Only 41 cases of the so-called delta plus, or Nepal variant, have been found in the UK, despite ministers blaming it for their decision to shut down travel with Portugal.
Public Health England revealed on Wednesday that the number of cases identified by genome sequencing had less than doubled in the three weeks since the controversial decision.
It came as independent scientists said the variant showed “no signs of expansion” in the UK, with no evidence that it is taking over in any country.
Known formally as delta plus, it is technically not a new variant, but the existing delta or Indian variant with a mutation, known as K417N to its spike protein.
Some scientists have speculated that this could make it more dangerous because it could allow viral particles to bind more strongly to cells in the lungs.
However, speaking at Wednesday night’s Downing Street press conference, Dr Mary Ramsay, the head of immunisation at PHE, said: “I think we are on top of the situation. We continue to be vigilant.”
The delta plus variant was explicitly blamed by Grant Shapps, the Transport Secretary, when Portugal was taken off the green travel list in the first week of June.
Where is the Indian (Delta) variant in the UK?
The decision was criticised as irrational by experts in Portugal, who pointed out that only 12 cases had been discovered there.
India has now classed delta plus, also known as AY.1, as a variant of concern.
Experts in the country believe that, as well as binding more easily to the lung, it might resist monoclonal antibody therapy, an infusion used to treat severely ill patients.
However, Professor Francois Balloux, from University College London, said there was “no particular cause for concern”.
“Given the tiny number of strains reported, nothing is known about the transmissibility, immune evasion or lethality of the delta plus strain,” he said.
“Though, given that it has remained at very low frequency everywhere where it has been identified strongly suggests it is not more transmissible than its delta progenitor.”
Only around 160 cases have been identified through genome sequencing to date across the world, he said, the first of which in the UK was observed on April 28.
He added: “The mutation may contribute to immune escape, though its impact on transmissibility is not clear-cut.”
Dr Ramsay said: “The good message is that what we expected to happen with delta was that the vaccines would prove to still work against the more serious disease and we expect the same for this other variant.
“There is also the option of having different vaccines in the future and that is something we will have to continue monitoring.”