The Indian variant renders the coronavirus vaccines less effective when it comes to reducing transmission.
As the number of cases of the mutant strain continues to rise sharply in the UK, questions around its infectiousness and deadliness have become more pressing.
Sir Mark Walport, a member of Sage, said that while vaccines still seemed to be protecting people from severe Covid cases, they appeared less good at stopping transmission of the Indian strain.
The deputy chair of the Joint Committee on Vaccination and Immunisation (JCVI), told BBC Radio 4's Today programme that the race between the spread of coronavirus and the vaccination rollout was "on a knife edge".
Prof Harnden said: "The vaccines may be less effective against mild disease but we don't think they're less effective against severe disease.
"But in combination with being less effective against mild disease, they're almost certainly less effective against transmission."
He added: "That's why we're imploring (local authorities) to get out there and vaccinate their unvaccinated people."
More positive is the fact that hospitalisations in the over-60s, who are most at-risk from serious infection but who are also most likely to be vaccinated, remain low.
Data also suggests that the AstraZeneca and Pfizer vaccines are effective against the variant.
Ministers are pushing on with a major easing of restrictions on Monday despite rising coronavirus case numbers.
Boris Johnson is believed to be sticking by plans to allow mixing indoors and greater physical contact in England as scientists warned the strain could be 50 per cent more transmissible than the Kent variety.
Health minister Edward Argar said on Saturday that the Government was acting "coolly" and "calmly" in carrying on with step three in the road map to ending lockdown restrictions.
However, the British Medical Association (BMA) said the move is a "real worry" while many are still awaiting vaccination.
Mr Argar was also forced to defend border restrictions, saying it is "impossible to completely hermetically seal" the nation amid criticism of the delay in adding India to the travel red list.
The Prime Minister warned on Friday the variant could cause "serious disruption" to plans to ease the lockdown and may delay the planned ending of all legal restrictions on June 21.
The Scientific Advisory Group for Emergencies (Sage) concluded there is a "realistic possibility" the strain is 50 per cent more transmissible than the one that emerged in Kent.
If the higher transmissibility is confirmed, the experts said moving to step three could "lead to a substantial resurgence of hospitalisations" that is "similar to, or larger than, previous peaks".
Mr Argar told BBC Breakfast: "All the evidence so far suggests there is no evidence of increased severity of illness or that it evades the vaccine.
"So, at the moment, on the basis of the evidence we are doing the right thing, coolly, calmly continuing with Monday, but keeping everything under review."
Monday's easing will allow people to socialise indoors in homes, pubs and restaurants, and will permit physical contact between households for the first time in more than a year.
Mr Argar said people should take personal responsibility when deciding whether or not to hug loved ones, when allowed to do so.
"You have to take all the facts into consideration," he said. "It's about personal responsibility, it's about making the right judgment call."
The BMA's public health medicine committee co-chairman Dr Richard Jarvis urged the public to take a "cautious approach" to social and physical contact.
"With key segments of the population still not vaccinated and clusters of variants, including the rapidly increasing Indian variant, becoming a growing concern, we must approach this next stage of easing lockdown with the utmost caution," he said.
"It is a real worry that when further measures lift on May 17, the majority of younger people, who are often highly socially mobile and could therefore be most at risk of a more infectious strain, are not yet vaccinated."
Meanwhile, Culture Secretary Oliver Dowden has warned the live events industry that the nation is entering a "period of heightened vigilance", as plans for the reopening of theatres, music venues and sports stadiums threaten to be derailed by the variant.
To combat its spread, people aged over 50 and the clinically vulnerable will have their second doses of a Covid vaccine accelerated.
Surge testing is also under way in several places in England including areas of Bolton, Blackburn, Sefton and London.
Public Health England data shows a rise in cases of the Indian variant of concern from 520 to 1,313 this week in the UK.
