What are vaccines?
Vaccines are proteins (antigens) that initiate a protective immune response within an animal. In most cases the immune response involves antibodies that can block either the disease agent (bacteria or virus) or its product (toxin), so disease does not occur.
The first time an animal is exposed to an antigen, the immune response takes 7-21 days and may not be strong. This is why in natural infection, animals still develop disease and why many recover from disease over time. The disease triggers an immune response that includes the production of antibodies, which then reduce or neutralise the disease.
However, if the disease is severe, death or production losses will already have happened as the immune response is occurring. If an animal does survive, invariably the next time they are faced with the same challenge, their immune system is primed and ready, and the antibody response is much quicker and stronger, resulting in the disease having little or no effect. Therefore, most animals are considered ‘immune’ to a disease after they have had it.
Bovine ephemeral fever is a classic example of a lifelong immunity acquired from natural infection. However, the animal’s age when first infected influences the severity of the clinical signs experienced.
Vaccination aims to generate this immune response and make animals immune without (in general) giving them the disease or experiencing the effects of the disease.