One figure in particular is being closely scrutinised as ministers decide when to end lockdown
R, or the “effective reproduction number”, is a way of rating a disease’s ability to spread. It’s the average number of people on to whom one infected person will pass the virus. For an R of anything above 1, an epidemic will grow exponentially. Anything below 1 and an outbreak will fizzle out – eventually.
At the start of the coronavirus epidemic, the estimated R for coronavirus was between 2 and 3 – higher than the value for seasonal flu, but lower than for measles. That means that each person would pass it on to between two and three people on average, before either recovering or dying, and each of those people would each pass it on to a further two to three others, causing the total number of cases to snowball over time.
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