UK health secretary Sajid Javid told the House of Commons on Friday the B.1.1.529 variant is of “huge international concern” and could “pose a substantial risk to public health” in the UK.
It was designated a Variant of Concern and named “Omicron” by the World Health Organisation on Friday evening.
Nicola Sturgeon has labelled the variant emergency the “most significant and concerning” development of the past few months, but said there was no need to “hit the panic button”.
The UK has already suspended flights from six African countries – South Africa, Namibia, Lesotho, Botswana, Eswatini and Zimbabwe – in a four-nation approach.
But while the new variant was first sequenced in South Africa, scientists cannot be sure it originated there.
There have already been reports of cases in other parts of the world, including Belgium, Israel and Hong Kong.
With travel bans on just six nations, there is a risk of the variant passing to the UK via an intermediary country without restrictions in place, said Professor Rowland Kao, chair of veterinary epidemiology and data science at Edinburgh University.
It comes as the head of the European Commission, Ursula von der Leyen, called for all EU nations to put on an "emergency brake" and stop all flights from southern Africa.
Israel's Prime Minister Naftali Bennett said on Friday the country was "on the verge of a state of emergency", and that he planned to "act fast, strong and now".
And the head of the UN World Tourism Organisation called for a quick decision on travel restrictions, telling Reuters on Friday that “if it continues to spread as we are expecting, then it will be late and will make no sense to apply restrictions”.
Some experts have discouraged travel bans. Mark Woolhouse, professor of infectious disease epidemiology at Edinburgh University, said there was an argument for making travel safer through screening and testing, rather than preventing it.
Others called for the strengthening of other protective measures, including the mandatory wearing of face coverings, which is not currently in force in England.
Prof Kao said a long-standing challenge of the Covid-19 pandemic was that by the time new threats were identified, it was often too late to mitigate them.
“The problem we're always going to have is that by the time you know it's a real problem, you should have done something already,” he said.
“You have to wait quite a while before you actually know the variant is worse than what you've got.
"You need to have enough cases to generate enough information about how likely it is that people are going to go into hospital, how likely it is to detect the vaccine, and all those various things just take more time to get the proper information.
“So you almost have to act before you actually have a good basis to act.”
Scientists are concerned because Omicron has appeared to spread rapidly in South Africa, and has mutations associated both with increased transmissibility and reduced vaccine efficacy.
But very little is yet known for certain.
Prof Kao said more time was needed to gather information on whether the new variant was more transmissible or more deadly than Delta, and whether it may reduce the efficacy of vaccines.
But enough cases have been identified in South Africa that there is a “good chance” that some infected individuals have been moving around, he said.
"That means geographical spread,” he said. “And geographical spread means that you can't just restrict flights from South Africa, for example, you need to think more broadly.
“Now is the point where if another country doesn't [restrict inbound travel from South Africa], then you get a cascading effect.”
The new variant could “leapfrog” its way into the UK via an intermediary country not on the UK red list, where there are no restrictions on travel from South Africa, Prof Kao said.
Another cause for concern is the variant may have originated in another country, with less well-developed genomic sequencing, and only been discovered when it travelled to South Africa, he added.
This would mean there has likely been wider spread than currently thought.
Prof Kao stressed while tightening restrictions would be preferable when considering only epidemiology, other factors would need to be considered by policy makers.
“Speaking purely about the epidemiology, you're better off putting in a lot of restrictions now while we have time to figure out whether or not it's spreading in other places,” he said.
“We've got a pretty good idea that it probably hasn't spread extensively, because we get a lot of sequences from around the world.
“It's probably not huge at the moment, but that doesn't rule out the possibility that some other country, particularly in Sub-Saharan Africa, because travel links are going to be greater in that area.”
He added: “There's all sorts of reasons why you might not want to do this unless you knew the risk was higher, but just based on the epidemiology, you might be saying let's impose some border controls now – more than what we've already got, so put everybody into quarantine when they enter the country at the very least, until you have a chance to see what's going on, until we're sure that it isn’t more widespread.
“If it's not more widespread, that gives you time.”
Prof Devi Sridhar, chair of global public health at Edinburgh University, said it was unlikely the UK would be able to prevent importation of the new variant, and that it may already have arrived.
Further restrictions to prevent spread may be on the horizon, she said, but this depends on hospitalisation rates and the ability of the NHS to cope.
Dr Jeffrey Barrett, director of the Covid-19 Genomics Initiative at the Wellcome Sanger Institute, said there may be some hope of containing this new variant better than Delta, which had spread rapidly before the alarm was raised.
Asked whether any further restrictions would be helpful, he told a panel about the new variant on Friday: “We’ve lived through almost two years of this, and so we probably have some sense of what are sensible choices to try to make, and hopefully decision makers will at least have the benefit of that past experience to help them.”
Discussions are ongoing over the prospect of adding further countries to the four-nation travel red list, Mr Javid said on Friday.
“We are keeping this under review and there’s very live discussions going on about whether we should and when we might add further countries, and we won’t hesitate to act if we need to do so,” he said.