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Cases of the Delta variant of coronavirus have more than tripled in a week with 42 people now confirmed to have died after testing positive for the variant, 12 of whom had had both vaccines doses.
Public Health England (PHE) said that as of Wednesday, the UK has seen 42,323 confirmed cases of the B.1.617.2 variant first identified in India, up 29,892 from 12,431 a week ago, and an increase of 240%.
PHE said this has been partly driven by a reduction in test turnaround times, and a faster process for identifying cases of the variant.
The figures are a further blow to Boris Johnson's plan to lift lockdown on 21 June.
The prime minister is set to make an announcement on Monday, and a delay to the full lifting of restrictions is looking increasingly likely.
PHE said research indicates the Delta variant has a 60% increased risk of household transmission compared to the previously-dominant Alpha variant first identified in Kent, which caused the UK's winter crisis of infections, hospital admissions and deaths.
Researchers added it is "encouraging" the huge increase in Delta variant case numbers has not yet translated into a similar increase in hospitalisations.
Of 383 people admitted to hospital with the Delta variant, 251 were unvaccinated, 66 were more than 21 days after their first dose and 42 were more than 14 days after their second dose.
As of 7 June, there had been 42 deaths of people who had tested positive with the variant. Of these, 12 were more than 14 days after their second dose.
Responding to the new case numbers, Dr Jenny Harries, chief executive of the UK Health Security Agency, said: "Vaccination is our best defence. If you are eligible, we urge you to come forward and be vaccinated. Remember that two doses provide significantly more protection than a single dose."
Watch: Friday's daily politics briefing
In a further sign the end of lockdown could be delayed, Dr Harries highlighted the restrictions currently in place.
"While vaccination reduces the risk of severe disease, it does not eliminate it.
"With data showing that Delta is significantly more transmissible than Alpha, it is just as important as ever to follow public health advice, which has not changed.
"Get vaccinated, work from home where you can and remember ‘hands, face, space, fresh air’ at all times. These measures work, and they save lives."
The Times has reported Boris Johnson is considering delaying the end of lockdown by four weeks in order to provide extra time for people to receive both their vaccine doses.
More than 20 million UK adults still need to receive a second dose.
Appearing on BBC Radio 4’s Today programme on Friday, COVID-19 vaccines minister Nadhim Zahawi effectively put forward a case for delaying.
“The vaccines have begun to sever the link between infection rates, cases, hospitalisations and deaths,” he said.
“It’s really important, with these hard-won battles against the virus… the virus hasn’t gone away, it will attempt to survive by mutating [and] it can only mutate if it has a population to infect."
He added: “Let’s not squander those hard-fought gains.”
The Delta variant is about 60% more transmissible than the Alpha variant. Health secretary Matt Hancock said on Thursday that the Delta variant now makes up 91% of the UK's COVID cases.
Overall infections in the seven days up to Thursday, meanwhile, increased by 17,036 (63.2%) on the previous seven days.
Previous modelling from University of Warwick researchers, in evidence supplied to the government last month, raised the prospect that a variant that is 50% more transmissible could lead to a peak of more than 20,000 daily hospital admissions in the middle of July if lockdown is lifted as planned.
The second wave peak on 12 January saw 4,578 patients admitted to hospital.
Speaking on Wednesday, Prof Neil Ferguson, who is one of the UK’s leading COVID experts and whose modelling convinced Johnson to impose the first national lockdown in March last year, was asked if delaying the end of lockdown will make a difference.
“Yes," he said, “because it allows more people to get second doses.”
Watch: PM talks up UK's 'colossal' global vaccine pledge