Vaccines

Vaccination is a way of preventing communicable disease. Rather than waiting for a dangerous pathogen to infect the body, a vaccine introduces a safe form of the pathogen, or material carrying its antigens. Depending on the vaccine, the pathogen may be dead, inactive or weakened. The important point is that its antigens can stimulate an immune response without causing the disease the vaccine is designed to prevent.

Antigens are molecules that the immune system recognises as foreign. White blood cells with matching receptors recognise the vaccine antigens and produce specific antibodies. Each antibody fits a particular antigen, so immunity to one pathogen does not automatically protect against a different one.

The safe material is removed, but some of the activated white blood cells remain as memory cells. The first response takes time to develop; its main value is that it prepares the body for the future.

What happens during a later infection

If the real pathogen later enters the body, memory cells recognise its familiar antigens. They produce the correct antibodies much more quickly and in greater amounts than during the first response. The pathogen can therefore be destroyed before it causes disease, or before serious symptoms develop. This protection is immunity.

Vaccines are therefore preventive rather than treatments for an infection that has already developed. They train the immune system in advance; they do not work by directly killing every pathogen that later enters the body.

From individual immunity to herd immunity

Vaccination can also reduce spread through a whole population. Imagine a chain of transmission: an infected person passes a pathogen to another susceptible person, who passes it to someone else. If many people are immune, the pathogen repeatedly reaches people in whom it cannot establish disease and continue spreading easily. Chains of transmission are more likely to break.

This population effect is called herd immunity. It gives some indirect protection to people who are not immune because they are less likely to encounter an infected person. This is particularly important for people who cannot receive a particular vaccine, or whose immune response is weak.

Herd immunity is not a guarantee that every unvaccinated person will be protected. An unvaccinated person can still become infected if exposed, and outbreaks become more likely when vaccine uptake falls. A large proportion of the population therefore needs to be vaccinated so that the pathogen has few susceptible hosts and finds it difficult to spread. The proportion needed is not the same for every disease because pathogens differ in how readily they are transmitted.

Why vaccination decisions affect other people

Vaccination benefits the individual by reducing their risk of disease, but high uptake also benefits society by limiting transmission. Decisions about vaccination can therefore involve effectiveness, benefits, risks and cost, as well as a balance between an individual’s right to decide and what is best for the wider population. This collective effect is why vaccination programmes aim for high coverage rather than relying only on people choosing protection after an outbreak has begun.