Live attenuated vaccines
Live vaccines contain weakened forms of the organism that causes the disease. Such organisms are also called attenuated. There are many examples of highly successful vaccines that have been developed using a live attenuated organism or virus. The current vaccine for tuberculosis, BCG, containes an attenuated form of a mycobacteria. Attenuated forms of many viruses have been prepared for use as vaccines to control diseases such as polio, measles, and mumps.
Often, the attenuated form of the organism [or virus] is obtained by serial passage [or culture] of the active organism in culture media or cells. In these cases, the molecular basis of attenuation is unknown. Today, it is likely that regulatory agencies would require an understanding of the basis of attenuation. Therefore, development of any new attenuated form of mycobacteria for use as a candidate vaccine is likely to include the introduction of one or more specific mutations into the genome of the pathogen. Likely candidates include mutations that interfere with synthesis of an amino acid or nucleic acid component essential for the growth of the organism.
Some examples of such live vaccines have already been prepared and evaluated in preclinical trials. One is a recombinant form of BGC called rBCG30. This strain overproduces and secretes a 30 kDa protein from M. tuberculosis.