Page 292 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 36 Viral Vaccines
There are three concerns about the use of live vaccines:
more easily in tropical climates.
(1) They are composed of attenuated viral mutants,
Most viral vaccines are usually given before a known
which can revert to virulence either during vaccine pro-
exposure (i.e., they are administered preexposure). How-
duction or in the immunized person. Reversion to viru-
ever, there are two vaccines, the vaccines against rabies and
lence during production can be detected by quality control
hepatitis B that are also effective when given postexposure
testing, but there is no test to predict whether reversion will
occur in the immunized individual. Of the commonly used
enough that the vaccine-induced immunity can prevent
live vaccines, only polio vaccine has had problems regard-
the disease. Thus the rabies vaccine is most often used in
ing revertants; measles, mumps, rubella, and varicella vac- because the incubation period of these diseases is long
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people after they have received a bite from a potentially
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mebooksfree.com mebooksfree.com mebooksfree.com tages of current vaccines will be bypassed by the use of mebooksfree.com
cines have not.
rabid animal, and the hepatitis B vaccine is used in people
Even if the virus in the live vaccine does not revert, it
who have sustained a needle-stick injury.
can still cause disease because, although attenuated (weak-
The prospect for the future is that some of the disadvan-
ened), it can still be pathogenic in a host with reduced
immunity. For this reason, live viral vaccines should not be
purified viral antigens produced from genes cloned in
given to immunocompromised people or to pregnant
either bacteria or yeasts by recombinant DNA techniques.
women because the fetus may become infected.
The advantages of antigens produced by the cloning
(2) The live vaccine can be excreted by the immunized
process are that they contain no viral nucleic acid and
person. This is a double-edged sword. It is advantageous if
so cannot replicate or revert to virulence, they have no
the spread of the virus successfully immunizes others, as
occurs with the live polio vaccine. However, it could be a
produced in large amounts. A disadvantage of these cloned
problem if, for example, a virulent poliovirus revertant
vaccines is that they are unlikely to stimulate a cytotoxic
spreads to a susceptible person. Rare cases of paralytic contaminating viruses from cell culture, and they can be
T-cell response because no viral replication occurs. The
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polio occur in the United States each year by this route of
two viral subunit vaccines containing purified anti-
infection.
gens produce by recombinant DNA techniques are the
(3) A second virus could contaminate the vaccine if it
was present in the cell cultures used to prepare the vac-
virus (HPV) vaccine.
cine. This concern exists for both live and killed vaccines,
Another prospect for the future is the use of “DNA vac-
although, clearly, the live vaccine presents a greater prob-
cines.” These vaccines contain purified DNA encoding the
lem, because the process that inactivates the virus in the
appropriate viral proteins genetically engineered into a
killed vaccine could inactivate the contaminant as well. It
viral vector or plasmid. Immunization with this composite
is interesting, therefore, that the most striking incidence
DNA elicits both antibody and cytotoxic T cells and pro-
of contamination of a vaccine occurred with the killed
polio vaccine. In 1960, it was reported that live simian
Certain live viral vaccines, such as the vaccines contain-
vacuolating virus 40 (SV40 virus), an inapparent “passen-
ing vaccinia virus, adenovirus, and poliovirus, are being
ger” virus in monkey kidney cells, had contaminated some tects against disease in experimental animals.
used experimentally to immunize against other viruses
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lots of polio vaccine and was resistant to the formaldehyde
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such as HIV. This is done by splicing the HIV gene into the
used to inactivate the poliovirus. There was great concern
live viral genome and then infecting the experimental ani-
when it was found that SV40 virus causes sarcomas in a
variety of rodents. Fortunately, it has not caused cancer in
cedure is that a cytotoxic T-cell response is elicited (because
the individuals inoculated with the contaminated polio
the virus is replicating), whereas if the purified antigen
vaccine.
alone were used to immunize the animal, an antibody
response but not a cytotoxic T-cell response would be
Certain viral vaccines, namely, influenza, measles,
mumps, and yellow fever vaccines, are grown in chick
elicited.
The viral vaccines currently in use are described in
embryos. These vaccines should not be given to those who
have had an anaphylactic reaction to eggs. People with
mended for children from 0 to 6 years of age are listed in
allergies to chicken feathers can be immunized.
Table 36–3.
In addition to the disadvantages of the killed vaccines
already mentioned—namely, that they induce a shorter Table 36–2. The vaccines, both viral and bacterial, recom-
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mebooksfree.com mebooksfree.com mebooksfree.com preformed antibody in preparations called immune globu- mebooksfree.com
duration of protection, are less protective, and induce
PASSIVE IMMUNITY
fewer IgA antibodies—there is the potential problem that
the inactivation process might be inadequate. Although
Passive immunity is provided by the administration of
this is rare, it happened in the early days of the manufacture
lins. The immune globulins useful in the prevention of viral
of the killed polio vaccine. However, killed vaccines do
diseases are described next. Passive–active immunity is
have two advantages: They cannot revert to virulence, and
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