How do vaccines work?

When a germ enters your body, it reproduces. Your immune system recognises these germs as foreign, and begin to make antibodies to get rid of the germs. Usually, the first time you come in contact with a particular germ, it takes a while for your immune system to recognise and attack the germ, and in the meantime, the germ causes illness. The illness could be mild, or could be very dangerous.

Your immune system remembers what the germ looks like. If you get infected again, your immune system recognises it quickly, and makes antibodies to get rid of the germ before it can make you sick.

A vaccine is a weakened or killed germ. When it is given to you, your immune system reacts to the vaccine in the same way it reacts to the original germ, and makes antibodies against it. But because the vaccine is a weakened or killed germ, it can't make you sick. Instead, your immune system learns how to recognise the real germ, and if you get infected, it can quickly get rid of it, before it has a chance to make you sick.