Vaccine is an immunological substance designed to confer specific protection against a given disease .It stimulates specific immune response (either humoral or cell mediated or both ) to generate specific protection against an infectious agent .
Vaccine may be prepared from
1- live modified organisms 2-inactivated 3-killed organism 4-toxoid
5-combination of these
live versus killed vaccines
live attenuated as well as inactivated vaccines for different diseases are available .The former are also called as replicating and the later as non replicating vaccines. The live vaccines are prepared from live and attenuated organisms which have lost their ability to induce disease but retain their immunogenicity .In general live vaccines are more potent immunizing agents because of the following reasons .