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What exactly is the R number and how useful is it?

The reproduction measure hides a wealth of complications but is indispensable to decision-making

By Philip Ball  

Illustration: Chris Tilbury/Prospect

Once you get past the astonishing mathematical illiteracy with which the UK government has tried to convey its Covid-19 “alert level” in preparation for an eventual exit from lockdown, it does make some sense. The two most relevant factors for the spread of the disease are indeed the level of infections and the reproduction number R, the latter being a measure of how readily the disease is being transmitted between people. (The government’s five criteria for relaxing the restrictions also take into account the availability of…

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