Researchers at Imperial College London said it appeared to be growing at a rate of around 2.8 per cent during the course of the study.
Experts believe it could be 10 per cent more infectious than other Delta variants. However, pre-print research from the latest round of the REACT-1 study suggested it was less likely to cause symptoms.
Two thirds (66.7 per cent) of those with the variant AY.4.2 in the study had some symptoms, compared to 76.4 per cent of those with its Delta parent, AY.4.
A third of those with the variant reported the classic Covid symptoms of a persistent cough, fever or loss or change to taste and smell.
This compared with 46.3 per cent of those who had the original Delta variant.
AY.4.2 does not appear to make vaccines any less effective than the original Delta strain.
“At present it seems to be about 10 per cent more infectious than the other delta variants, though I am not sure whether we know why this is yet,” said Prof Paul Hunter, a Professor in Medicine at the University of East Anglia.
Scientists say it is too soon to say for sure whether the new sub-variant is less likely to make people sick.
Other factors could be at play, such as more cases recorded in those who already have partial immunity.
“If this association is confirmed then there are several possibilities, one of which is the variant is indeed less virulent or that it is more likely to cause infections in people who are partially immune,” said Prof Hunter, who added more research needed to be done.
Director of the REACT programme, Professor Paul Elliott, told the I newspaper that if the association is confirmed it could give the variant a “transmission advantage” as fewer people will know they have Covid if they do not have symptoms.
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