Are there any side effects?

No significant safety concerns have been reported, but nothing, including paracetamol, is 100% safe.

Short lived fatigue, headache and pain were reported after the injection in some patients.

"These effects are what we would expect with a vaccine that is working and inducing a good immune response," said Prof Peter Openshaw, from Imperial College London.

Image copyright MODERNA Moderna labs
Image caption Moderna scientists used an innovative technique for developing the vaccine so quickly
How does this compare to the Pfizer vaccine?

Both vaccines use the same approach of injecting part of the virus's genetic code in order to provoke an immune response.

The preliminary data we have seen so far is very similar - around 90% protection for the Pfizer/BioNTech vaccine and around 95% for Moderna's.

However, both trials are still taking place and the final numbers could change.

Moderna's vaccine appears to be easier to store as it remains stable at minus 20C for up to six months and can be kept in a standard fridge for up to a month.

Pfizer's vaccine needs ultra-cold storage at around minus 75C, but it can be kept in the fridge for five days.

The Sputnik V vaccine, developed in Russia, has also released very early data which suggests it is 92% effective .