BY APRIL this year, around half of the world's population was under some kind of lockdown. Such restrictions helped slow the spread of the coronavirus. As new cases decline in many places, countries are beginning to ease restrictions. How can we know it is safe to do so? The World Health Organization's principal recommendation is that, in order to move to a sustainable level of virus transmission, countries should have the spread of the virus under control. In practice, this means seeing a robust decline in the number of cases. The WHO also advises that countries use testing and contact tracing to identify and isolate new cases of covid-19. Without screening and isolation, easing restrictions will inevitably lead to the number of new infections rising again. When New Scientist went to press, the UK government appeared to be on course to restart contact tracing imminently, after controversially abandoning it in March. Yet to ease restrictions, a country's number of cases also needs to be at a manageable level, says Christina Pagel at University College London. A lot of attention has been paid to the R, or reproduction number: the number of people each person with the virus is likely to infect. If this is above one, cases will continue to rise exponentially, so the aim is to keep it below this. But that alone isn't enough, says Pagel.
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