The most important fact about coronavirus that the government does not know, writes Robert Peston

I understand a bit more than I did about what the prime minister, the chief medical officer Chris Whitty and the chief scientific adviser Sir Patrick Vallance are trying to achieve with their Covid-19 policy, what that policy actually is, what they actually know about the illness and - importantly - what they don't know.

Let's start with the most important thing they don't know. This is the proportion of people who will get the virus but will show no symptoms.

It is a hugely important ratio, because it conditions how far the government should take measures to restrict our ability to move around and socialise.

The point is that the more people who get the illness without showing any symptoms, the fewer people will need hospitalisation - which in turn means that the fewer actions the PM would need to take to curb our precious freedoms to be with who we want, where we want and when we want.

But the more people who would have symptoms when ill with the virus as a proportion of the population, the more urgent it is to delay its spread - because a massive surge of seriously sick people would overwhelm our healthcare system.

Just to explain with notional numbers, based on one statistic that Whitty and Vallance seem to have some confidence in, namely that 5% of those with symptoms will need hospitalisation. On that basis, if 30m people got Covid19, but half of those were asymptomatic (with no symptoms), then 750,000 of us would require help in a hospital during the epidemic's duration. But if the asymptomatic ratio were just 20%, then the Coronavirus burden on the health service would be a near impossible 1.2m newly sick people.

So as I hope you can now gather, a massively important statistic is this ratio of asymptomatic sufferers to those showing any kind of symptom. But Whitty and co have little idea what it is, and the problem is that there is no available serological test, or test for the viral antibodies in our blood, that would conclusively show who has had the virus, whether or not they ever manifested symptoms.

Huge amounts of work are going on to establish such a test. But it almost certainly won't be ready in time to inform the decision of Whitty, Vallance and Johnson about how much so-called "social distancing" - staying away from people - to enforce on all of us.

Boris Johnson said he wants to delay the peak of the virus outbreak. Credit: PA

If you are with me so far, let us now examine what Whitty and Johnson want to achieve with social distancing.

Now I understand Whitty has not been instructed by Johnson or the health secretary to restrict his advice to policies that are affordable or do less damage to the economy. That may happen at some point, but so far Whitty has given the advice he believes is best for our health and has encountered no push back.

Which then brings us to his core ambition.

It is, as far as possible, to manage the rate of infection through the population so that it is not so rapid that the NHS can't cope at all, such that far too many people die of the virus in a short time because the NHS can't cope, but is not so slow that the ability of the NHS to help people with other illnesses and life-threatening conditions is impaired for months and even years.

To put this in the grimmest possible terms, Whitty's primary function is to minimise deaths from coronavirus, though these deaths may not be directly from the virus itself but also from the inability to treat those who suffer strokes, cancer, heart attacks and so on.

Prof Chris Whitty is helping to formulate the response to the virus outbreak. Credit: PA

Calibrating all this is spectacularly hard, not just because the government does not know the asymptomatic ratio, but it also doesn't know whether the virus will automatically calm down in summer, only to rear up again next winter, nor does it know whether those who suffer it are likely to acquire immunity for a short while or a long time.

For what it's worth, Whitty's central planning assumption is that this virus will have a spike in numbers soon, and then come back - probably less aggressively - in the winter.

There has been a reduced number of flights from Heathrow due to the coronavirus outbreak. Credit: PA

He makes another important assumption - which is that the probability of there being an effective vaccine within 18 months is less than 50% (largely because no vaccination for any form of coronavirus has ever been made yet).

Where is this leading the government?

Given that the government has been widely criticised for not taking dramatic enough prophylactic action, surprisingly it will - I think - lead Whitty to pretty soon recommend measures that will feel severe and unkind, namely that older people and those with chronic illness should be barred from any but essential physical contact with people.

Which in crude terms means the elderly being prevented from mixing with their children, grandchildren and younger friends.

Why?

Because the statistics are clear that the likely need for hospitalisation as result of catching Covid-19 rises very sharply over the age of 70.

Coronavirus: Everything you need to know

Now you may feel this is common sense.

But for those who are old, and may in any case have not much longer to live, it is miserable to be cut off from those they love.

In fact it is worse than that. For some old and infirm, ending contact for an extended period with their loved ones can lead to chronic depression, and cause some to give up the will to live.

So the cost of this sort of social distancing would be big - if, in the view of many, necessary.

As for other forms of social distancing, I would be surprised if at some point there were not temporary closures of pubs and restaurants, because Whitty and Vallance believe the risk of transmitting the virus is much greater in smaller social venues than in giant ones like stadia.

And at some point, probably at the peak of the epidemic, schools are likely be closed for a couple of weeks.

All of which is to make one simple and depressing caveat about Johnson's repeated statement, and that of his health secretary, that they are being guided always by the science: which is that Whitty and Vallance would admit that what they don't know about this virus is probably as significant as what they do know.