Boris Johnson has revealed the new more infectious 'Kent variant' has a higher degree of mortality.

The announcement came during a press conference on Friday (January 22) where the prime minister appeared alongside Sir Patrick Vallance and Chris Whitty. 

But what is the 'Kent variant' and why is it more deadly? 

The so-called 'Kent variant' is a new strain of coronavirus which scientists believe could be twice as deadly. 

The UK’s chief scientific adviser Sir Patrick Vallance told a press briefing tonight that the coronavirus variant which emerged in Kent is “a common variant comprising a significant number of cases” and transmits up to 70% more easily than the original virus.

He told a Downing Street press conference: “We think it transmits between 30% and 70% more easily than the old variant. We don’t yet understand why that is the case.

“It doesn’t have a difference in terms of age distribution … it can affect anybody at any age, similarly to the original virus.”

He added that among people who have tested positive for Covid-19, there is “evidence that there is an increased risk” of death for those who have the new variant compared with the old virus.

Sir Patrick cautioned, however, that this is based on evidence which is “not yet strong” and there is “no real evidence of an increase in mortality” among those hospitalised by the variant.

“These data are currently uncertain and we don’t have a very good estimate of the precise nature or indeed whether it is an overall increase, but it looks like it is,” he added.

Three groups of experts advising the Government have looked at the impact of the more contagious Kent variant on mortality.

Researchers concluded the new strain is between 29 and 91 per cent more likely to kill.

The prime minister told the press conference tonight: "We've been informed today in addition to spreading more quickly, it appears there is some evidence the new variant may be associated with a higher degree of mortality."

Mr Vallance said that 13 or 14 people per 1000 would die of the new strain, compared to around 10 of the old strain.

There isn't much information about the South Africa and Brazil variants, he explained.

But he added: "We are more concerned they have more features they might be less susceptible to vaccines.

"They are definitely of more concern and we need to keep looking at it and studying it."