Community immunity: how vaccination stops a disease spreading

0:00Does a vaccine protect only the person who gets it?

0:04Each dot is a person. Each line is a contact between two people who meet often.

0:10At first, everyone is susceptible, which means they can catch the disease.

0:16Some people cannot be vaccinated, such as babies who are too young and some people who are very ill.

0:23Now one person is infected. A germ, a micro-organism that causes disease, is in their body, and they can pass it on.

0:32The germ passes from person to person along chains of contacts.

0:37Soon it reaches everyone, even the people who cannot be vaccinated.

0:43Start again, but now half the people get a vaccine. It trains the immune system, the body's defence against germs.

0:51This usually makes them immune: if the germ gets in, the body destroys it quickly, so it is not passed on.

1:00But with only half immune, the germ can still find a chain around them, and reach someone who cannot be vaccinated.

1:08Measles spreads very easily, so about 95 in 100 people need to be immune to stop it spreading.

1:16Now the germ reaches one more person, and then every chain is broken.

1:22The people who cannot be vaccinated are protected.

1:26This is called community immunity, or herd immunity.

1:30So a vaccine protects you, and when enough people are immune, it protects the people around you too.