Video report by ITV News Science Editor Tom Clarke
Six cases of a "variant of concern", first identified in the Brazil city of Manaus, have been detected in the UK, however one person did not complete their Covid-19 test registration, meaning officials have been unable to locate them.
The health secretary said officials had identified the batch of home testing kits in question, meaning the search "narrowed from whole country down to 379 households in the South East and we're contacting each one".
The government and many of its scientific advisers are extremely concerned about the emergence of the strain, also known as the P.1 variant, because it is not yet clear whether vaccines protect against it.
Is the easing of lockdown still on course? Tom Clarke breaks it down
Mr Hancock said current coronavirus vaccines being used in the UK "have not yet been studied against this variant and we're working to understand what impact it might have".
"This variant has caused significant challenges in Brazil," he added.
Dr Susan Hopkins, strategic response director at Public Health England, said officials are tracking the "variant of concern" very closely in a bid to contain its spread in a bid to ensure it does not interfere with the UK's vaccine rollout.
Experts are concerned, she said, because the Brazil strain "shares some important mutations with the variants first identified in South Africa", another "variant of concern" which spreads much more quickly than the original Covid-19 virus.
All you need to know about the P.1 variant:
"These and other mutations are associated with reduced impact of antibodies against the virus in laboratory experiments," she told Monday's Downing Street press conference."The current vaccines have not yet been studied against this variant and we will need to await further clinical and trial data to understand the vaccine effectiveness against this variant."
Mr Hancock said the government is "stepping up testing and sequencing in South Gloucestershire" where two cases were identified, to contain the spread of the variant.
There is "no information to suggest the variant has spread further" than that area, Mr Hancock said.
The other three cases of the variant were identified in Scotland. All those aside from the missing case have been told to self-isolate.
The government is urging anyone who took a test on February 12 or 13 and has not received a result, or has an uncompleted test registration card, to contact the NHS on either 119 for England, Wales and Northern Ireland, or 0300 303 2713 for Scotland.
England's policy of mandatory hotel quarantine for arrivals from red-list countries was brought in five days after the Brazilian variant was imported to the UK.
Had the policy, which was announced weeks before it was implemented, been in force sooner it is likely it would have contained the Brazilian variant, critics have contended.
Shadow health secretary Jon Ashworth said tougher border controls should have been in place sooner before adding: "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, because £22 billion has been allocated to this system - it feels to me that somebody has vanished into thin air."
Watch Matt Hancock's Covid update in full:
The health secretary said a case going under the radar due to an incomplete test registration is "extremely rare".
A number of experts, including at least one on the government's Scientific Advisory Group for Emergencies (Sage), have said the variant's arrival in the UK could cause a delay to the roadmap for relaxing coronavirus restrictions.
Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, said: "It is a variant of concern but we are going to be faced with these in the next six months as we move towards relaxing measures.
"There are going to be challenges on the way - and there is always a risk that we might have to go backwards, and that's what nobody wants to do is to actually open up and then have to close down again," he told the BBC.