CLINICAL PHARMACOLOGY
Typhoid fever is an infectious disease caused by S
typhi. Humans are the only natural host and reservoir for S typhi;
infections result from the consumption of food or water that has been contaminated
by the excretions of an acute case or a carrier. S typhi organisms
efficiently invade the human intestinal mucosae ultimately leading to
bacteremia; following a typical 10- to 14-day incubation period, a systemic
illness occurs. The clinical presentation of typhoid fever exhibits a broad
range of severity and can be debilitating. Classical cases have fever, myalgia,
anorexia, abdominal discomfort and headaches; the fever increases step-wise
over a period of days and then may remain at 102°F to 106°F over 10 to 14 days
before decreasing in a step-wise manner. Skin lesions known as rose spots may
be present. Constipation is common in older children and adults, while diarrhea
may occur in younger children. Among the less common but most severe complications
are intestinal perforation and hemorrhage, and death. The course is typically
more severe without appropriate antimicrobial therapy. The case fatality rate
was reported to be approximately 10% to 20% in the pre-antibiotic era. 1,2,3
During the period of 1983 to 1991 in the US, the case fatality rate reported to
the Centers for Disease Control and Prevention (CDC) was 0.2% (9/4010). 4
Infection of the gallbladder can lead to the chronic carrier state.
Typhoid fever is still endemic in many countries of the
world where it is predominantly a disease of school-age children and may be a
major public health problem. Most cases of typhoid fever in the US are thought
to be acquired during foreign travel. During the periods of 1975 to 1984 and 1983
to 1984, respectively, 62% and 70% of the cases of typhoid fever reported to
the CDC were acquired during foreign travel; this compares to 33% of cases
during 1967-1972. 5
In 1992, 414 cases of typhoid fever were reported to the
CDC. Of these 414 cases, 1 (0.2%) case occurred in an infant under one year of
age; 77 (18.6%) cases occurred in persons one to nine years of age; 81 (19.6%)
cases occurred in persons 10 to 19 years of age; 251 (60.6%) cases occurred in
individuals ≥ 20 years of age; the age was not available for 4 (1%) cases.
One death was reported in 1991. (4) Domestic surveillance could underestimate
the risk of typhoid fever in travelers since the disease is unlikely to be
reported for persons who received diagnosis and treatment overseas.6
Approximately 2% to 4% of acute typhoid fever cases
develop into a chronic carrier state. The chronic carrier state occurs more
frequently with advanced age, and among females than males.2,7 These
non-symptomatic carriers are the natural reservoir for S typhi and can
serve to maintain the disease in its endemic state or to directly infect new
individuals. Outbreaks of typhoid fever are often traced to food handlers who
are asymptomatic carriers.8
Two formulations were utilized in studies of the Typhoid
Vi Polysaccharide Vaccine. These included the liquid formulation which is
identical to Typhim Vi vaccine and a lyophilized formulation.
The protective efficacy of each of these formulations of
the Typhoid Vi Polysaccharide Vaccine was assessed independently in two trials
conducted in areas where typhoid fever is endemic. A single intramuscular dose
of 25 mcg was used in these efficacy studies. A randomized doubleblind controlled
trial with Typhim Vi vaccine (liquid formulation) was conducted in five
villages west of Katmandu, Nepal. There were 6,908 vaccinated subjects: 3,454
received Typhim Vi vaccine and 3,454 in the control group received a 23-valent
pneumococcal polysaccharide vaccine. Of the 6,908 subjects, 6,439 subjects were
in the target population of 5 to 44 years of age. In addition, 165 children
ages 2 to 4 years and 304 adults over 44 years of age were included in the
study. The overall protective efficacy of Typhim Vi vaccine was 74% (95%
confidence interval (CI): 49% to 87%) for blood culture confirmed cases of
typhoid fever during 20 months of post-vaccination follow-up.9,10,11
The protective efficacy of the Typhoid Vi Polysaccharide
Vaccine, lyophilized formulation, was evaluated in a randomized double-blind
controlled trial conducted in South Africa. There were 11,384 vaccinated
children 5 to 15 years of age; 5,692 children received the Vi capsular polysaccharide
vaccine and 5,692 in the control group received Meningococcal Polysaccharide (Groups
A+C) Vaccine. The protective efficacy for the Vi capsular polysaccharide
(lyophilized formulation) group for blood culture confirmed cases of typhoid
fever was 55% (95% CI: 30% to 70%) overall during 3 years of post-vaccination
follow-up, and was 61%, 52% and 50%, respectively, for years 1, 2, and 3.
