The researcher, Kamil Khafizov, said the AY.4.2 subvariant may be around 10% more infectious than the original Delta - which has driven new cases and deaths to a series of record daily highs in Russia - and could ultimately replace it.

However, he said this was likely to be a slow process.

"The vaccines are effective enough against this version of the virus, which is not so different as to dramatically change the ability to bind to antibodies," he said.