The Indian variant of Covid-19 may not be as transmissible as first thought, according to some experts.
The B1617.2 strain, a sublineage of a new variant of concern, is growing rapidly in the UK, as it did in India, where it was first detected.
However, some scientists said the surge may actually be the result of a phenomenon called the "founder effect".
That happens when a small number of people infect a higher proportion than normal.
Experts suspect this is because cases are increasing in the UK in areas with high numbers of large, multi-generational households but are now levelling off in some parts.
Prof Carl Heneghan, director of the Centre for Evidence-Based Medicine at the University of Oxford, said numbers were “falling off at speed” in India.
"We could be looking at a founder effect and where you've got a small number of people having a bigger impact," he told The Telegraph.
“India is actually looking more like the natural curve which happens in winter and has a high drop-off, as opposed to flattening the curve.”
Speaking from Downing Street on Wednesday evening, Prof Jonathan Van-Tam, the UK's deputy chief medical officer, said that the additional transmissibility fell within in a wide range.
"We have a credible range which goes from a few per cent through to 50 per cent more transmissible," he said.
"I think most people feel it’s going to be in the middle, but it is just too early."
Opinion divided over strength of variant
Other experts, however, do not agree, citing models that suggest the strain has a significant growth advantage.
Tom Wenseleers, a biology and biostatistics professor at KU Leuven in Belgium, said the latest data showed B1617.2 was still "spreading like wildfire and outcompeting the Kent variant, B117".
He pointed to a model which showed the Indian variant had a growth advantage of 11 per cent a day over the Kent strain.
“Assuming a constant and unaltered generation time of 4.7 days, a growth advantage of 10 per cent a day would translate to a 60 per cent transmission advantage,” he wrote on Twitter.
He said that could be because of several reasons.
“In practice, a mix of increased contagiousness, longer duration of infectiousness and/or some immune escape could explain this growth advantage. This remains to be studied further.”