Living Organism as Vaccines (with Attenuated Virulence) -

Some vaccines involve use of organisms, whose virulence is greatly attenuated or diminished. These organisms are unable to produce disease but induce immunity when injected in the host. There are at least five different ways of producing attenuated strains of pathogens:

(i) many successive passages of pathogen in some animals other than the usual host of the pathogen or in cultured cells.

(ii) Selection of mutants of low virulence.

(iii) Treatment of pathogens with chemicals.

(iv) Cultivation of pathogens under un favourable conditions such as high temperature.

(v) Deletion or insertion of genes leading to attenuation of pathogens as tried for vaccinia virus.