The first consignment of the Pfizer/BioNTech coronavirus vaccine has arrived in the UK.
It has been taken to a central hub at an undisclosed location, and will now be distributed to hospital vaccination centres around the UK. The UK has ordered 40 million doses - enough to vaccinate 20 million people.
England's deputy chief medical officer said the first wave of vaccinations could prevent up to 99% of Covid-19 hospital admissions and deaths.
Professor Jonathan Van-Tam said that would be possible if everyone on the first priority list took the vaccine and it was highly effective. He said it was key to distribute the vaccine "as fast" and at the "highest volume" as possible, but he acknowledged there would need to be some flexibility in the list.
The Pfizer/BioNTech vaccines are made in Belgium and have travelled to the UK via the Eurotunnel. The order in which people will get the jab is recommended by the Joint Committee on Vaccination and Immunisation (JCVI) and decided by the government.
Elderly people in care homes and care home staff have been placed top of the priority list, followed by over-80s and health and care staff.
However, because hospitals already have the facilities to store the vaccine at the necessary -70C, the very first vaccinations are likely to take place there - for care home staff, NHS staff and patients - to lower the risk of wasting doses.
Prof Van-Tam said: "If we can get through phase one [of the priority list] and it is a highly effective vaccine and there is very, very high up take, then we could in theory take out 99% of hospitalisations and deaths related to Covid 19.
"That is why the phase one list is what it is, that is the primary ambition." The UK became the first country in the world to approve the Pfizer vaccine.
Dr Anthony Fauci, the top infectious disease expert in the US, has since said that the UK was not as rigorous as the US in its Covid-19 vaccine approval process.
"The UK did not do it as carefully," he told Fox News. "If you go quickly and you do it superficially, people are not going to want to get vaccinated." But the UK has defended its process, and said the jab is safe and effective.
Dr June Raine, the head of the UK medicines regulator, said "no corners had been cut" in vetting the jab. The Medicines and Healthcare Products Regulatory Agency (MHRA) reviewed preliminary data on the vaccine trials dating back to June.
The Pfizer/BioNTech vaccine offers up to 95% protection against Covid-19.
The UK's 40 million doses will be distributed as quickly as they can be made by Pfizer in Belgium, with the first load rolled out next week and then "several millions" throughout December, Health Secretary Matt Hancock has said. But the bulk of the roll-out across the UK will be next year.
And it could take until April for all those deemed most at-risk to receive the new vaccine, according to NHS England chief executive Sir Simon Stevens.