Vaccines produce their protective effect by inducing active immunity and providing immunological memory. Immunological memory enables the immune system to recognise and respond rapidly to exposure to natural infection at a later date and thus to prevent or modify the disease.
There are two forms of immunity, acquired and innate. Acquired immunity can be from a vaccination or transferred across the placenta at birth, innate immunity is something that the individual is born with and includes physical barriers such as the skin and chemical barriers such as the digestive system.
From birth and in early infancy and childhood, people are exposed to countless numbers of foreign antigens and infectious agents in the everyday environment. Responding to these stimuli helps the immune system to develop and mature. Compared with exposure in the natural environment, vaccines provide specific stimulation to a small number of antigens.
The main aim of vaccinations is to protect the individual who receives the vaccination against specific diseases. In addition to the immunity gained by the individual, they are also less likely to be a source of infection to others therefore reducing the risk to unvaccinated individuals. This concept is called population or herd immunity. An example of one such programme is the whooping cough vaccination programme, babies under the age of two months are too young to be immunised but are at the greatest risk of dying if they contract whooping cough, however these babies are protected because their older siblings and children have been routinely immunised as part of the whooping cough programme.
It is possible to eliminate the presence of a disease if the vaccination coverage is high enough, for example diphtheria.
However if high vaccination coverage were not maintained, it would be possible for the disease to return.