Vaccination was associated with an increase in anti-Vi antibodies as measured
by radioimmunoassay (RIA) and enzyme-linked immunosorbent assay. Antibody
levels remained elevated at 6 and 12 months post-vaccination. 11,12
Because of the low incidence of typhoid fever, efficacy
studies were not feasible in a US population.
Controlled comparative efficacy studies of Typhim Vi
vaccine and other types of typhoid vaccines have not been performed.
An increase in serum anti-capsular antibodies is thought
to be the basis of protection provided by Typhim Vi vaccine. However, a
specific correlation of post-vaccination antibody levels with subsequent
protection is not available, and the level of Vi antibody that will provide
protection has not been determined. Also, limitations exist for comparing
immunogenicity results from subjects in endemic areas, where some subjects have
baseline serological evidence of prior S typhi exposure, to naive
populations such as most American travelers.
In endemic regions (Nepal, South Africa, Indonesia) where
trials were conducted, pre-vaccination geometric mean antibody levels suggest
that infection with S typhi had previously occurred in a large
percentage of the vaccinees. In these populations, specific antibody levels
increased fourfold or greater in 68% to 87.5% of older children and adult
subjects following vaccination. For 43 persons 15 to 44 years of age in the
Nepal pilot study, geometric mean specific antibody levels pre- and 3 weeks
post-vaccination were, respectively, 0.38 and 3.68 mcg antibody/mL by RIA; 79%
had a four-fold or greater rise in Vi antibody levels.9,12
Immunogenicity and safety trials were conducted in an
adult US population. A single dose of Typhim Vi vaccine induced a four-fold or
greater increase in antibody levels in 88% and 96% of this adult population for
2 studies, respectively, following vaccination (see Table 1). 10,13
Table 1 10,13: Vi ANTIBODY LEVELS IN US
ADULTS 18 TO 40 YEARS OF AGE GIVEN TYPHIM Vi VACCINE
|
N |
GEOMETRIC MEAN ANTIBODY LEVELS (mcg antibody/mL by RIA) |
% ≥ 4 FOLD INCREASE (95% CI) |
Pre (95% CI) |
Post (4 weeks) |
Trial 1 (1 lot) |
54 |
0.16
(0.13 to 0.21) |
3.23
(2.59 to 4.03) |
96%
(52/54)
(87% to 100%) |
Trial 2 (2 lots combined) |
97 |
0.17
(0.14 to 0.21) |
2.86
(2.26 to 3.62) |
88%
(85/97)
(81% to 94%) |
No studies of safety and immunogenicity have been
conducted in US children. A double-blind randomized controlled trial evaluating
the safety and immunogenicity of Typhim Vi vaccine was performed in 175
Indonesian children. The percentage of 2- to 5-year-old children achieving a four-fold
or greater increase in antibody levels at 4 weeks post-vaccination was 96.3%
(52/54) (95% CI: 87.3% to 99.6%), and in the study subset of 2-year-old
children was 94.4% (17/18) (95% CI: 72.7% to 99.9%). The geometric mean
antibody levels (mcg antibody/mL by RIA) for the 2-to 5-year-old children and
the subset of 2-year-olds were, respectively, 5.81 (4.36 to 7.77) and 5.76
(3.48 to 9.53). 10,11
In the US Reimmunization Study, adults previously
immunized with Typhim Vi vaccine in other studies were reimmunized with a 25
mcg dose at 27 or 34 months after the primary dose. Data on antibody response
to primary immunization, decline following primary immunization, and response
to reimmunization are presented in Table 2. Antibody levels attained following reimmunization
at 27 or 34 months after the primary dose were similar to levels attained following
the primary immunization.10,13 This response is typical for a T-cell
independent polysaccharide vaccine in that reimmunization does not elicit
higher antibody levels than primary immunization. The safety of reimmunization
was also evaluated in this study (see ADVERSE REACTIONS section).
