Both of those peaks would far exceed those in previous waves,

Analysis: Concern over Delta variant means decision on ending restrictions on 21 June hangs in balance

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While experts have stressed increases in transmissibility are more concerning than increases in disease severity, this data suggests the outlook could be more worrying than earlier models suggested. Photograph: James Veysey/Rex/Shutterstock

Nicola Davis Science correspondent@NicolaKSDavisMon 7 Jun 2021 09.11 EDT

Summer has nearly arrived and the UK is beginning to unlock from coronavirus restrictions, with a full lifting still on the cards in England on 21 June.

Yet the spectre of the Delta variant is casting an ominous shadow, with concerns it could fuel a third wave. So just how serious could the next peak be – and could it be more serious than Britain’s first two waves?

In May members of the Scientific Pandemic Influenza Group on Modelling (Spi-M) from the University of Warwick released results from models that suggested a variant 40% more transmissible than the Alpha variant – first detected in Kent and known as B.1.1.7 – could result in up to 6,000 hospital admissions a day.–166250853/–166250864/–166250859/–166250853/–166249790/–166249795/

This rises to 10,000 admissions a day if the variant is 50% more transmissible – assuming full relaxation went ahead. Both of those peaks would far exceed those in previous waves, which peaked at more than 4,000 people admitted to hospital in a single day and left the NHS at risk of being overwhelmed.

The Spi-M projections matter because the Delta variant, first discovered in India and also known as B.1.617.2, is believed to be more transmissible than the Alpha variant – though by how much remains unclear.

On Sunday Matt Hancock said the it was about 40% more transmissible, while just two days earlier Prof Neil Ferguson – a member of Spi-M – said the best estimate was 60%, though he added the possible range spanned 30-100%.

But this is not the only concern. Work from Public Health England (PHE) suggests that after a single dose of the Pfizer/BioNTech or Oxford/AstraZeneca Covid vaccines, there was a 17 percentage point reduction in effectiveness against the Delta variant compared with the Alpha one – though only a modest reduction after two doses.AdvertisementBoko Haram leader killed on  direct orders of Islamic State

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Experts have suggested this, as well as demographic considerations, may be among the factors contributing to the transmissibility of the variant – although experiments suggest the variant is also likely to have an inherent advantage.

Further analysis by PHE has also suggested infection with the Delta variant is associated with a more than twofold higher risk of hospitalisation than the Alpha variant, although they caution more research is needed to confirm this and explore the link to vaccination.

While experts have stressed increases in transmissibility are more concerning than increases in disease severity, this data suggests the outlook could be more worrying than earlier models suggested.

“If you do have a variant that is more transmissible and has a higher severity then that is more of a concern, but I think there is still some uncertainty around exactly how much more transmissible the Delta variant is and whether there is higher severity – and if so how much,” said Dr Michael Tildesley of the University of Warwick and a member of Spi-M.

Those uncertainties make it hard to put exact figures on just how bad a third wave could be.

“While the evidence that the Delta variant is both more transmissible and more likely to cause hospitalisation, we are still unsure as to its impact on ICU [intensive care unit] burdens and mortality,” said Prof Rowland Kao of the University of Edinburgh, who is also a Spi-M member.

“Should our existing vaccines remain highly protective even against the Delta variant, then this should alleviate at least some of the impact of any third wave of infections,” he said, adding another question is whether the impact of the Delta variant seen in current hotspots will be replicate across the country.

“Nevertheless,” Kao said, “the evidence is concerning … It does lend extra credence to the idea of slowing down easing restrictions and, in limited cases, considering stronger local restrictions.”

Experts have also raised concerns that a rise in infections could lead to many more cases of long Covid, and increased pressure on NHS services, potentially worsening the surgery backlog.

Prof Graeme Ackland, of the University of Edinburgh, whose team has verified the Imperial College Covid models, said his calculations suggest the R number is currently far above 1 in all areas of Britain.

“I think what is likely to happen is there will be a third wave of cases that will be just as big as the second wave,” he said, adding he did not think that would translate into hospitalisations and deaths on a par with, or exceeding, the second wave. “But I say that with a very high level of uncertainty,” he said. “There will still be a sizeable peak.”

François Balloux, director of the UCL Genetics Institute, and a professor of computational biology at UCL, said the next week would be crucial when it came to predicting how a third wave might play out. “If the growth rates of this [Delta] variant tends to go down over the coming weeks, then I think the situation looks not too bad. If it doesn’t, then yes I worry that restrictions may have to stay in place for the time being.”

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