The Moderna coronavirus vaccine is the third jab to be rolled out in the UK, alongside the Pfizer/BioNTech and Oxford University-AstraZeneca jabs.
It will be administered to people in Wales from today.
Here is what we know about the vaccine :
How effective is it against coronavirus?
The phase three results suggested vaccine efficacy against coronavirus was 94.1%, and vaccine efficacy against severe COVID-19 was 100%.
More than 30,000 people in the US took part in the trial, from a wide range of age groups and ethnic backgrounds.
How many doses of Moderna does the UK have?
The government has bought 17 million doses - enough to vaccinate about 8.5 million people because two doses are required.
How much does it cost?
The Moderna vaccine is the most expensive, per dose, of the vaccines created so far.
It was pitched for $38 (£28) a dose during last summer - much higher than Pfizer, at $20 (£15).
How does the vaccine work?
The Moderna jab is a messenger RNA (mRNA) vaccine.
Conventional vaccines are produced using weakened forms of the virus, but mRNAs use only the virus's genetic code.
An mRNA vaccine is injected into the body, where it enters cells and tells them to create antigens.
These antigens are recognised by the immune system and prepare it to fight coronavirus.
No actual virus is needed to create an mRNA vaccine.
This means the rate at which the vaccine can be produced is accelerated.