Immunity and vaccination : live or inactivated?

Vaccines are complex products comprising a number of different components. In their simplest form, we can regard them as containing one or more antigens - the main 'active ingredients' designed to stimulate the immune system.

Veterinary vaccines are traditionally classified in two main categories – modified live (attenuated) and killed (inactivated).

Modified live vaccines typically stimulate broader immune response because their antigens are presented in a way which mimics natural infection. However, they are sufficiently weakened (attenuated) to avoid causing disease.

Killed vaccines tend to take longer to stimulate the immune system and many require at least two initial doses for optimal protection. In some, adjuvants are added to enhance the immune response. Although adjuvants improve the effectiveness of many vaccines, they can sometimes increase the risk of reactions, such as transient local nodules at the site of infection or temporary malaise post vaccination.

Increasingly, some modern vaccines utilise genetic engineering techniques in their production. These can broadly be grouped into three main sub categories, namely subunit, gene deletion and vector vaccines.

Subunit vaccines contain a component of the infectious agent rather than the whole organism. The component used is a protein which is purified and is responsible for stimulating protective antibodies.

Gene deletion vaccines irreversibly attenuate a live vaccine organism so it lacks the ability to induce disease, whilst the vector vaccines use the genetic code for the desired antigens and insert them into a harmless virus which is then used to infect the vaccinated animal but does not cause disease.

For further information on vaccine types, please contact your local veterinary practice.