The new variant is being called VUI-202012/01 -- the first "Variant Under Investigation" in the UK in December 2020. While scientists hunt for more information about the variant, its impact is already being felt.

Multiple countries have now imposed restrictions on travelers from the UK. British Health Secretary Matt Hancock said Sunday that the variant was "out of control" and Prime Minister Boris Johnson chaired an emergency meeting Monday as his government tried to manage the fallout.

Here's what you need to know.

What is a variant and why are officials concerned about this one?

A variant occurs when the genetic structure of a virus changes, according to the US Centers for Disease Control and Prevention. All viruses mutate over time and new variants are common, including for the novel coronavirus.

As with other new variants or strains of Covid-19, this one carries a genetic fingerprint that makes it easy to track, and it happens to be one that is now common. That alone does not necessarily mean the mutation has made it spread more easily, nor does it not necessarily mean this variation is more dangerous.

However, the UK government's New and Emerging Respiratory Virus Threats Advisory Group said it had "moderate confidence" that this new variant "demonstrates a substantial increase in transmissibility compared to other variants."

Chris Whitty, England's chief medical officer, said this particular variant "contains 23 different changes," which he described as an unusually large number.

Whitty said the variant was responsible for 60% of new infections in London, which have nearly doubled in the last week alone.

On Monday, Chief Scientific Adviser Patrick Vallance added that he anticipated the spread of the new variant would increase after Christmas.

"Given that we're entering a period of inevitable mixing, I think there will be some increases in numbers over the next few weeks," he said, adding that local restrictions are likely to be tightened rather than eased in the new year.

That finding has immediate implications for virus control. More cases could place an even greater strain on hospitals and health care staff just as they enter an already particularly difficult winter period, and ultimately lead to more deaths.

Public Health England (PHE) has said that a mutation in the Covid-19 spike protein, the part of the virus that attaches itself to host cells, could increase its transmissibility. Scientists across the UK are conducting more research on this issue.

Where did the variant originate and how has it taken hold?

The new variant of Covid-19 originated in southeast England, according to the World Health Organization.

PHE have said that backwards tracing, using genetic evidence, suggests the variant first emerged in England in September. It then circulated in very low levels until mid-November.

"The increase in cases linked to the new variant first came to light in late November when PHE was investigating why infection rates in Kent [in southeast England] were not falling despite national restrictions. We then discovered a cluster linked to this variant spreading rapidly into London and Essex," PHE said.

Multiple experts have also suggested that this new variant could have been amplified because of a superspreader event, meaning the current spike in cases could also have been caused by human behavior.

"A higher genomic growth rate in the samples sequenced, may not necessarily mean higher transmissibility, e.g. if there was a rave of several thousand people where this variant was introduced and infected many people mostly in that rave, this may seem very high compared to a lower background of non-variant virus," Julian Tang, clinical virologist at the University of Leicester, told the Science Media Centre.

Which countries are affected?

The variant has already spread globally. As well as the UK, the variant has also been detected in Denmark, Belgium, the Netherlands and Australia, according to the WHO.

Australia has identified two cases of the variant in a quarantined area in Sydney and Italy has also identified one patient infected with the variant.

A similar but separate variant has also been identified in South Africa, where scientists say it is spreading quickly along coastal areas of the country.

Is the new variant more deadly?

There is no evidence to suggest that the new variant is more deadly as of now, according to Whitty, who said that "urgent work" was underway on Saturday examining the implications for mortality.

"We are not seeing any increased virulence (clinical severity) or any gross changes in the [spike protein] that will reduce vaccine effectiveness -- so far," Tang told the Science Media Centre (SMC.)

Multiple experts have pointed out that for some viruses increasing transmissability can accompany decreasing virulence and mortality rates. This may mean that the variant is less lethal, though it's currently too early to tell.

"New viruses will adapt to a new host over time -- with decreasing mortality, and possibly increasing transmissibility," Tang said.

"As viruses are transmitted, those that allow for increased virological 'success' can be selected for, which changes the properties of the virus over time. This typically leads to more transmission and less virulence," Martin Hibberd, professor of emerging infectious disease at the London School of Hygiene &

Tropical Medicine, said to the SMC.

Will the developed vaccines work against this variant?

Whitty said Saturday that current vaccines should still work against the new variant.

His remarks were echoed in the US by the head of Operation Warp Speed. "Up to now, I don't think there has been a single variant that would be resistant to the vaccine," Moncef Slaoui told CNN on Sunday. "We can't exclude it, but it's not there now."

The UK, the US and the EU have authorized the Pfizer/BioNTech Covid-19 vaccine and several others are in development.

What measures are being taken to contain the variant?

England's chief medical officer has urged people in Britain to take steps to reduce the virus' spread.

"Given this latest development it is now more vital than ever that the public continue to take action in their area to reduce transmission," Whitty said on Saturday.

Large swathes of England, including London and the southeast, are now under strict Tier 4 Covid-19 restrictions, which is only the latest disruption to a Christmas holiday shadowed by the pandemic.

Dozens of countries across Europe, the Middle East and the Americas have also announced travel bans for the UK.

Others, such as Greece and Spain, have imposed restrictions that require travelers arriving from Britain to undergo coronavirus tests or quarantine.

America's top infectious disease expert, Dr. Anthony Fauci, told CNN on Monday that he would advise against additional restrictions on UK travel. The US must "without a doubt keep an eye on it," Fauci said, but "we don't want to overreact."

The US has maintained a ban on travel from the UK, Ireland, and Europe's Schengen zone as well as number of other countries since March.