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Killed vs. Modified-Live Vaccines


For some antigens, veterinarians have to decide whether to use a killed vaccine or a modified-live vaccine. Each vaccine has its advantages, and each has its disadvantages. Most killed vaccines contain an adjuvant system, whose purpose is to stimulate an immune response. Without an adjuvant, the body is less likely to produce an effective response to the disease antigen. This discussion will cover the differences between killed and modified-live vaccines.


In order to make an informed decision regarding killed and modified-live vaccines, it is important to know the generally accepted characteristics of each type of vaccine advantage and disadvantage.

Killed Vaccine (inactivated)


No reversion to virulence

Safe in pregnant animals

Safe in immunocompromised animals

No shedding of the virus



Two doses often needed for initial response


Modified-live Vaccine


Less frequent transient adverse responses than killed vaccine

Parenteral or mucosal administration



Higher potential for vaccine-induced disease

Vaccinal virus shed

Viral replication implies:

  • Possible risk in pregnant animals

  • Possible risk in immunocompromised animals


Short shelf life when reconstituted

Easily inactivated by mishandling

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