THE Indian Covid variant could be 50 per cent more infectious than the dominant Kent strain, Sage has warned.
The panel of Government scientists admit it could spark a "much larger" wave.
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Meanwhile, the Chief Medical Officer for England, Professor Chris Whitty, said it could threaten the UK's path out of the crisis.
But Boris Johnson said he did not believe the easing of lockdown needed to be delayed, and the relaxation in England on Monday would go ahead.
The Indian variant could, however, "pose a serious disruption" to unlocking progress and puts the June 21 relaxation into doubt.
Sage documents published this evening said the Indian variant, called B.1.617.2, could be up to 50 per cent more transmissible than the Kent one – which in itself is up to 70 per cent faster-spreading than the original Wuhan virus.
The Sage paper, detailing the group meeting on May 13, said: "Transmission of this variant is currently faster than that of the [Kent] variant most prevalent in the UK (high confidence).
"Observed doubling times are around a week or shorter for some of the largest clusters but slower in others.
"It is therefore highly likely that this variant is more transmissible than B.1.1.7 (high confidence), and it is a realistic possibility that it is as much as 50 per cent more transmissible.
"There are also plausible biological reasons as to why some of the
mutations present could make this variant more transmissible."
Sage said going ahead with step two and three of the lockdown lifting "at the earliest dates" – May 17 and June 21 – "could lead to a much larger peak".
"If this variant were to have a 40-50 per cent transmission advantage nationally compared to [Kent]… it is likely that progressing with step 3 alone would lead to a substantial resurgence of hospitalisations similar to, or larger than, previous peaks."
The warning is based on modelling given to Sage from Warwick University on May 5.
It showed that if a new variant is up to 50 per cent more transmissible, it could cause daily hospitalisations to reach 5,500 by late summer, even with vaccines.
This level could overwhelm the NHS.
It comes as:
- Boris Johnson reveals millions of Brits will get second Covid jab SOONER amid Indian variant fears
- The PM admits the variant may make June 21 lockdown lift "more difficult" and we face ‘hard choices’ on roadmap
- An army is deployed to Bolton in race to crackdown on Indian variant – as Covid cases soar 158%
Speaking at the Downing Street Briefing this evening with the PM, Prof Whitty said: "There is no doubt this [variant] is going up.
"Earlier this week we said we thought it was as transmissible as B.1.1.7 [Kent] and possibly more so.
"There is now confidence that this variant is more transmissible than B.1.1.7.
"The question in practical terms over the two to three weeks is whether it is somewhat more transmissible than B.1.1.7, or a lot more.
"That will have long term implications for the long term prospects of this epidemic in the UK and indeed the pandemic internationally."
He later said: "The thing which has changed is the very clear view of everyone who has looked at it, that it's more transmissible than B.1.1.7.
"We expect, over time, this variant will overtake and come to dominate in the UK in the way that B.1.1.7 [did]."
Mr Johnson said this evening the overall case numbers are low and therefore do not change the next step in unlocking.
But he added: "What we are saying is the public needs to be aware of this variant, particularly in areas it is more prevalent.
"And that's because of these questions about increased transmissibility. We just need a bit more time to see how that pans out.
"If [transmissibility] is at the lower end, we don't have very much to worry about. But if it’s at the highest end, we’ll certainly have to think about the extra measures we will have to take to protect the public."
Why would a new variant cause a wave, even with vaccines?
A new variant that is highly transmissible will easily spread among an unvaccinated population that have the freedom to meet with no restrictions.
At the same time, there will be some people in the community who are vaccinated but still vulnerable.
The vaccines do not prevent all infection. And they are not 100 per cent effective at preventing severe disease and death.
There will also be people who could not take the vaccine due to medical reasons.
And some would have refused the jab for personal reasons, with low uptake among people in ethnic minority communities.
Together, these people make up a considerable fraction of the population, experts say.
A variant which can dodge immunity from vaccines poses another threat.
Because the jabs were created to fight the original virus strain, they may not work as well against new strains which have "immunity escaping" features.
The Sage document said although the number of hospitalisations has remained low, "this could be because the number of infections has only recently increased".
But it confirmed that there is currently no "clear evidence of any difference in disease severity".
It comes after the PM said the Government was "anxious" about the new Indian strain, B.1.617.2.
Cases have suddenly and rapidly grown, with 1,313 spotted in the UK so far – more than double the figure given by PHE a week ago (520).
Four deaths have also been recorded since the variants' emergence on April 10, compared with 12 from the South African variant found in mid-December.
Cases are most prevalent in London and the North West, PHE revealed, with Bolton highlighted as a hotspot.
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Bolton has been highlighted as a hotspot in recent days leading to a testing blitz, along with Sefton and Blackburn.
There is evidence it can reinfect people who have immunity against Covid from prior infection – but PHE said this is “expected with any prevalent variant”.
It does not necessarily mean that vaccines will not work against it, as officials say there is "no evidence" the variant weakens jab efficacy.
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