But during a press briefing in Downing Street on Friday, Johnson and his advisers gave the first indication that the strain could also be deadlier.
England’s chief scientific adviser Patrick Valance gave an example. He said that out of 1,000 men in England aged 60 or older, the original virus would kill 10. The new variant, he said, would kill 13 or 14.
Since the new variant was discovered in Britain last year, public health officials have pointed out that the new mutation does not appear to make people sicker or increase the number of deaths – and this small, measurable increase in mortality is therefore potentially worrying.
The Prime Minister and his scientific adviser reiterated that current evidence shows that existing vaccines remain effective against the original virus and the new variants.
The coronavirus, like all viruses, is replicating and changing all the time – with errors and mutations in its genetic material.
Most of these mutations are not important, but some could cause a virus to become more or less contagious, or make people more or less sick.
In addition to the so-called “British variant”, new mutations discovered in South Africa and Brazil are being watched closely.