current form.16-18 The civilian campaign,
likely impacted by these concerns, concluded in late 2003 after having vaccinated
more than 39 000 persons,19 a total well
below initial targets. Newer vaccine candidates, such as those derived from
cell-culture grown vaccinia virus, more attenuated vaccinia virus strains,
and component vaccines, are under development.
The results of our study show that a frozen preparation of APSV has
a high vaccination success rate and is an available option for smallpox vaccination
of vaccinia-naive persons, even at 10-fold diluted doses. This allows for
amplification of the current smallpox vaccine stockpile, if needed.