There are several factors that can have a negative influence on the efficacy of a vaccination. When storage and handling are excluded, the next factor of importance is the animal itself.
Vaccination mimics natural infection. It is true for the majority of vaccines that both protective antibodies and protective immune cells are formed after natural infection and after vaccination.
A certain class of protective antibodies is especially important – the so-called neutralising antibodies. As the word says the purpose of these antibodies is to neutralise the invading bacteria orviruses and thereby protect the animal from infection.
This is a desired effect. However, when thein vading bacteria or virus is a vaccine strain, then this is an undesired effect because the neutralised bacteria or virus will not stimulate the immune system.