Vaccines may comprise living, attenuated microorganisms, killed microorganisms, or purified bacterial and viral components (component vaccines).
Types Of Vaccine
Vaccines may comprise living, attenuated microorganisms, killed
microorganisms, or purified bacterial and viral components (component
vaccines). Recent innovations include the development of DNA vaccines that
encode for the transcription of antigen when introduced directly into host
tissues or vaccines that might be delivered nasally by non-pathogenic bacteria
(e.g. Lactococcus lactis) or otherwise by viral vectors (e.g. adenovirus). Some
aspects of these vaccine classes are discussed below.
Live, infective microorganisms, attenuated with respect to their
pathogenicity but retaining their ability to infect, can be used to confer
protective immunity. Two major advantages stem from the use of live vaccines.
First, the immunization mimics the course of a natural infection such that only
a single exposure is required to render an individual immune. Secondly, the
exposure may be mediated through the natural route of infection (e.g. oral),
thereby stimulating an immune response that is appropriate to a particular
disease (e.g. secretory antibody as a primary defence against poliomyelitis
virus in the gut with oral polio vaccine (OPV); see below). Disadvantages
associated with the use of live vaccines are also apparent. Live, attenuated
vaccines, administered through the natural route of infection, will replicate
in the patient and could be transmitted to others. If attenuation is reduced
during the replication process, infections might result (see OPV below). A
second, major disadvantage, of live vaccines is that the course of their
action, and possible side effects, might be affected by the infection and
immunological status of the patient.
Since these vaccines are unable to evoke a natural infection profile
with respect to the release of antigen, they must be administered on a number
of occasions. Immunity may not reach optimal levels until the course of
immunization is complete and, with the exception of toxin-dominated diseases
such as diphtheria and tetanus where the immunogen is a toxoid, are unlikely to
match the performance of a live vaccine. The specificity of the immune response
generated in the patient may initially be low. This is particularly the case
when the vaccine is composed of a relatively crude cocktail of killed cells,
where the immune response is directed only partially towards antigenic
components of the pathogen. This increases the possibility of adverse reactions
in the patient. Release profiles of these immunogens can be improved through
their formulation with adjuvants (Chapters 9 and 24), and the immunogenicity of
certain purified bacterial components such as polysaccharides can be improved
by their conjugation to a carrier.
A development associated with research into gene therapy has been the
use of DNA encoding specific virulence factors of defined pathogens to evoke an
immune response. The DNA is introduced directly into tissue cells by means of a
transdermal ‘gene gun’ and is transcribed by the recipient cells. Accordingly,
the host responds to the antigenic material produced as though it were an
infection. The course of release of the antigen reflects that of a natural
infection and, therefore, a highly specific response is invoked. Eventually the
introduced DNA is lost from the recipient cells and antigen release ceases. To
date, few experimental trials have demonstrated convincing protection in
humans, but this remains a promising approach. Protective immunity has been reported
in a trial of a human vaccine for bird flu and a West Nile virus vaccine for
horses has been approved.
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