Types of Vaccine

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.

1)  Live Vaccines

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.

2)  Killed And Component Vaccines

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.

3)  DNA Vaccines

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.