Since the coronavirus pandemic erupted, a lurking fear for scientists and politicians has been the emergence of a “mutant” strain making it more dangerous. The variant that has prompted the UK to impose a Christmas clampdown takes the health emergency into a new phase. It does not, so far, appear to cause more serious symptoms or higher mortality rates, or be more resistant to vaccines. But its higher transmission rate makes containing it even more demanding. Since the same variant has been found in the Netherlands, Denmark, Australia and elsewhere, more countries internationally may soon be grappling with the same problems.

The new variant’s unusually large number of mutations appear to help it to infect human cells. Since emerging in Kent in September it has rapidly supplanted older variants in south-east England. Modelling suggests it raises the R value, or average number of people to whom an infected person passes on the virus, by at least 0.4.

Since even strict lockdowns in the UK have struggled to bring the R value much below 1.0 — where the pandemic starts to shrink — that suggests, worryingly, that the “armoury” of existing measures may be exhausted. A particular concern is that, apparently thanks to the new variant, cases were latterly rising in the south-east even during England’s November lockdown.