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What is the R value and why is it important? 

The nation’s armchair experts on epidemiology are familiar with the R value. The number of new infections that result from each existing case is the essential fact about the progress of an outbreak.

The government’s modellers are focused now on one crucial question: which elements of normal life can restart without allowing R to go above one?

Social-distancing measures have pushed the value well below one, probably to about 0.6, meaning that the outbreak is reducing in scale. That means there is not much leeway for R to increase without undoing the nation’s good work.

Keep R below one, and the outbreak will die; allow it to nudge above and exponential growth means that it is a matter of time before the NHS is overwhelmed. There is all the difference in the world between 0.99999 and 1.00001.

The bad news is that so far the modelling has found that releasing any of the restrictions wholesale would put R back above one.

Reopening schools, allowing people to go back to work, a return to pubs and restaurants — all would restart the outbreak and, eventually, require a second lockdown. Only outdoor work could safely resume while keeping R below one.

Scientists have therefore been asked to look at much more limited options to resume normal life. Rather than reopening all schools, could these made be accessible to some age groups only? Are there particular sectors of the economy that might safely start again?

No clear answers have emerged, and when they do they will, in common with all models, be estimates. If they say that R will be just below one, they do not need to be very far out to be completely wrong.

A cautious approach to continuing restrictions is not without its own risks. As the chief medical officer for England said yesterday, lockdown itself kills: through essential treatment missed now and the long-term effects of increased deprivation in the future. Balancing these two dangers in a situation where the answers are inherently uncertain is a wicked problem.

Little wonder that while the scientists are drawing up the options, they say that in the end only Boris Johnson can decide.                       (Source:  The Times  23/4)