Patients in Carmarthenshire will be the first in the UK to receive the vaccine on Wednesday.
A third Covid-19 vaccine will be rolled out across Wales from Wednesday with patients in Carmarthenshire becoming the first in the UK to receive it.
The Moderna vaccine was approved in January as safe and effective for use in the UK.
Supplies arrived in Wales on Tuesday, with 5,000 doses sent to Hywel Dda University Health Board vaccination centres.
It has not yet been confirmed when the rest of the UK will start using it.
The UK has ordered 17 million doses of the Moderna vaccine, with Scotland receiving its first batch on Monday.
The jab is the third of seven vaccines that the UK has ordered. Like the Pfizer-BioNTech and Oxford-AstraZeneca jabs which are already in use, the Moderna jab is given in two doses, several weeks apart.
The first doses will be administered at Carmarthen’s Glangwili Hospital.
Health Secretary Matt Hancock said he was delighted the UK rollout of the Moderna vaccine was starting.
“The UK government has secured vaccines on behalf of the entire nation and the vaccination programme has shown our country working together at its best,” he said.
Wales’ Health Minister Vaughan Gething said a third vaccine for use in Wales “significantly adds” to the nation’s defences against Covid-19.
Across the whole of the UK, more than 31.6 million people have now had a first dose of a Covid vaccine – about three in five adults – while 5.4 million have been fully vaccinated.
In Wales, more than 1.49 million people (47.4% of the population) have had a first dose, while more than 469,000 people have had both doses.
It comes as regulatory bodies from the UK, Europe and the World Health Organization continue to assess data on the Oxford-AstraZeneca jab and a potential association with a rare form of blood clot.
Updates from the regulatory bodies are expected in the coming days.
On Tuesday, the trial of the Oxford vaccine on children stopped giving out jabs as a precaution.
The Moderna vaccine is a two-dose jab given at an interval of between four and 12 weeks.
Like Pfizer’s, it is an RNA vaccine and works by injecting part of the virus’s genetic code 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, meaning the rate at which it can be produced is accelerated.
It requires temperatures of around -20C for shipping – similar to a normal freezer.
Trial results suggested efficacy against the disease 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.
The government has bought 17 million doses – enough to vaccinate about 8.5 million people.
Scotland is due to receive more than one million of the UK’s order, and the doses will arrive over a period of months.
First Minister Nicola Sturgeon cautioned that the supply does not mean the vaccine programme will be accelerated.
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 fatigue, muscle pain, joint pain, headache, other pain and redness at the injection site.
But these were generally short-lived.
Ros Jervis, director of public health for Hywel Dda University Health Board, said the Moderna vaccine would be used alongside the Oxford AstraZeneca jab in the rollout to communities in Carmarthenshire, Ceredigion and Pembrokeshire.
“We are incredibly lucky to have a third vaccine in Wales, with a long shelf life and the ability to be easily transported, to help deliver the vaccination programme to small clinics across our rural communities,” he said.
Darren Hughes, director of the Welsh NHS Confederation, which represents health boards, told BBC Radio Wales the Moderna rollout would help the country make up for a reduction in Oxford-AstraZeneca supplies in April.
Mr Gething said every vaccine given in Wales was “a small victory” against the virus and people should go for their jab when invited.
A Welsh Conservatives spokesman said the UK government’s vaccination strategy had been “fully vindicated”.
Plaid Cymru’s health spokesman Rhun ap Iorwerth said: “We need to know what this milestone now means for further easing of restrictions.”
The Welsh Liberal Democrats said the third vaccine came as “welcome news”, adding the priority should be to vaccinate everyone “as quickly and safely as possible”.