The dominance of the variant, first identified in India, was confirmed by Matt Hancock as it was questioned by deputies on its management of the pandemic.
He said Thursday that the 91% figure came from an assessment he saw “last night”.
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The delta is believed to be about 40 to 60 percent more transmissible than the Kent (Alpha) variant, and appears to have helped push cases to their highest level since February – more than 7,500 were reported as of Wednesday. And nearly 7,400 were announced on Thursday.
Additional tests have been deployed in some regions, such as the Northwest, in an attempt to limit its spread.
The Health Secretary also appears to have left the door open for future lockdown measures if current vaccines are not effective against the new variants.
Mr Hancock told MPs the plans involve changing vaccines “as quickly as possible” – with a goal “to have a variant vaccine, treatment and diagnosis within 100 days”.
He said he was “very confident” that a vaccine could be developed quickly but “in the meantime, the tools we have are the tools available”.
He did not deny that this included lockdowns and social distancing and said he would “consider” releasing plans to deal with a new variant.
However, people who have received both doses of a Pfizer or AstraZeneca jab are believed to have strong protection against the Delta variant.
Meanwhile, young people are behind the current increase in cases, according to the medical director of Public Health England (PHE).
“Once again, we are seeing a rapid increase in cases across the country and the Delta variant is now dominant,” Dr Yvonne Doyle said Thursday.
“The increase is mainly in the younger age groups who have not yet received the vaccine and we are seeing more hospitalizations. “
Case rates have increased in most age groups, with the highest level of 121 per 100,000 people among 20-29 year olds.
The lowest rates were among those over 80, where it was 6.7 per 100,000.
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Regionally, the North West of England had the highest case rates – 149.6 per 100,000. The South West was the lowest, at 20.8.
Hospital admissions also edged up from 0.90 per 100,000 to 1.09 in one week. However, like the daily cases, they are well below the peak of the second wave in mid-January.
The number of people who have died with COVID has also fallen again according to The latest report from PHE, which runs from May 31 to June 6.
This supports UK-wide death figures which show the seven-day moving average has remained stable for the past month or so, standing at 9.4 per day.
Hospitalization and death data will be one of the key criteria the government uses to judge whether the removal of most other restrictions can take place on June 21.
The government is expected to announce its decision on Monday.