Each virus and bacterium triggers a unique response in the immune system involving a specific set of cells in the blood, in the bone marrow and all over the body, called T-cells, B-cells, among others.
A vaccine stimulates an immune response, and a ‘memory’ of the body to a specific disease, without causing the disease.
Vaccines contain a greatly weakened or an inactivated (killed) form of a virus or bacterium that usually causes a disease, or a small part of the virus or bacterium. This is called the antigen.
When a person is given the vaccine, their immune system recognises the antigen as ‘foreign’. The immune cells are trained to kill and make antibodies against the disease-causing agent. Antibodies are special proteins that help kill the infectious agent.
Later, if the person comes into contact with the actual infectious virus or bacterium, their immune system will ‘remember’ it. It will then quickly produce the right antibodies and activate the right immune cells to kill the virus or bacterium, protecting the person from the disease.
Immunity usually lasts for years, and sometimes for a lifetime. The length of time varies by disease and by vaccine.
Immunity through vaccination protects not only the immunised individual but also protects unvaccinated people in the community, such as infants who are too young to be vaccinated. This ‘community immunity’ can only work if enough people are vaccinated.
In contrast, a person who becomes immune by getting the disease can expose other unvaccinated people to the disease. The person is also at risk of complications.