The prime minister backed a warning from the Welsh government – telling MPs there were “logistical challenges” in distributing it widely.
Just hours after ministers hailed the approval as the UK “leading” the world, Mr Johnson sought to temper expectations that the jabs would be widely available soon.
“It is very, very important that we do not get hopes up too soon,” he told prime minister’s questions in the Commons.
Mr Johnson ducked questions from Keir Starmer about which 400,000 people will receive a jab before the end of the year – the maximum possible, given the 800,000 doses available.
The priority list states that frontline NHS staff, and care home residents and their carers, will be the first in the queue, but the Cardiff government cast doubt on that happening.
Mr Johnson agreed, saying, of the Pfizer vaccine: “It does need to be kept at minus 70C, so there are logistical challenges to be overcome to get vulnerable people the access to the vaccine that they need.
"We are working on it with all four devolved administrations in order to ensure that the NHS across the country is able – and it's the NHS that will be in the lead – to distribute it as fast and as sensibly as possible to the most vulnerable groups.”
The prime minister did confirm that vaccines would not be compulsory, telling MPs “that's not how we do things” in Britain.
The mRNA vaccine from Pfizer/BioNTec has to be stored at around -75 C and transported at that sub-zero temperature to the locations where it will be used.
The jabs - in nearly 1,000 doses per package - have to be used within five days once opened and, crucially, the thawing and preparation process can take up to five days from when they arrive.
The Welsh government believes that means people on the priority list will need to go to special vaccination centres – preventing its use on most care home residents.
It appears likely that care homes might have to await the go-ahead for use of the AstraZeneca/Oxford vaccine, which is some way behind in the approval process.
It can be stored at fridge temperature and there will be many more doses available, with the UK government having ordered 100 million of them.
In the Commons, Sir Keir also raised concerns about public confidence in the vaccine, adding: “That's going to be crucial to the success of getting this rolled out across the country, getting our economy back up and running.”
Mr Johnson replied: “We are, of course, working to tackle all kinds of disinformation across the internet and he's right to single out the anti-vaxxers and those who I think are totally wrong in their approach.
“He's right to encourage take-up of vaccines across the country and we'll be publishing a paper very shortly on online harms designed to tackle the very disinformation that he speaks of.”