CLINICAL PHARMACOLOGY
Salmonella typhi is the etiological agent of
typhoid fever, an acute, febrile enteric disease. Typhoid fever continues to be
an important disease in many parts of the world. Travelers entering infected
areas are at risk of contracting typhoid fever following the ingestion of
contaminated food or water. Typhoid fever is considered to be endemic in most
areas of Central and South America, the African continent, the Near East and
the Middle East, Southeast Asia and the Indian subcontinent (3). There are
approximately 500 cases of typhoid fever per year diagnosed in the United
States (4). In 62% of these patients (data from 1975â⬓1984) the disease was
acquired outside of the United States while in 38% of the patients the disease
was acquired within the United States (5). Of 340 cases acquired in the United
States between 1977 and 1979, 23% of the cases were associated with typhoid carriers,
24% were due to food outbreaks, 23% were associated with the ingestion of
contaminated food or water, 6% due to household contact with an infected person
and 4% following exposure to S. typhi in a laboratory setting (6).
The majority of typhoid cases respond favorably to
antibiotic therapy. However, the emergence of multi-drug resistant strains has
greatly complicated therapy and cases of typhoid fever that are treated with
ineffective drugs can be fatal (7). Approximately 2â⬓4% of acute typhoid cases
result in the development of a chronic carrier state (8). 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 individuals
(3).
Virulent strains of S. typhi upon ingestion are
able to pass through the stomach acid barrier, colonize the intestinal tract,
penetrate the lumen and enter the lymphatic system and blood stream, thereby
causing disease. One possible mechanism by which disease may be prevented is by
evoking a local immune response in the intestinal tract. Such local immunity
may be induced by oral ingestion of a live attenuated strain of S. typhi
undergoing an aborted infection. The ability of S. typhi to cause
disease and to induce a protective immune response is dependent upon the
bacteria possessing a complete lipopolysaccharide (1). The S. typhi Ty21a
vaccine strain, by virtue of a reduction in enzymes essential for lipopolysaccharide
biosynthesis, is restricted in its ability to produce complete
lipopolysaccharide (1,2). However, a sufficient quantity of complete
lipopolysaccharide is synthesized to evoke a protective immune response.
Despite low levels of lipopolysaccharide synthesis, the cells lyse before regaining
a virulent phenotype due to the intracellular build-up of intermediates during lipopolysaccharide
synthesis (1,2).
Results from clinical studies indicate that adults and
children greater than 6 years of age may be protected against typhoid fever
following the oral ingestion of 4 doses of Vivotif (Typhoid Vaccine Live Oral
Ty21a). The efficacy of the S. typhi Ty21a strain has been evaluated in
a series of randomized, double-blind, controlled field trials. Suspected
typhoid cases, detected by passive surveillance, were confirmed bacteriologically
either by blood or bone marrow culture. The first trial was performed in
Alexandria, Egypt with a study population of 32,388 children aged 6 to 7 years.
3 doses of vaccine, in the form of a freshly reconstituted suspension
administered after ingestion of 1 g of bicarbonate, were given on alternate
days. Immunization resulted in a 95% decrease [95% confidence interval (CI) =
77%â⬓99%] in the incidence of typhoid fever over a 3-year period of surveillance
(9). A series of field trials were subsequently performed in Santiago, Chile to
evaluate efficacy when the vaccine strain was administered in the form of an
acid-resistant enteric-coated capsule. The initial trial involved 82,543
school-aged children, and compared 1 or 2 doses of vaccine given one week
apart. After 24 months of surveillance vaccine efficacy was 29% (95% CI =
4%â⬓47%) for the single dose schedule and 59% (95% CI = 41%â⬓ 71%) for the 2-dose
schedule (10). A further field trial was performed in Santiago, Chile involving
109,594 school-aged children (11). 3 doses of enteric-coated capsules were
administered either on alternate days (short immunization schedule) or 21 days
apart (long immunization schedule). Following 36 months of surveillance
vaccination resulted in a 67% (95% CI = 47%â⬓79%) decrease in the incidence of
typhoid fever in the short immunization schedule group and a 49% reduction (95%
CI = 24%â⬓66%) in the long immunization schedule group. After 48 months of surveillance
the short immunization schedule resulted in a 69% (95% CI = 55%â⬓80%) decrease
in typhoid fever (12). An undiminished level of protection was observed during
the fifth year of surveillance. A field trial was next conducted in Santiago,
Chile to determine the relative efficacy of 2, 3 and 4 doses of enteric-coated
vaccine administered on alternate days to school-aged children. Relative
vaccine efficacy as determined by comparison of disease incidence within the 3
vaccinated groups was highest for the 4 dose regimen (13). The incidence of
typhoid fever per 105 study subjects was 160.5 (95% CI = 130â⬓191) for the 3
dose regimen versus 95.8 (95% CI = 71â⬓121) for the 4 dose regimen (p < 0.004).
An additional field trial to determine vaccine efficacy was conducted in Plaju,
Indonesia involving 20,543 individuals approximately 3 to 44 years of age (14).
Due to logistical considerations 3 doses of enteric-coated capsules were
administered at weekly intervals, a schedule known to provide suboptimal
protection (11). After 30 months of surveillance vaccine efficacy for all age groups
was 42% (95% CI = 23%â⬓57%). Vaccine organisms can be shed transiently in the
stool of vaccine recipients (16). However, secondary transmission of vaccine
organisms has not been documented. Ty21a has not been isolated from blood
cultures following immunization. At present, the precise mechanism(s) by which
Vivotif confers protection against typhoid fever is unknown. However, it is
known that immunization of adult subjects can elicit a humoral anti-S. typhi
LPS antibody response. Taking advantage of this fact, the seroconversion rate
(defined as a ≥ 0.15 increase in optical density units over baseline
determined in an ELISA) was compared in an open study between adults living in
an endemic area (Chile) and non-endemic areas (United States and Switzerland)
after the ingestion of 3 doses of vaccine. Comparable seroconversion rates were
seen between these groups (15). S. typhi Ty21a cultured in medium not
containing BHI induced an anti-S. typhi LPS antibody response comparable
to that obtained with vaccine organisms cultured in medium containing BHI (15).
Challenge studies in North American volunteers have shown that the Ty21a strain
is capable of providing significant protection to an experimental challenge of S.
typhi (16). Because of the very low incidence of typhoid fever in United
States citizens, efficacy studies are not currently feasible in this
population. However, the above observations support the expectation that
Vivotif will provide protection to recipients from non-typhoid endemic areas
such as the United States.
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