VMC 221: Veterinary Immunology and Serology (1+1)

LIVE ATTENUATED VACCINES

• The virulence of the pathogen is reduced and immunogenicity is maintained by adapting the pathogen in an unfavourable condition and the organism still replicates.
• Attenuation is achieved by growing the pathogens in an unnatural host, by passaging in non homologous host (host/cell culture) for repeated period of time (i.e. 70-80 times) or in different physiological conditions or in different environment.
• Attenuation may also be done by adapting the virus to grow in a temperature lower than the normal called cold adapted virus and the process is called cold adaption.
• Thermo stable vaccine strain grows at elevated temperature.
• Temperature sensitive mutants cannot grow at slightly elevated temperature.
• The process of reducing the virulence and retaining the immunogenicity is called as attenuation so that the pathogen changes its habit of growing.
• Advantages 
  • Replication provides large quantities of immunogen
  • There is no need for adjuvant
  • Single dose often produce long lasting immunity
  • Whole organism has both T and B epitopes 
  • The vaccine is cost effective and often does not require booster vaccination
  • Can be effective against intracellular pathogens
• Disadvantages
  • Chance of reversion to virulence
  • There may be shedding of virus
  • Can induce transient immunosupression
  • Cold chain required for transport
  • Possible contamination with other animal viruses
  • There may be side effects due to unwanted parts of the vaccines.