Downing Street confirmed the programme plans to have reached all those aged 50 and over, as well as adults aged 16 to 65 in an at-risk group, by May – having previously said it aimed to do so ‘by the spring’.

Speaking on the BBC Radio 4 Today programme, Dr Clive Dix said: ‘I’m very optimistic we’ll definitely meet the May target. Every time we’ve been set an objective on the taskforce, we’ve met it, and we’ll work day and night to ensure we meet whatever target that’s feasible.’

Health Secretary Matt Hancock warned that ‘lots of things have got to go right’ to hit the goal, including supply, but he said he was ‘sure’ it was achievable.

Dr Dix said it was possible a new variant could emerge that is able to evade the jabs currently available, but added that scientists are working to try and ‘second guess’ future mutations in order to create a library of new vaccines which can be scaled up if they are ever needed.

He said: ‘The UK is properly at the forefront of surveying all of these variants.

‘We have actually sequenced nearly 50% of all the virus that has been sequenced in this pandemic at the Sanger centre in Cambridge.

NHS staff administer the Oxford-AstraZeneca Covid-19 coronavirus vaccine at Totally Wicked Stadium

‘Taking that data and having scientists look very seriously at what’s emerging – where the mutations are occurring, what they might do to the protein – we can kind of second guess some mutations that haven’t even occurred yet and we can go ahead and make those.

‘And that’s part of the collaboration – we’ll make libraries of future vaccines, just small amounts, enough to then, if it does occur, do a quick clinical study to see that it works and then start manufacturing.’

These studies of coronavirus will help the country and the world get ‘ahead of the game’ on vaccine-evading new variants, Dr Dix added.

Boris Johnson visits a manufacturing facility for the Oxford/Astrazeneca vaccine at Oxford Biomedica in Oxfordshire

Asked whether there could be a mutation capable of escaping the current vaccines on offer, he told Today: ‘Of course – when it will occur and whether it will occur is one thing.

‘That’s what happened with flu, we get these pandemic threats with flu.

‘We should learn from flu… I believe this virus will be very similar – it will last a long time, it will be travelling around the world in different places, it will be endemic in certain countries and we need to do that work, yes.

‘I think there is the possibility but we will be ahead of the game.

‘We’re not going to wait for it to happen – we now have capabilities in the UK to be responsive and that capability won’t just be for the use of the UK of course.

‘Once we’ve done it, it will actually help the whole world because it will be part of that whole surveillance and reaction.’

The government’s coronavirus dashboard shows that as of Saturday 11,465,210 people in the UK have received their first dose of the vaccine, with 510,057 having had their second.