Table 2 10,13: US STUDIES IN 18- TO
40-YEAR-OLD ADULTS: KINETICS AND PERSISTSENCE OF Vi ANTIBODY* RESPONSE TO
PRIMARY IMMUNIZATION WITH TYPHIM Vi VACCINE, AND RESPONSE TO REIMMUNIZATION AT
27 OR 34 MONTHS
|
PREDOSE 1 |
1 MONTH |
11 MONTHS |
18 MONTHS |
27 MONTHS |
34 MONTHS |
1 MONTH POST-REIMMUNIZATION¶ |
GROUP 1† |
N |
43 |
43 |
39 |
ND§ |
43 |
ND |
43 |
Level* 95% CI |
0.19 (0.14-0.26) |
3.01 (2.22-4.06) |
1.97 (1.31-3.00) |
|
1.0711 (0.71-1.62) |
|
3.04 (2.17-4.26) |
GROUP 2‡ |
N |
12 |
12 |
ND |
10 |
ND |
12 |
12 |
Level 95% CI |
0.14 (0.11-0.18) |
3.78 (2.18-6.56) |
|
1.21 (0.63-2.35) |
|
0.7611 (0.37-1.55) |
3.31 (1.61-6.77) |
* mcg antibody/mL by RIA
† Group 1: Reimmunized at 27 months following primary immunization.
* Group 2: Reimmunized at 34 months following primary immunization.
§ Â Not Done.
|| Antibody levels pre-reimmunization.
¶ Includes available data from all reimmunized subjects (subjects initially
randomized to Typhim Vi vaccine, and subjects initially randomized to placebo
who received open label Typhim Vi vaccine two weeks later). |
Concurrently Administered Vaccines
Concomitant Administration of Typhim Vi and Menactra vaccine
In a double-blind, randomized, controlled clinical trial,
945 participants aged 18 through 55 years received Typhim Vi and Menactra
vaccines concomitantly (N=469), or Typhim Vi vaccine followed one month later
by Menactra vaccine (N=476). Sera were obtained approximately 28 days after
each respective vaccination. The antibody response to Typhim Vi vaccine and to Menactra
vaccine components were similar between groups.
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47: 510-550, 1983
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Vaccines, Plotkin SA, Mortimer EA, eds. W.B. Saunders, 1988
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and Immunotherapy. Stanley J. Cryz, Jr., Editor. pp 59-72, 1991
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11:1-8, 1989
6 Woodruff BA, et al. A new look at typhoid vaccination.
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7 Ames WR, et al. Age and sex as factors in the
development of the typhoid carrier state, and a method for estimating carrier
prevalence. Am J Public Health 33: 221-230, 1943
8 CDC. Typhoid fever - Skagit County, Washington. MMWR
39: 749-751, 1990
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with the Vi capsular polysaccharide of Salmonella typhi. N Engl J Med 317:
1101-1104, 1987
10 Unpublished data available from Sanofi Pasteur Inc.,
compiled 1991
11 Unpublished data available from Sanofi Pasteur SA
12 Klugman KP, et al. Protective activity of Vi capsular
polysaccharide vaccine against typhoid fever. The Lancet, 1165-1169, 1987
13 Keitel WA, et al. Clinical and serological responses
following primary and booster immunization with Salmonella typhi Vi capsular
polysaccharide vaccines. Vaccines 12: 195-199, 1994