The Brazilian coronavirus variant now in the UK can spread more easily and might overcome immunity from past infection, a study showed on Tuesday.
Research on the P1 variant in the Brazilian city of Manaus, where it was first identified, found it to be twice as transmissible as the original virus.
Results showed the chance of Covid-19 reinfection with the strain was between 25 and 60 per cent.
But experts warned the data could not predict what might happen in the UK after six cases of the strain were discovered last month.
The search continued for one of those cases after the patient failed to fill out their contact details on their Covid-19 test documents.
Health Secretary Matt Hancock said on Tuesday that the search had narrowed to 379 households in the south-east of England.
“We’re contacting each one,” Mr Hancock said.
He said there was “no information” to suggest further cases of the Brazilian variant in the UK.
One of the lead researchers on the Manaus study said it was unlikely the strain would spread quickly in Britain given the containment measures in place.
"You need many introductions of a virus to start an epidemic,” said Prof Ester Sabin from the University of Sao Paulo.
"Six is very few. I would say if you take care and do contact tracing, this is going to decrease."
What did the study find?
Blood test samples indicated more than 67 per cent of Manaus’s population had Covid-19 by October 2020.
Scientists were surprised when the city suffered a major surge in cases at the start of this year despite most of the population having been previously infected.
They believe the new variant emerged in November and quickly became the dominant strain, accounting for 87 per cent of all cases in Manaus in eight weeks.
The variant was found to be up to 2.2 times more transmissible than other coronavirus strains in the city.
Dr Nuno Faria from Imperial College London said there was no evidence to suggest vaccines would be ineffective against the Brazilian strain.
“We know that vaccines are effective and they can protect us from infection, disease and death,” Dr Faria said. “This is a period to be optimistic about the future.
"The more we know about the virus, the better we’re able to protect against it.
"And I think there’s no concluding evidence to suggest at this point that the current vaccines won’t work against P1.”