A Covid-19 expert has said there is no rule that variants of viruses become milder over time.
Professor Peter Openshaw from Imperial College London said the issue of mutating viruses becoming less severe has been hotly debated.
But he told BBC Radio 4's Today programme that he would love to be able to say that was the case.
The professor of experimental medicine at Imperial College London said: "But the conclusion we have reached after many discussions is that we have no way of concluding that viruses have to become less severe over time.
"It could be the next variant - and there will be another - could be more severe for all we know.
"There's no rule about this."
Prof Openshaw is a member of the Government’s New and Emerging Respiratory Virus Threats Advisory Group and was made a CBE in the New Year's Honours.
Earlier this month, he said a fourth dose of vaccine against the virus might be needed, with jabs possibly rolled out annually as they are against the flu, but added that more data is needed on the “virus evolutionary pattern”.
He said: “I can quite imagine that it could be necessary to go for a fourth dose in order to maintain and broaden vaccine-induced immunity, particularly to strengthen immunity against the new variants.
