Here are the numbers that show why schools are very unlikely to re-open any time soon in London and the south east and why within a week or so the whole country may be in a lockdown that includes school closures.
Tier 4, the so-called "stay-at-home tier", is broadly equivalent to the four-week circuit-breaking lockdown that was imposed in November.
It did not include school closures, but it suppressed the rate of transmission of Covid-19 to 0.85 or 0.9. In other words it led to the infection gradually shrinking in the community.
Unfortunately, since then we’ve witnessed the explosive growth of the new strain of Covid-19.
Here is the point: the accumulating evidence, analysed by experts at Imperial College, Edinburgh University, the London School of Hygiene and Tropical Medicine, Public Health England among many others, is that the increased transmissibility of the new strain, its infectiousness, is at the top end of initial estimates.
So it is probably 70% more transmissible than the old strain.
Which means that measures equivalent to the November lockdown would only see the rate of transmission or "R" cut to 1.4 or 1.5. The virus would still be growing exponentially.
This means any suppressive measures that don’t include the closure of schools would see the virus continuing to rage like wildfire in the community; it would continue to spread.
Which is why, with the virus rampant in London, the south east and the east of England, it is very difficult to see schools reopening within weeks there.
And as for the rest of the country, well there is evidence the virus is growing everywhere, which means those primary schools that are opening on Monday may not be open for all that long.
The same depressing logic applies to secondary schools. And it is more and more debatable whether it is rational and sustainable for Boris Johnson and Gavin Williamson to continue to insist that GCSE and A Level exams will take place.
Unless that is more-or-less the entire British army was deployed into schools to administer Covid-19 tests to every single student two or three times a week.
As I understand it, the government has given itself a week to review the post-Christmas data before making a definitive decision on whether to impose a national lockdown that includes school closures.
But few of its advisers are hopeful the data will improve enough to prevent a lockdown.
Even so, the depressing statistics don’t end there.
Even shutting schools might not reduce the R number to less than 1. Analysis in September by the government’s scientific advisers on Sage suggested school closures would reduce R by a bit less than 0.4.
So it is possible that a new lockdown, involving the closure of schools, would reduce the rate of growth of the virus but wouldn’t see the virus diminishing.
That implies any new national lockdown may have to include draconian additional measures like curfews, or imprisoning us in our homes between - say - 8pm and 6am, as is the case in France.
The libertarian wing of the Tory Party will go nuts if that happens. The prime minister will be reluctant to enrage them to that extent.
All of which is to reinforce what I’ve been banging on about for weeks. There is no amount of cash or resource that could be deployed on speeding up vaccination that most reasonable people would deem as anything but amazing value for money.