Should we worry about the emergence of a new variant of SARS-CoV-2, the virus which causes Covid-19? News of the new variant – which, it seems, might transmit more easily than previous versions – was the big surprise of Matt Hancock’s statement to the Commons this afternoon. The other big announcement – that London and parts of Essex and Hertfordshire will be going into Tier 3 – was a foregone conclusion.

As I wrote here in May, the SARS-CoV-2 virus has already mutated once into a form that might be more transmissible. This could possibly explain why Europe and North America have found it harder to contain the virus than have Asian countries. Were we fighting a slightly different disease to the one which emerged in Wuhan in January?

At this evening’s press briefing, Chris Whitty, the Chief Medical Officer, played down fears about the new mutation. Mutations are only to be expected, he said, and many have already emerged. It isn’t clear, he added, whether the new variant is more transmissible than previous ones – while it is especially common in parts of London and the South, that may simply be a coincidence that these are the parts of the country where rates of infection are currently rising fastest.

There is no evidence, he said, that the new variant is more dangerous to humans than previous versions, and no reason to suspect that this would be the case – there would be no evolutionary advantage in the virus evolving to be more fatal to the people it infects. On the contrary, it is advantageous to a virus to evolve into a more benign form over time, although it would be over-hopeful to imagine that has happened yet.

Nor, said Whitty, is there any reason to imagine that the new variant will be any more resistant to the Pfizer vaccine or any other vaccine – too few people have yet been given the vaccine for the virus to start developing its own immunity to the vaccine.

He did, however, raise the prospect that SARS-CoV-2 will develop resistance to vaccines in the longer run – one reason why we need more than one vaccine. Not only that, we may need regularly to redevelop vaccines, to deal with the most recent variants – and then to revaccinate the population. He also suggested that the new variant is slightly less sensitive to one of the tests that Public Health England currently uses to detect the virus. Given that the lateral flow tests used for the government’s new mass testing programme have been found to detect fewer than half the cases of Covid-19 in some instances, it is a job to see this changing the picture very much.

The new variant may not turn out to be of great significance. In a recent study of 46,723 people with Covid-19 from 99 countries, researchers identified more than 12,700 mutations. ‘None of these mutations are making Covid-19 spread more rapidly,’ according to Lucy van Dorp, a professor at University College London’s Genetics Institute and one of the co-lead researchers on the study.

But what today’s news has done – in particular the decision to shift the capital into a higher tier – is to change the mood. From a picture of declining infections in late November and early December, we are heading back, once more, into a period of tighter restrictions.

Once again, as throughout this crisis, questions at this evening’s briefing focused on whether tighter lockdowns would be imposed. This time, there were also repeated questions on whether the relaxation of rules on households mixing over Christmas ought now to be revisited. Given that Germany and other countries have started imposing lockdowns across Christmas, I give it until about Thursday until Boris is back at the lectern announcing that, regrettably, it is going to be necessary to cancel Granny’s visit.