How vaccines work

Vaccines are widely and routinely administered around the world, based on the common-sense principle prevention is better than treatment.

Vaccines typically provide the immune system with harmless copies of an antigen: a portion of the surface of a bacterium or virus that the immune system recognizes as foreign (an antigen often plays a role in causing disease – for example by enabling a virus or bacterium to attach to cells). A vaccine may also provide a non-active version of a toxin – a poison produced by a bacterium – so that the body can devise a defence against it.

Once the immune system detects an antigen, white blood cells called B-lymphocytes create a protein called an antibody that is precisely designed to attach to that antigen. Many copies of this antibody are produced. If a true infection with the same disease occurs, still more antibodies are created, and as they attach to their targets they may block the activity of the virus or bacterial strain directly, thus combating infection. In addition, antibodies, once in place, make it much easier for other components of the immune system (particularly phagocytes) to recognize and destroy the invading agent.

Building up immunity

Immune systems are designed to remember; once exposed to a particular bacterium or virus, they retain immunity against it for years, decades or even a lifetime and are thus prepared to defeat a later infection quickly. A body encountering a germ for the first time may need 7–12 days to mount an effective defence, by which point serious illness and even death can occur. It is therefore hugely beneficial to use the immune system’s “memory” of a bacterium or virus to quickly and effectively defeat it.


All vaccines used for routine immunization are very effective in preventing disease, although none is 100% effective. Thus, more than one dose of a vaccine is usually given to increase the chance of developing immunity.