Key variants have emerged in areas that already had higher immunity, an expert said (Image: Phil Harris)

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The coronavirus R rate would be as high as 7 if there were no vaccines due to a more transmissible strain, a top expert has said.

Public Health England's Dr Susan Hopkins said the virus reproduction rate could be more than double the initial estimate at the start of the pandemic – without controls like jabs.

She told the Commons Science and Technology Committee that the Delta strain – first identified in India – was 66% more transmissible than the Alpha variant, which was detected in Kent last winter.

Public Health England estimates that 15,000 to 25,000 new infections are taking place each day in the UK.

But the vaccines have proved to be more than 90% effective against hospitalisation, in a major boost to defences against the virus.

Dr Hopkins told MPs: "We're seeing it as much greater transmissibility than Alpha, which had greater transmissibility than the viruses that had gone before unmitigated – so if we were in the real world where we had none of the measures that we were seeing right now – we would estimate R greater than five and maybe up to seven.

Vaccines have proved to be more than 90% effective against hospitalisation
(Image: AFP via Getty Images)

"So compared to where we were right back at the beginning of this where we thought the R value was 2.5.

"That's why we need people to have vaccination because that's a clear mitigation measure, that's why we need people to take care, take caution, particularly in healthcare settings."

Dr Hopkins backed Boris Johnson's plan to delay freedom day to July 19 – and said some restrictions may remain in place afterwards, like wearing masks on public transport.

"I think we will all need to make decisions for ourselves, particularly on wearing masks, using better ventilation, hand hygiene," she added.

The growth rates of the Delta variant has "increased by 40 to 80%", compared to the Alpha variant, which first emerged in Kent, she said.

Transmission rates are 66% greater for the Delta strain while the "secondary attack rate – so how many people go on to infect another individual, is also about 30 to 40% greater", Dr Hopkins added.

She said: "All of which, along with the biological data and the mutations that we've identified, determine this as much greater transmissibility than the Alpha variant."

Scientists are monitoring around 25 variants, with eight under investigation.

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"All of them have mutations that we're concerned about, but mutations alone is not enough to predict whether it's really going to impact on our journey through vaccines and impact on the public health risk of hospitalisation," she added.

But rising Covid immunity could force even worse variants into existence as the virus adapts.

SAGE member Prof Wendy Barclay, of Imperial College London, said some of the key new variants have emerged in areas that already had higher immunity to the disease due to the previous outbreaks.

The virus is still adapting in its human host to become more transmissible, Prof Barclay told MPs.

She said there is currently no proof it will become more deadly – but there is no guarantee it won’t either.

Prof Barclay said: “It has been noted that variants of concern such as Beta and Gamma have emerged apparently from parts of the world which did experience fairly intensive infection rates in the first wave.

"And so there is a feeling that the immunity of a population as it increases may drive selection of viruses with different properties.”

The Delta variant is much more transmissible that the previous strains
(Image: AFP via Getty Images)

Immunity to Covid is rising because of the vaccine rollout – but scientists fear the virus could evolve to resist the jab.

Professor Sir Andrew Pollard said “it will escape from vaccines” partially but “this will happen, it will continue to happen” and will not be the focus forever.

“In the end we’re going to have to come back to focusing on the really important public health issue, which is the hospitalisation and the death,” he said.

Prof Barclay said the “constellation” of genetic changes in new variants suggested they may have emerged in people who harboured the disease for a long time – such as those with a compromised immune system.

Scientists tracked the evolution of the virus in an HIV-infected person in South Africa and the result was like the Beta variant, she said.

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Prof Barclay said the virus becoming more transmissible is separate to whether or not it becomes more deadly. But there is no guarantee it will not evolve to kill a higher percentage of people, she warned.

“There is perhaps a fallacy around, that as the virus adapts to humans it will get less severe for example. I don’t think that’s necessarily the case,” she said.

“The overarching pressure on the virus is transmissibility… the two properties are separate, they don’t necessarily get linked together.

“So I think it is possible that as virus becomes more transmissible, it could cause more severe disease, or it could go the other way.

“At the moment, the virus is adapting to become fitter in the human host.”

The Government is ramping up the vaccination roll out
(Image: Daily Mirror/Andy Stenning)

Vaccine effectiveness against symptomatic illness will reduce over time, but the focus will be on whether protections remain against hospital admissions, according to Prof Pollard.

He said: "What the virus is doing is it's evolving away from immunity, and we're seeing lower effectiveness against symptomatic infection.

"If we wind the clock forward to a year or two from now, one would expect those numbers to get lower and lower, because unless the virus disappears from the planet, which I don't think is to happen, it will have to be able to survive in vaccinated populations.

"If we focus on effectiveness against symptomatic disease in the future, we'll go mad because those numbers will get lower and lower over time because that is the only way the virus survives.

"So the really important question is: 'What does effectiveness against hospitalisation look like?' And it's really encouraging so far, and I think we're all hoping it will stay like that."

Prof Pollard said work around booster doses and whether new variant vaccines were needed will be critical in future.