Why Boris Johnson's coronavirus lockdown update is all about the R number - the reproduction number of Covid-19

There were three messages in Boris Johnson's address to the nation, and quite a lot of important gaps.

The messages were:

1) because the Covid-19 epidemic has been tempered but not eliminated, lockdown continues though will be modified very gradually;

2) it would be a jolly good thing if a few more of us could return to work, especially on construction sites and in factories, so long as that can be done in a way that does not imperil health;

3) the pace at which lockdown is modified, and whether it is modified at all, is in the collective hands of the British people, and will be wholly determined by whether we continue to obey the social-distancing rules.

So what were the gaps? Here are a couple:

1) We don't know how the financial incentive to stop work, to stay home and save lives - the £40bn job retention scheme that pays people to cease engaging in economic activity - will be changed so that it becomes rational for employers and employees to turn on the commercial ignition switch;

2) We don't yet have a finely calibrated system to tell us precisely what the rate of viral transition is for the UK as a whole, let alone for individual nations, or regions, or localities, and until we do we will be stumbling slightly precariously in the dark as economic and social activity very slowly resumes (the PM today says the R is in the range of 0.5 to 0.9, which does not suggest that the boffins have a high degree of confidence in the current monitoring arrangements).

With regard to why we should leave home for work when we are being paid to stay home, I expect the Chancellor to say something about how the furloughing scheme will be changed later this week - though changes won't take effect until July.

As for letting us all know how risky it is to step outside the front door, the government's ambition is for the ONS survey of the spread and prevalence of the virus, coupled with data harvested with the NHSX's viral alert app, to generate separate R values initially for individual nations, then in succeeding weeks for regions and finally for our respective local areas.

In theory you will be able to look up the R for Bognor or Arbroath in the way you can currently check the pollen count.

And when that happens, the government should be able to take evasive action at the local rather than national level, to protect the freedoms and health of the majority at the temporary cost of a return to lockdown for the few.

Or to put it another way, whether five and six-year-olds return to school on June 1, and more shops open then, and secondary school pupils facing exams get some face-to-face help, and whether any kind of hospitality businesses are allowed to open in some modified way in July, all of that will be dependent on the R - on the rate of transmission staying safely below 1.

Which is logically coherent, given that when R is less than 1, the incidence of the illness will be diminishing.

But whether it will work in practice depends on a whole host of imponderables, most notably whether the government really can in just a few weeks create its new “Covid Alert System” run by a “Joint Biosecurity Centre” that is robust and reliable at determining in real time how many of us have the virus, rather than with the very serious lags that prevail today.

We are pinning our hopes on the technical competence of the government to tell the difference between its Rs and its elbow.