Boris Johnson says he was pinned, as he put it, because of a meeting he had last week with the conservative MP, Lee Anderson, who since tested positive for Covert 19.

Mr.

Johnson said in a video message that he felt great and was bursting with antibodies after having the virus earlier this year.

But the order to stay at home for another 10 days has come at a crucial time for the prime minister as he tries to move on from the sudden departure of two of his closest advisers and enters an important week on Brexit negotiations.

Well, this was Boris Johnson's announcement on Twitter.

The good news is that NHS tests and traces working evermore efficiently.

The bad news is that they picked me and I've got to self isolate because somebody I was in contact with a few days ago has developed co vid on bond.

It doesn't matter that we were all doing social distancing.

It doesn't matter that I'm fit as butcher's dog.

Feel great so many people do in my circumstances and actually doesn't matter that I've had the disease and I'm bursting with antibodies.

We've got to interrupt thes spread of the disease on one of the ways we can do that now is by self isolating for 14 days.

When you get contacted by NHS Tests and trace, let's talk to our political correspondent, Chris Mason.

He's outside number 10 for us, where Boris Johnson, of course, is self isolating Chris.

He sounds very chirpy.

How frustrated do you think he really is, though I suspect his mighty frustrated, I guess any of us would be.

When you get that notification that you gotta hold yourself up, W f h working from home s Oh, yeah, I guess three human reaction would be one off.

Frustration on the political context is one that will underline that frustration, I think because this was meant to be the kind of big reset moment before we heard from Downing Street last night that the prime minister would have to self isolate.

We've been told that there was a big plan for a kind of energetic relaunch.

That's my word rather than theirs.

But that was the clear subtext after all, of the turbulence and vitriol of the last few weeks with those departed advisors on Friday, there was talk that he'd be out meeting.

Conservative MPs in the north of England have been grumbling about a lack of attention, the imminent need for a Brexit deal in the eyes off many with that deadline just around the corner.

Ah, big plan on climate change.

Talk about meeting the chancellor to talk about the spending review a real sense of plastering up on a billboard but like you might get outside a kebab shop saying under new management when they had a recent visit from environmental health.

None of that, though, is going to happen at least physically.

Instead, this is going to be a remote reset.

All that said.

The health secretary for England, Matt Hancock, said, That shouldn't make too much difference because we're all getting used.

Thio working remotely Well, it doesn't change the amount of work the prime minister will be able to do, driving forward the the agenda at all.

I mean, the the wonders of videoconferencing mean that you could be incredibly effective, even when self isolating.

If you're in the sort of job where you where you could do that, where you can communicate.

In fact, I'd say that the, you know, probably the majority of my meetings with the prime minister are over video conference.

Of course, I see him as well on do you could be really effective that way.

Mr.

Hancock has also been defending the prime minister after a photo was dredged out of Facebook that have been put up there by Lee Anderson, the conservative MP in question who met the prime minister on Thursday morning and then went on over the weekend to test positive.

The two of them are stood further apart than would have been conventional in 2019, but perhaps a little closer together than the 2 m regulation given that they were inside and weren't wearing a face covering Downing Street.

Insist that through that door is a co vid secure working environment, even though for at least the second time now we know that the coronavirus has managed to make its way through that door.

I didn't so doing disrupt the prime minister's plans, Chris Good to talk to You Christmas Mason.

There are political correspondent Well, Danny Altman is professor of immunology at Imperial College, London, and he joins us now, picking up on what Chris was saying there.

We know that Boris Johnson has to self isolate because he's come into contact with someone who's tested positive.

Those are the rules, aren't they?

Are they good rules?

I think they're very good rules, and I was pleased to hear him taking it seriously, as in test trace and isolate means what it says.

It's there for a reason.

Even if you've had the infection or think you've had the infection or think you're everybody's on board, you know we can't be blood there about this.

If he waas social distancing properly, would it be possible to pick up the virus or the terribly hard for me to say, isn't it?

But you know, this is a moveable feast where we're trying to assess the risks all the time.

And, you know, I take quite a kind of conservative you that this is a very infectious virus, a very dangerous virus.

And if I'm in an enclosed space, I want to be in a really well ventilated space wearing a mask, probably more than 2 m.

Boris Johnson, we know, had covered in April.

How likely do we know?

Is he still toe have antibodies?

I suppose I'm asking you, What do we know about how people can be re infected?

Yeah, I think it's a really good question.

So obviously there's been lots and lots of work on measuring people's antibody levels and trying to work out what this means and who's protected and how long for.

And the simple answer is that people are very variable that antibodies do Wayne on that.

There's a significant minority of people perhaps 10% plus who have three or four months have virtually no protection on board and around the world.

We are seeing some cases of reinfection.

Can you pass on covert 19?

Even if you don't have symptoms?

Absolutely.

And I think that's one of the reasons that this has been so devastating.

Compared to, for example, SARS or merge.

You know, the last coronaviruses that you just can't see where it's coming from.

You know you can't you can't spot it because it's it's spreading before you know you have it.

We know that other people were in that meeting room.

Someone must have taken that picture, mustn't they?

Of the prime minister on the Conservative MP.

He was meeting.

Will they all need to self isolate us?

Well, well, you know, I don't I don't know any of the details.

And it's certainly not for me to be the policeman pronounced, but one of one would assume so, Yes.

My question is, if you've all bean in a room together, then you're all going to get pinned, as Boris Johnson would say, Yes.

Yes, yes.

Yeah.

Okay.

Danny Altman from Imperial College.

Really good to have your thoughts.

Thanks for joining us on BBC News.