New coronavirus variant in UK prompts lockdown, travel bans -- but evidence so far is thin

Fears that a new variant of the novel coronavirus is behind a rise in infections in London and parts of southeast England has caused dozens of countries around the world to suspend travel to and from the country, but there is still a great deal that is not known about the new variant which has caused such international alarm, according to several experts.

Speaking to the nation on Saturday, Prime Minister Boris Johnson announced a U-turn on a Christmas relaxation of the rules, new restrictions on foreign travel from areas where the new variant was most prevalent and effective lockdowns in the locations where the new variant is most prominent. The variant, which was first detected in the U.K. months ago, has also been found in patients in Australia, Denmark and the Netherlands.

"There is no evidence the variant causes more severe illness or higher mortality, but it does appear to be passed on significantly more easily," Johnson said.

"Although there is considerable uncertainty, it may be up to 70% more transmissible than the old variant," Johnson added, citing an early analysis from NERVTAG, the advisory group on New and Emerging Respiratory Virus Threats.

Johnson is expected to address the nation again later Monday.

However, according to Ian Jones, a professor and virologist at the University of Reading, the evidence that the new variant is behind the rise in infections is "not proven and "remains hypothetical."

"The new strain, the B.1.1.7 variant, is notable for the number of changes it has (17) compared to "normal" strains and for its rapid rise," Jones told ABC News. "These two points are usually discussed together on the assumption that the changes cause the rise, that it is more transmissible. But this is not proven and remains hypothetical. I am not persuaded it warrants the current concern but I accept you have to err on the side of caution."

"As of today I am not aware of any independent measure of the virus replication or transmission so it is very speculative currently. Importantly the virus is seen elsewhere in the UK but appears not to be spreading rapidly so overall the jury's out," he said.

London and parts of southeast England were placed into a newly created Tier Four of restrictions on Saturday evening, which effectively mirrors the measures imposed during the national lockdown of November, although areas of communal worship will still be allowed to remain open during the holidays. Domestic travelers rushed to train stations to leave London just hours before Tier Four came into effect. Health Secretary Matt Hancock said on Sunday that the new variant was "out of control."

French President Emmanuel Macron, who was diagnosed with COVID-19 on Thursday, said, "All this shows the complexity of the virus, its aggressiveness, dare I say its inventiveness and the humility that we must always have."

There is so far no evidence the variant would prove resistant to vaccines, and the data, which has been shared with the World Health Organization, Johnson said, was "early" and "subject to review," but he had "to act on information as we have it because this is now spreading very fast."

Responding to the news over the past few days, a number of countries have introduced travel bans to the U.K. France, Germany and several other European countries have suspended flights, while similar measures have been taken around the world – as far as Canada and Hong Kong. The U.S. has not yet joined the list of countries banning travel to the U.K., though New York Gov. Andrew Cuomo has called on the federal government to impose restrictions in light of the new variant.

When asked if the potential for the variant to be more transmissible could make it more deadly, Jones said, "Almost certainly no. The press speculation currently equates new and higher transmission with more deadly. There is no evidence at all for this (i.e. higher death rates). In fact the general rule in virology is that higher transmission is associated with a less severe infection."

The news comes as Europe continues to battle its second wave of coronavirus infections. Several countries have announced a brief amnesty on restrictions to allow people to meet with their loved ones on Christmas Day, but it remains to be seen whether the presence of the new variant will lead to further restrictions across the continent.

ABC News' Ibtissem Guenfoud contributed to this report