There could soon be several effective vaccines available to fight Covid-19.
But while many people want an injection as soon as possible, others are worried about putting something unknown into their bodies.
How do we know a vaccine is safe? This is the first and most important question scientists ask when they start designing and testing a new vaccine or treatment.
Safety trials begin in the lab, with tests and research on cells and animals, before moving on to human studies.
The principle is to start small and only ever move on to the next stage of testing if there are no outstanding safety concerns.