Should we be relieved to be governed by the scientists? Asks Robert Peston

What we learned on Thursday is that, at least while the Prime Minister is convalescing, the boffins of the government's Scientific Advisory Group for Emergencies are - in effect - running the country.

Dominic Raab, who is deputising for Boris Johnson, made it crystal clear that he and his fellow ministers - who met on Thursday in Cabinet and the Cobra committee - simply followed the advice of SAGE, which is chaired by Sir Patrick Vallance, in extending total lockdown for a minimum of three weeks.

As other ministers have confirmed to me, there was no pushing back on SAGE's view that easing any of the current unprecedented constraints on our basic freedoms would lead to another surge in Covid-19 infections that would damage health and the economy.

"SAGE is the clear arbiter/adviser," said a senior minister.

This is not government by our elected representatives as we conventionally know it. And maybe, given the gravity of what's happening, that is both understandable and sensible.

There is no issue of principle here so long as Raab is on firm ground when saying that, despite the massive economic costs of this lockdown, there is no trade off between mortality and prosperity to consider.

Raab said "the advice from SAGE is that relaxing any of the measures currently in place would risk damage to both public health and our economy" and that "early relaxation would do more damage to the economy over a longer period".

Which of course implies that there is no political choice here, just data analysis.

That said, what we don't know, for what it's worth, is whether there are any credible economists - or any economists, credible or otherwise - on the committee, because the membership of the committee is a secret.

I understand that Treasury officials, at the least, feed in their views. So we have to assume Raab's statement, that a continued lockdown is for the best in all senses, would be supported by technocrats other than the medical and epidemiological ones.

But it would be helpful to see the models on which these sweeping judgements are based.

Let's give SAGE the benefit of the doubt that it would be nuts to even contemplate easing any of the constraints on our freedom of movement now.

But what will determine when it makes sense to follow the example of most other big European countries and start setting out some kind of very long path towards more normal living?

Raab set out "five specific things" which the Government will need to be satisfied of before it "considers it safe to to adjust any of the current measures.

These are :

1) ministers must be confident that the NHS is no longer at risk of being overwhelmed by the virus;

2) the daily death rate must have fallen in a sustained and consistent way;

3) there must be "reliable data from SAGE" showing that "the rate of infection is decreasing to manageable levels across the board";

4) operational challenges, such as ramping up the number of tests and the supply of PPE protective equipment, are being met;

5) and in Raab's words, "we need to be confident that any adjustments to the current measures will not risk a second peak of infections that overwhelm the NHS."

At least with condition 2), there is reasonably reliable and objective data - although there is some doubt about quite how many deaths in the community that are due to Covid-19 are going unreported (as the chief medical officer Chris Whitty pointed out, there has been a surge in deaths well beyond those provably attributable to the coronavirus).

An analysis by the distinguished statistician David Spiegelhalter makes the point graphically that the actual death toll from Covid19 may be significantly higher than official figures show.

All that said, it is perfectly possible to assess how many people are dying. And there is quite a lot of evidence that the death rate has reached a plateau, although it has certainly not fallen in any significant way.

But although the high death rate is traumatising it is a lagging indicator - and there is much more encouraging data about numbers being admitted to hospital or to intensive care, both of which are falling.

What about the other four tests? Well perhaps I am missing something, but 1), 3), and 5) seem to me to be saying more or less the same thing, though using different words. And 4) - more tests for everyone and protective equipment for healthcare workers - is obviously critical to all three of those near identical conditions being met.

They are all different ways of saying "no end to lockdown until we're sure that there won't be another surge of infections that would eviscerate the NHS's capacity to save lives".

Which is a statement of the blindingly obvious and is no help to any of us, since the whole premise of the lockdown was to prevent the NHS from falling over.

The fundamental challenge of this virus has not changed, namely that none of us can be confident of having any immunity to it, because - as the full scientific version of its name says - it is novel or new.

And we can't be confident that we are safe to go back to normal life till we know how many of us have had it, how many are immune, and whether we have an effective vaccine or other therapeutic drugs.

So social controls of some sort or other will be with us - almost certainly - till well into next year, because it will be months, possibly well over a year, till we know precisely who is at risk and who not.

Which does not mean those social controls won't change progressively, albeit very modestly and slowly, rather sooner.

The point is that Vallance said he and the chief medical officer Chris Whitty were confident that the rate of viral transmission, so called "R0", is now less than 1 - which is another way of saying that anyone infected will probably on average infect fewer than one other person.

And he said that if R0 is 0.5 - that is, if it would take two infected people to generate any new case - SAGE would probably feel it was safe to have some very limited relaxation of the lockdown constraints, because even if that led to some increase in transmission of the virus, it would not return to being an uncontrollable epidemic.

The most revealing thing said today at the press conference was also by Vallance and it was that he thinks the UK's infection spread is still about four weeks behind that of Italy.

And the point is there are small incremental relaxations of the lockdown in Italy - forestry work is restarting, shops selling books and children's clothes have been given permission to re-open.

So just possibly lumberjacks and booksellers will be able to return to work here some time in May (please take that as an analogy or proxy for incredibly modest though unspecified restoration of social and economic norms).

But the big point is that we are being ruled by scientists and data, Which is not how democracies traditionally function,

That would be less concerning if the scientists actually had thoroughly reliable data on how many of us have had the illness and may or may not be immune. They don't - and they don't even know when that data will be available.

Just so you know.

Coronavirus: Everything you need to know