The Delta coronavirus variant doubles the risk of patients needing hospital treatment compared with the previously dominant variant in Britain, suggests a Scottish study released on Monday.
But two doses of vaccine still provide strong protection, the study found.
It said early evidence suggested protection from vaccines against the Delta strain, first identified in India, might be lower than the effectiveness against the Alpha variant, discovered in Kent, in south-east England.
British Prime Minister Boris Johnson delayed the ending of Covid-19 restrictions in England on Monday after a rapid rise in cases of the Delta variant, which is also more transmissible than the Alpha strain.
The study, published in a research letter in The Lancet, looked at 19,543 community cases and 377 hospital admissions among 5.4 million people in Scotland.
It found 7,723 cases and 134 hospital patients had the Delta variant.
Chris Robertson, professor of public health epidemiology at the University of Strathclyde, said that adjusting for age and comorbidities, the Delta variant roughly doubled the risk of hospital admission, but vaccines still reduced that risk.
"If you test positive, then two doses of the vaccine or one dose for 28 days roughly reduces your risk of being admitted to hospital by 70 per cent," Prof Robertson said.
Two weeks after the second dose, Pfizer BioNTech's vaccine was found to have 79 per cent protection against infection from the Delta variant, compared to 92 per cent against the Alpha variant.
For Oxford-AstraZeneca's vaccine, there was 60 per cent protection against Delta compared with 73 per cent for Alpha.
The researchers cautioned against using the data to compare the inoculations because of differences in the groups who received each type of shot, and in how quickly immunity is developed with each vaccine.
They said two doses of vaccine provide much better protection than one dose against the Delta variant.
The researchers said that a delay to easing lockdown in England would help more people to receive second doses and for their immune responses to build up.
"I think any sort of increase in the window of opportunity before lockdown measures are completely brought to an end will be helpful," said Aziz Sheikh, director of the Usher Institute at the University of Edinburgh.