The health secretary says 379 households in south-east England are being contacted.
The search for an individual infected with the Covid variant first found in Brazil has narrowed to 379 households in south-east England, Health Secretary Matt Hancock has said.
He said the batch of home testing kits in question had been identified and each household was being contacted.
It is one of six cases of the P.1 variant found in the UK in February.
Mr Hancock told MPs the UK’s current vaccines had not yet been tested against this variant.
But work is already under way to redesign or tweak the vaccines to make them a better match for some of these new “variants of concern”, although existing ones should still provide some protection, particularly against severe disease.
Updated vaccines could be ready within months, meaning the UK could have millions of doses ready to give people a booster shot before next winter to make sure the population is protected.
The first detailed study of the Brazil variant suggests it is more contagious and may evade immunity provided by past Covid infection.
This raises questions about how well current vaccines, which were designed around earlier versions of the virus, might work.
Mr Hancock said the variant had caused “significant challenges” in Brazil.
He added: “We’re doing all we can to stop the spread of this new variant in the UK, to analyse its effects, and to develop an updated vaccine that works on all these variants of concern and protect the progress that we’ve made as a nation.”
The person infected with the variant who has not been traced used a home testing kit but did not complete their contact details, Mr Hancock said.
“Incidents like this are rare and only occur in around 0.1% of tests,” he added.
News of the mystery case prompted an appeal for anyone without a result from a test on 12 or 13 February to come forward immediately by calling 119.
The case is not linked to five other cases of the Brazil variant in the UK and Mr Hancock said all these individuals quarantined at home as they were legally required to.
Two of the cases are from the same household in South Gloucestershire. They tested positive after someone returned from Brazil on 10 February – five days before the government’s hotel quarantine rule came into force for arrivals high-risk countries.
Everyone in five postcode areas of South Gloucestershire is now being invited to take a Covid test, even if they do not have symptoms.
Mr Hancock said testing and sequencing in South Gloucestershire was being stepped up “as a precaution” and there was “no information to suggest the variant has spread further”.
Two other people in the same household have also since tested positive for Covid – but tests are still ongoing to check if it is the same variant.
The other three cases are Scottish residents who flew to Aberdeen from Brazil via Paris and London, the Scottish government said.
Other passengers who were on the same flight to Aberdeen are now being contacted and there is no reason to believe the variant is in circulation in Scotland, Health Secretary Jeane Freeman said.
Labour’s shadow health secretary Jonathan Ashworth said tougher border controls should have been in place sooner.
He asked: “How on earth can a test be processed that doesn’t collect the contact details and what mechanisms will be put in place to fix this in the future?”
Preliminary data from Manaus – the Amazonian city hit hard by the P.1 variant – suggest it could be up to twice as transmissible as earlier variants of Covid there.
But experts say this should not be used to predict what may happen in the UK.
One of the lead researchers said it was unlikely the variants would quickly take off in Britain, when only six cases had been identified and these were being closely monitored.
Meanwhile, Mr Hancock said the government was “on track” to meet its target of offering a vaccine to the top nine priority groups, including all over 50s and those with underlying health conditions, by 15 April and all adults by the end of July.
“Although the day-to-day figures of supply are lumpy, we have some bumper weeks ahead later this month,” he added.