Britain said on Friday that a newly identified coronavirus variant spreading in South Africa was considered by scientists to be the most significant one yet found and so it needed to ascertain whether or not it made vaccines ineffective.
Defending a ban on flights from South Africa, Namibia, Botswana, Zimbabwe, Lesotho and Eswatini, Transport Secretary Grant Shapps said that the lesson of Covid was that early action was essential.
The UK Health Security Agency said that the variant - called B.1.1.529 - has a spike protein that was dramatically different to the one in the original coronavirus that Covid-19 vaccines are based on.
"As scientists have described, (this is) the most significant variant they've encountered to date in their research," Shapps told Sky News.
Officials have advised the government on the need to act swiftly and pre-emptively in case the concerns over the impact of the variant are borne out, even though it could take weeks to generate all the information needed about its characteristics.
The variant has also been found in Botswana and Hong Kong, but the UK Health Security Agency said that no cases of the variant had been detected in Britain.