Question: what is a vaccine?

Anne Moore answered on 6 Nov 2015:

A vaccine is a medicinal product that is used to prevent, rather than cure disease. All vaccines in use to date are used to prevent a person getting an infectious disease, such as influenza, measles, whooping cough,pneumonia etc. There are 26 human diseases that we can prevent with vaccines, there are also alot of veterinary vaccines.
A vaccine educates the immune system; it teaches it how to respond to virus or bacteria or parasite or fungus. Every vaccine contains a substance, called an “antigen” that is generally a protein, but can sometimes be a polysaccharide. This antigen is the same as the protein from the virus, bacteria or parastie and it is the part of that bug that the immune system recognises very well. The immune system then responds when it sees this antigen in an appropriate way. The vaccine will also sometimes contain substances called “adjvuants” which teach the immune system how to respond in the most appropriate way. For example, sometimes we need an antibody response to clear an invading bacteria, other times we need T cells to kill off infected cells.
The immune system develops a memory of how to respond to that antigen, so the next time it sees the antigen, when it’s in the bug, there is a higher and more rapid response. This better prepared, more educated response provides immediate protection from that infection and therefore the person doesn’t get sick. If enough people are protected against the bug, then the bug cannot live in the environment and go from person to person, this is called “herd immunity”. This gives a community-wide sheild against the bug.
Vaccines have been one of the most successful public interventions in history. Immunization against smallpox has eradicated it from the planet. This is the only example of a disease (any disease) which mankind has eradicated. Globally, polio virus is almost eradicated.
As vaccines are given to millions of healthy people, and to alot of babies, they must be incredibly safe to use. They also must be easy and cost-effective to make. But they still have to do their job of inducing an immune response!