Up to six in ten people who have already had Covid-19 could be vulnerable to a second infection from the new Brazilian variant.
Early research from the city of Manaus showed that protective antibodies did not work against the dangerous P1 variant.
The findings outline the shocking speed at which P1 took over - becoming the dominant variant in just eight weeks.
The Covid-19 Genomics UK Consortium (COG-UK) said it is too early to say whether this would be the case against recent immunity developed from the Kent variant now dominant in the UK.
Both are known to be more contagious than earlier Covid-19 variants.
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Prof Ester Sabino, of the University of São Paulo, said between 25% and 61% of protective antibodies from early Covid-19 variants did not work against P1.
He told a UK media briefing: “In other words if 100 people were infected in our house last year, somewhere between 25 and 61 of them are susceptible to infection by P1.
“Strikingly we found that cases were increasing rapidly.”
Public Health England has flagged six cases of the P1 strain in the UK and a hunt is underway to one of them who has so far not been identified.
They arrived back in the UK while the Government delayed introducing tougher hotel quarantine rules on new arrivals.
Prof Sharon Peacock, director of COG-UK and microbiologist at Cambridge University, said there was no evidence yet whether or not immunity from the Kent variant could protect against P1.
She said: “We know that P1 has spread to quite a few countries and 25 countries today have reported the presence of it. So it is being distributed around the world.”
Scientists were shocked when Manuas was swept by a coronavirus second wave because the first wave had been so severe.
By October 76% of the population had developed Covid-19 antibodies which was expected to have generated a high degree of herd immunity.
Alarm bells were raised when infections surged again in 2021 and genomic sequencing identified P1 as the culprit.
It displayed 17 mutations, 10 of which were on the crucial spike protein linked to how the virus enters human cells.
Asked whether the P1 variant could delay the lifting of lockdown, Dr Thomas Mellan, of Imperial College London, said: “It is difficult to say.
“I think we have to see what happens over the next couple of weeks whether or not there are more cases detected.”
Examination of old viral samples suggested it first emerged in Brazil in November.
Experts believe natural waning of immunity may have contributed to P1 evading antibodies and triggering a second wave.
The Brazilian research suggests P1 is between 1.2 and 1.4 times more transmissible than early Covid-19 variants.
It is not known yet whether this could allow it to “out compete” the Kent variant now dominant in the UK.
The Kent variant can be easily vaccinated against but mutations on P1 suggest this may not be the case for it.
Prof Peacock added: “What what I would say is that we need to see whether this data is generalizable to other settings.
“I think that how they relate to United Kingdom is yet to be determined.”
