Britain on Monday appealed for a person infected with a powerful Covid-19 strain from Brazil to come forward as experts fretted about its impact on new vaccines.
The public appeal came a week before England is due to start unwinding its third Covid lockdown, with progress hinging on the vaccines' ability to curtail the pandemic.
The variant that emerged in Manaus, northern Brazil, has been detected in six people in the UK, one of whom cannot be located after they failed to fill in their contact details on a form after taking their coronavirus test.
"We are working with several data points to try and locate them, not least, of course, to try and highlight anyone who has had a test on (February) 12 to come forward if they haven't had a result," vaccines minister Nadhim Zahawi told BBC television.
He said community-wide "surge testing" was starting in South Gloucestershire, western England, after two of the Manaus cases were confirmed there.
Experts warned that based on data from Brazil, the variant was both more transmissible and better at evading antibodies than the UK's predominant strain which emerged in the southeastern county of Kent last September.
Danny Altmann, professor of immunology at Imperial College in London, said the public should feel "somewhat worried but not total panic" about the emergence in Britain of the Manaus variant.
"It's not that all immunity is gone, it's that the vaccines look so much less potent, so there'll be more people who have low antibody responses where it can break through and get affected," he told Times Radio.
In mid-January, Britain banned flights from South America including Brazil.
But the new cases arrived on a flight from Sao Paulo via Zurich around February 10, triggering complaints from opposition parties about gaps in the border protection regime.
Britain has Europe's highest death toll from the Covid pandemic.
On Sunday the government said it had given more than 20 million people at least one dose of a coronavirus vaccine, providing some hope that the end is in sight -- unless new strains take hold.
Next Monday, schools across England are due to reopen to pupils for classroom teaching as part of a staggered return to normality over the coming months.
In this file photo taken on December 21, 2020 Travellers wearing face coverings walk with their luggage in the almost deserted departures hall at Terminal 5 of Heathrow Airport in west London, as a string of countries around the world banned travellers arriving from the UK, due to the rapid spread of a new, more-infectious coronavirus strain