Communicable Diseases » Vaccination

What is vaccination?

Your white blood cells can make antibodies to fight a pathogen, but the first time they meet a new pathogen they are slow. It can take days to make enough of the right antibodies, and in that time the pathogen multiplies and you become ill. Vaccination lets your body practise before the real pathogen arrives.

Vaccination means putting small quantities of dead or inactive forms of a pathogen into the body. The pathogen cannot make you ill, because it is no longer able to reproduce and cause disease. Your white blood cells still treat it as an invader, so they are stimulated to make antibodies.

Key terms:

• Vaccination: introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate white blood cells to make antibodies.
• Vaccine: the dead or inactive pathogen that is given to a person.
• Immunised: protected against a disease, usually by vaccination.
• Herd immunity: protection of a whole population from a disease because most people in it are immunised.

How vaccination protects one person

The key idea is that the vaccine gives your white blood cells the antibody training without the illness. Follow the steps in order.

1. A vaccine containing a dead or inactive pathogen is put into your body, usually by an injection.
2. Your white blood cells detect the pathogen and are stimulated to produce antibodies against it.
3. The pathogen is not active, so you do not become ill.
4. Later, the same pathogen enters your body. Your white blood cells respond quickly and produce the correct antibodies.
5. The correct antibodies are ready at once, so the pathogen is dealt with before it can multiply enough to make you ill. Infection is prevented.

Notice that a vaccine only works against the pathogen it was made from. A vaccine made from one pathogen does not protect you against a different one.

How vaccination protects a whole population

Vaccination does more than protect the person who is vaccinated. Pathogens need to pass from one person to another to spread. If most people in a population are immunised, a pathogen that enters the population finds very few people it can infect. It cannot be passed on easily, so the spread of the pathogen is reduced.

This is called herd immunity. It also helps to protect people who cannot be vaccinated, such as babies who are too young, because there are very few infected people around to pass the pathogen on to them.

Think of a pathogen as a spark and the population as a pile of paper. If the paper is all dry, the fire spreads quickly. If most of the paper has been soaked in water, the spark has almost nothing to light and the fire dies out. The soaked paper is the immunised people.

Herd immunity needs a large proportion of the population to be immunised. If only a few people are vaccinated, the pathogen can still find plenty of people to infect and it keeps spreading.

Evaluating the global use of vaccination

In the exam you may be asked to evaluate how vaccination is used around the world. This means weighing up the advantages and the problems and then reaching a judgement.

A good evaluation uses both sides. For example: "Vaccination prevents disease and has removed smallpox, so it is very effective. However, it only works when enough people are vaccinated, so it needs to reach people everywhere to give the best protection."

Exam-style question

(a) Describe what is put into the body in a vaccination. [2 marks]

(b) Explain how vaccination prevents a person becoming ill if the same pathogen enters their body later. [3 marks]

(c) Explain why vaccinating most of the people in a town reduces the spread of a pathogen. [2 marks]