"'TIS the season to be jolly, but it is also the season to be jolly careful".

That was the message from Prime Minister Boris Johnson during this evening's news conference as he again promised a relaxation of restrictions to allow families to get together over the festive period.

He spoke of there being "light at the end of the tunnel" with the positive vaccine news from the Oxford-AstraZeneca team, but reiterated the importance of having a regional tiered approach in place to tackle the virus.

England’s chief medical officer Professor Chris Whitty and director of the Oxford vaccine group Professor Andrew Pollard also spoke at the press conference.

Mr Johnson, who is still self-isolating so took part remotely, said the details of how the rules would be relaxed over the Christmas period had not yet been finalised with the administrations in Scotland, Wales and Northern Ireland.

He said "very good discussions" had taken place and an update would be provided soon.

"I know that many of us want and need Christmas with our families, we feel this year we deserve it", Mr Johnson said.

"But this is not the moment to let the virus rip for the sake of Christmas parties.

"'Tis the season to be jolly, but it is also the season to be jolly careful, especially with elderly relatives."

Prof Whitty was asked if there had been any modelling of how many cases and deaths could take place in a worst-case scenario as a result of easing restrictions at Christmas.

He said: "In terms of putting a number on it, to be honest, I wouldn’t put a number on it, because if people do all those things very seriously we will have much less impact from Christmas whilst people are still being able to enjoy it, than if people choose to actually take a very much less public-spirited approach to it and go wild over that period.

"And so I think what we really need to do is we need to say, look if everyone is serious before, during and after then we can minimise the amount of impact it’ll have.

"Of course it will have an impact, and that’s accepted. And this is all part of the balance of having to keep the virus under control but trying to do so in a way that is the least socially and economically damaging.

"It’s a very difficult balance, as we all know, for which there is no perfect answer. It’s trying to get the balance as best we can between these competing difficulties."

During the conference professor Andrew Pollard said trial data hinted the Oxford vaccine could help prevent asymptomatic infections – something that could "stop the virus in its tracks".

"We have this amazing set-up here in the UK where we have been able to swab over 8,000 people in the trials every week, so we can also pick up the cases of asymptomatic infection,” he told a Downing Street press conference.

"There is a hint in the data that, in one of our groups that had the higher efficacy, we were able to reduce the amount of asymptomatic infection.

"That may mean that there could be fewer people in the population who are spreaders.

"And that starts to stop the virus in its tracks, if we can get there.

"But it’s only a hint at the moment, we need more information."