Vaccines - The principle of vaccination is to induce a "primed" state in the vaccinated subject so that, following exposure to a pathogen, a rapid secondary immune response is generated leading to the accelerated elimination of the organism and protection from clinical disease. Success depends on the generation of memory T and B cells and the presence in the serum of neutralizing antibody.
- Attributes of a good vaccine
- 1.Ability to elicit the appropriate immune response for the particular pathogen:
- Tuberculosis - cell mediated response
- most bacterial and viral infections - antibody
- 2. Long term protection ideally life-long 3. Safety vaccine itself should not cause disease 4. Stable retain immunogenicity, despite adverse storage conditions prior to administration 5. Inexpensive
Live Vaccines - 1. Live attenuated organisms
- Organisms whose virulence has been artificially reduced by in vitro culture under adverse conditions, such as reduced temperature. This results in the selection of mutants which replicate poorly in the human host and are therefore of reduced virulence. Replication of the vaccine strain in the host reproduces many of the features of wild type infection, without causing clinical disease. Most successful viral vaccines belong to this group.
- The immune response is usually good - when the virus replicates in the host cells, both antibody as well as cell mediated immune responses are generated and immunity is generally long lived. Often, only a single dose is needed to induce long term immunity.
- Schematic Diagram of Development of Attenuated Cell Strain
- 2. Heterologous vaccines
- Closely related organism of lesser virulence, which shares many antigens with the virulent organism. The vaccine strain replicates in the host and induces an immune response that cross reacts with antigens of the virulent organism. The most famous example of this type of vaccine is vaccinia virus: Both cowpox virus and vaccinia virus are closely related to variola virus, the causitive agent of smallpox. Widespread use of vaccinia virus as a vaccine has lead to the world-wide eradication of smallpox.
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