Britain is braced for a lengthy lockdown as Boris Johnson prepares not to announce any major changes tonight.
Even when restrictions are finally eased - which No10 hints will be in June at the earliest - they won't all be swept away in one go.
But the Prime Minister still faces calls to at least spell out how he'll decide when it's time to give the public a break. Labour call it "light at the end of the tunnel".
So how will scientists and ministers decide when to relax the lockdown?
Well, for weeks now they've had five tests: the NHS is able to cope; a sustained fall in deaths and new infections; adequate testing and PPE; and no risk of second peak that overwhelms the NHS.
But the problem those tests are hard to measure.
For example, not even the first peak "overwhelmed" the NHS - but it still killed more than 25,000 people and counting, and required a nationwide lockdown.
So tonight the Prime Minister is expected to set out a little more of his thinking - and his spokesman says it all boils down to the crucial 'R0' number.
R is the number of people who are infected, on average, by each person carrying Covid-19.
At the start of the outbreak without any controls in place, reports suggest it was between 2 and 3.
That meant the virus spread exponentially and terrifyingly, doubling in the community every few days.
Lockdown measures mean the R number is now below 1, because hardly anyone is mixing with anyone else. So when they get sick, the virus bounces around their home then dies out.
The PM's spokesman said: "We don’t want to relax the social distancing measures which could lead to the virus - which the British public have done so much to suppress - being able to spread in an exponential way again.
"That will guide our approach as we move forward."
The key to easing lockdown will be to work out changes that are enough to open up society, but not enough to push that crucial R number back above 1.
The PM's official spokesman said: "The difference between an R rate above 1 and below 1 is that it reflects the number of people who an infected person is passing the disease onto.
"So if it is below one it means you are having success in suppressing the spread of the disease.
"If it’s above 1 it means every person who has the virus is passing it on to others."
So why is this such a big focus?
Well, because it is key to every one of those five tests.
The NHS will only be able to cope if the R number is low enough. There will only be a sustained fall in new deaths, and new infections, if the R number remains low.
Adequate testing and quick isolation of infected people will help keep the R number lower.
And of course, if the R number is lower, it will help avoid a second peak that overwhelms the NHS.
That's why government ministers are planning for a partial easing of lockdown - where it could be relaxed in some ways and tightened in others.
So, for example, there'll be a "phased" reopening of schools where some pupils go back to class but others have to stay at home.
And ministers are looking at the idea of "bubbles", where people can choose to expand their social contact to a few chosen people, especially if they live alone.
Meanwhile, pubs and restaurants look set to be among the last to reopen, with hair and nail salons also in it for the long haul.
All these factors are being looked at by the government's scientific advisors, SAGE.
They will report back by May 7 on whether the lockdown should be eased.
But government figures have suggested it will remain in place, taking us to the next three-week review at the end of May.
A contact-tracing app which will be crucial to stopping the R-number shooting up again also won't be ready until around mid-May.
So whatever Boris Johnson says tonight, we're in it for the long haul - and we're at the mercy of the letter R.
