Surge testing has been rolled out in targeted areas around England (Image: Daily Mirror/Andy Stenning)
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Public health bosses believe there are up to 25,000 new Covid cases a day in the UK as the Delta variant spreads in the community.
Dr Susan Hopkins, from Public Health England, said around 7,000 to 8,000 infections were being detected per day – but the true number is believed to significantly higher.
She told MPs that PHE was monitoring 25 variants and investigating eight strains as experts battle to prevents new mutations of the virus from taking hold.
Appearing before the Commons Science and Technology Committee, Dr Hopkins also revealed that experts believe the virus R rate could be as high as 7 without controls like the vaccine.
The Delta variant, first detected in India, is 66% more transmissible than the previously dominant Alpha strain, which ripped through the UK in the winter, she said.
Dr Susan Hopkins said the true number of Covid cases was much higher
Dr Susan Hopkins told MPs: "What we are seeing at the moment are about 7,000 to 8,000 infections per day – that's what we're detecting.
"But we know that that is less than half of what the true infections are in the community, and we've measured that a number of ways.
"So the estimate for current infections today is probably in the order of between 15,000 and 25,000 new infections today.
"It doesn't take very many doublings to get to very large numbers.
"But what we won't see – or what we hope we won't see and I think we won't see because of vaccine – is the same numbers of hospitalisations.
"So we will have a much much greater amount of infection in the community without seeing the same impact of hospitalisations.
Covid R number would now be 'up to 7' if we didn't have vaccine, top expert says
"But the more infections we have, the more impact there will be – so 90% effectiveness, that means 10% could actually come into hospital.
"So that means that we do need to have some measures in place, both social responsibility measures and the measures that are in place right now, to try and hold that peak down so that we can get as much vaccine into the individuals to reduce symptomatic disease, reduce transmission, and then clearly severity and hospitalisation."
Dr Hopkins said if the Delta variant was "unmitigated" – left to spread without any controls – the R value could be "greater than five and maybe up to seven".
"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," she said.
But vaccines expert Professor Sir Andrew Pollard said the crisis would be over if the vaccines continue to prevent against hospitalisations.
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He told MPs that the emergence of new variants "will happen, is going to continue to happen", but added: "But 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.
"And if those are disconnected – if transmission is disconnected by vaccine immunity from the severe disease to a large extent – then we'll need to monitor new variants perhaps if we need to find new vaccines and so on, but we are going to have to live with it being in in our communities and transmitting."
He continued: "That's the key bit that we have to look at with future variants: if that very high protection against hospitalisation continues, despite spread in the community, then the public health crisis is over.
"And so far, up to Delta, we're in a very good position, as long as we've got people vaccinated."