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.
Most 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’. This activates the immune system cells so that they kill the disease-causing virus or bacterium and make antibodies against it. Antibodies are special proteins that help kill the virus or bacterium.
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.
Some newer vaccines contain a molecule called mRNA instead of an antigen. This mRNA has instructions for making an antigen that is identical to a small part of a real virus.
When a person is given an mRNA vaccine, some of their cells read the mRNA instructions. These cells then produce the antigen for a short time before they break the mRNA down. The person’s immune system recognises the antigen as ‘foreign’ in the same way, activating immune cells and making antibodies. Some COVID-19 vaccines are mRNA vaccines.