A new vaccine is first tested in a very small group of people to rule out major safety problems and helps doctors work out the right dose.
Next it is tested in a larger group to check that the vaccine works consistently, and scientists watch for any side effects.
Then the vaccine is tested in tens of thousands of healthy volunteers. This shows whether the vaccine protects against natural infection and gives a better chance of discovering rare problems not seen in smaller studies.
But it doesn’t end there. If the vaccine passes all of these tests and is approved by regulators it is continuously monitored.
The vaccines available to use today have been given to millions of people, preventing illness and saving lives.