Reports around the emergence of a possible new mutation of coronavirus - which could be more resistant to vaccines - was cited on Thursday as a factor Portugal was taken off the 'green list' of travel destinations for Brits.
Transport secretary Grant Shapps said concerns around "a sort of Nepal mutation of the Indian variant that's been detected" and a rise in case numbers were a cause for concern.
In recent weeks the highly transmissible Indian variant - which has been renamed the 'delta' variant by the World Health Organization - has become the dominant strain in the UK, accounting for more than three-quarters of new infections.
However, the WHO has said it is “not aware” of any new strain.
What's going on?
The Nepal strain is a mutation of the Indian variant. It contains properties of the Indian variant but also a mutation labelled K417N. This mutation has been found in the Beta variant first identified in South Africa, and is believed to be part of why that variant is less well neutralised by vaccines.
The UK government revealed this week they in talks with AstraZeneca for additional doses of its COVID-19 vaccine that will have been specifically modified to better target the Beta variant.
Who is worried?
Professor Neil Ferguson, who sits on the government's New and Emerging Respiratory Virus Threats Advisory Group (NervTag), said on Friday that the mutation "may compromise vaccine effectiveness".
He said: "K417N, now the Beta variant which was known as the South African variant, may compromise vaccine effectiveness more, but we really don't have enough data at the moment. It is a variant of interest and of concern."
Asked if the latest data could affect the further unlocking of the country on 21 June, Prof Ferguson said: "I think the data is pointing this week in a more negative direction than it was last week so it points towards the direction of being cautious.
"I think balancing people's desire - and there clearly is a built-up desire to get back to normal - against the potential risk is a very difficult judgement call."
Dr Jeff Barrett, director of the COVID-19 Genomics Initiative at the Wellcome Sanger Institute, said there have been 91 sequences observed of the Indian variant containing this specific mutation.
He said that this dual threat - that the new mutation could make the virus more transmissible and more resistant to vaccines - means scientists are monitoring it carefully.
He said Indian variant plus the K417N mutation had been seen in numerous countries, including the UK, Portugal, the US, India, Nepal and Japan.
Watch: Nepal variant blamed for Portugal decision
What has the UK government said?
Transport secretary Shapps said of the decision to remove Portugal from the travel green list: “I want to be straight with people, it’s actually a difficult decision to make, but in the end we’ve seen two things really which caused concern.
“One is the positivity rate has nearly doubled since the last review in Portugal and the other is there’s a sort of Nepal mutation of the so-called Indian variant which has been detected and we just don’t know the potential for that to be vaccine-defeating mutation and simply don’t want to take the risk as we come up to 21 June and the review of the fourth stage of the unlock.”
Communities secretary Robert Jenrick said on Friday he was not aware of any cases of the Nepal variant in the UK, however, Public Health England has said it is investigating 20 cases.
Jenrick said Portuguese scientists had detected the variant, but added: “What we don’t yet know is how prevalent it is in Portugal – but more importantly than that, whether this really is a significant problem.
“You could of course wait whilst we do that research and then, God forbid, we learn that it is very virulent, that it does compromise our vaccines, or we can take a safety-first approach.”
What has the WHO say?
The World Health Organization has said it is “not aware” of any variant of concern detected in Nepal. A post on the WHO official Twitter account on Thursday read: "WHO is not aware of any new variant of SARS-CoV-2 being detected in Nepal.
“The three confirmed variants in circulation are: Alpha (B.1.1.7), Delta (B.1.617.2) and Kappa (B.1.617.1). The predominant variant currently in circulation in Nepal is Delta (B.1.617.2).”
There are thousands of variants of coronavirus but public health experts focus only on those that have worrying looking mutations or are showing signs of being more dangerous. These are called "variants under investigation" or "variants of concern".
Watch: What UK government COVID-19 support is available?