Vaccine action

One of the most effective ways of preventing the spread of infectious disease is to prime the immune system through vaccination. A vaccine trains the immune system to launch a fast and furious attack on a pathogen. 

Herd immunity

Vaccinating a significant portion (around 80 per cent) of a population can help provide immunity even to those who have not been vaccinated. When the disease is passed to vaccinated individuals, their primed immune system destroys it, preventing it from spreading further. This can help protect people who can't be vaccinated due to age or illness. Widespread vaccination can eliminate diseases entirely, such as smallpox. 

Safety first

Contagious diseases can be contained if a sufficient number of people are vaccinated. Vaccination also helps people who have an existing medical condition that may be worsened by the effects of the disease. 

To vaccinate or not?

Controversy exists over the use of vaccines. Scares over possible side effects have led some parents to refuse to have their children vaccinated, which has resulted in outbreaks of preventable diseases, such as measles and whooping cough. If only a small portion of the population is vaccinated, herd immunity breaks down. 

Types of vaccine

Each vaccine is developed for a specific pathogen and is designed to kickstart the immune system. This is done by injecting a harmless version of the pathogen that the immune system will remember if attacked by the real pathogen. This can be difficult - killing the pathogen may make it safe, but the vaccine may not produce an immune response. There are also some diseases that progress so quickly, the immune's memory system may not respond in time, so booster immunizations are given to keep reminding the immune system. 

Original disease-causing pathogen

• Inactivated - The pathogen is killed using heat, radiation, or chemicals. Used for influenza, cholera and bubonic plague vaccines. 
• Alive, but not dangerous - The pathogen is kept alive but the parts that make it harmful are removed or disabled. Used for measles, rubella and mumps vaccines. 
• Pieces of pathogen - Fragments of the pathogen, such as proteins on the surface of the cell, are used instead of the whole pathogen. Used for vaccines against hepatitis B and human papiloma virus (HPV)
• Tame toxins - Toxic compounds released by the pathogen, which are responsible for the illness, are deactivated using heat, radiation or chemicals. Used for tetanus and diphtheria vaccines. 
• DNA - DNA from the pathogen is injected into the body, whose own cells take up this DNA and start to produce proteins from the pathogen, which triggers an immune response. Used for Japanese encephalitis vaccine. 
• Related microbe - A pathogen that causes disease in another species, but few or no symptoms in humans, is sometimes used. For example, tuberculosis vaccine is made from a bacterium that infects cattle. 

Why do vaccines make you ill?

Vaccinations stimulate an immune response, which can produce symptoms in some people - but it means the vaccine is doing what it's supposed to.