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.
Is the vaccine safe?
Moderna said the vaccine was generally well tolerated, with no serious safety concerns identified.
Severe events after the first dose included injection-site pain, and after the second dose included fatigue, myalgia (muscle pain), arthralgia (pain in a joint), headache, other pain and redness at the injection site.
These side effects were generally short-lived.
How is it transported and stored?
The Moderna vaccine has been shown to last for up to 30 days in household fridges, at room temperature for up to 12 hours, and remains stable at -20C (-4F) - equal to most household or medical freezers - for up to six months.
The company claims mRNA-1273 can be distributed using widely available vaccine delivery and storage infrastructure - with no dilution required prior to vaccination.
Is the Moderna vaccine effective against variants?
In late January, the company behind the vaccine said it was effective against both the strain first detected in south east England and the mutation which first emerged in South Africa.
Moderna said laboratory tests found no significant impact on antibodies against the UK variant relative to prior variants.
While there was a six-fold reduction in neutralising antibodies produced against the South African variant, the levels remained above those that are expected to be protective, the company said.
What stage is the Moderna rollout at in each of the four UK nations?
People in Wales will get first doses of the vaccine, at the West Wales General Hospital in Carmarthen.
The rollout will begin in England "as soon as possible this month", a Department of Health and Social Care spokesperson said.
Scotland's First Minister Nicola Sturgeon said the first batch of Moderna vaccines had arrived in the country on 5 April and will be delivered over the coming months.
It has not been confirmed when the rollout of Moderna will begin in Northern Ireland.
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