Professor John Edmunds - a member of the government's Scientific Advisory Group for Emergencies (SAGE) - says the new Bristol variant of Covid-19 may be able to reinfect people who’ve already been infected or who’ve been vaccinated.
Prof Edmunds, from the Faculty of Epidemiology and Population Health at the London School of Hygiene and Tropical Medicine, said: "I don't know whether the Bristol variant is any more transmissible than the Kent variant. I suspect it isn't.
"Where it has an advantage it is may be able to infect people who were previously infected, or have been previously vaccinated. That's the worry with that particular virus."
Asked if he had a sense of what kind of time scale there would be for a phased return to normal, Prof Edmund said: "It's to do with how fast you vaccinate people. That's really the great limiting factor. The faster we vaccinate the high risk groups, the better, so the quicker we can ease up.
"Eventually, we're going to switch to vaccinating lower risk groups, and then we will really have an impact on transmission.
"At the moment, we're vaccinating the elderly, who don't really play a great role in transmission. They tend to be at the end of transmission chains, not in the middle."
He warned against easing restrictions too early, before enough people are vaccinated.
He told the Peston show: "Most people have just had a single dose, they're not fully protected. So I think easing up now would be really dangerous."
Since Sunday 7 February more than 15,000 COVID-19 tests have been carried out or collected for home testing across Bristol and South Gloucestershire, as part of the community surge testing programme.
A total of 21 cases of the new variant have been found nationwide, with 14 detected in the Bristol and South Gloucestershire area.
People living or working in one of 24 specific postcodes have been invited to get a test as part of the investigation help Public Health scientists better understand what is happening as the virus changes.
Director of Public Health at South Gloucestershire Council Sara Blackmore said: “The response to our community surge testing programme has been nothing short of incredible.
"We wish to thank all those residents who have taken the time to attend a testing site or Collect and Drop location, often in bitterly cold conditions.
"It has been a challenge to get these sites set up and working efficiently so we are grateful for everyone’s effort and patience."
Talking about the new variant she says, "Their work so far indicates that the growth rate of this form of the virus is relatively small, but we are only into day four of our two-week programme.
"We expect case numbers to increase but are confident that if everyone continues to follow public health guidance and national lockdown rules the number of new cases can be kept at a minimum.”