Page 818 - Veterinary Immunology, 10th Edition

VetBooks.ir Vaccine Assessment

To assess the efficacy of a vaccine, animals must first be vaccinated
and then challenged. The percentage of vaccinated animals that

survive this challenge can then be measured. It is important,
however, to determine the percentage of non-vaccinated control
animals that also survive the challenge. The true efficacy of a
vaccine, called the preventable fraction (PF), is calculated as
follows:

For example, a challenge that kills 80% of controls and 40% of

vaccinates shows that the PF of the vaccine is as follows:

Good, effective vaccines should have a PF of at least 80%.
Obviously, less effective vaccines are acceptable if safe and if

nothing better is available. In determining vaccine efficacy,
however, large challenge doses may overwhelm any reasonable
level of vaccine-induced immunity. It is also important to

determine what degree of protection is desired. It may be much
easier to prevent deaths rather than illness.

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