The Delta variant first identified in India is associated with about double the risk of hospital admission compared with the Kent variant, new data suggests.
But researchers found that two vaccine doses still provided strong protection against the variant which is now dominant in the UK.
However, the level of protection may be at a lower level compared with the Alpha variant first detected in Kent, early research suggests.
Based on data analysed from 5.4 million people in Scotland, the Delta variant is now the dominant form of Covid-19 in the country.
For the study period: April 1 and June 6 this year; there were 19,543 community cases and 377 admissions to hospital where a specific variant of Covid-19 was confirmed.
Of these totals, 7,723 cases and 134 hospital admissions were found to have the Delta variant, which is thought to be about 60% more transmissible than the previously dominant Alpha variant.
As with other variants, people with underlying conditions were more at risk of being admitted to hospital, researchers said.
While vaccines were found to reduce the risk of being admitted to hospital, strong protective effects against the Delta variant were not seen until at least 28 days after the first vaccine dose, researchers found.
In community cases at least two weeks after the second dose, the Pfizer/BioNTech jab was found to provide 79% protection against infection from the Delta variant, compared with 92% against the Alpha variant.
While the Oxford/AstraZeneca vaccine offered 60% protection against infection with the Delta variant compared with 73% for the Alpha variant.
Experts say this lower vaccine effect may reflect that it takes longer to develop immunity with the Oxford jab.
Professor Aziz Sheikh, director of the University of Edinburgh’s Usher Institute and EAVE II study lead, said: “Over a matter of weeks the Delta variant has become the dominant strain of SARS-CoV-2 in Scotland.
“It is unfortunately associated with increased risk of hospitalisation from Covid-19.
“Whilst possibly not as effective as against other variants, two doses of the Pfizer/BioNTech and Oxford/AstraZeneca vaccines still offer substantial protection against the risk of infection and hospitalisation.
“It is therefore really important that, when offered second doses, people take these up, both to protect themselves, and to reduce household and community transmission.”
Prof Sheikh told a press briefing that any delay in lockdown measures being relaxed, as Boris Johnson is expected to announce, will enable more people to get the greater protection that two vaccine doses offer.
He said: “If there is a delay, I think that will give us the opportunity to widen coverage, which is incredibly important for those who at the moment have only got one dose.
“It will give the opportunity to increase the proportion of the population with two doses and then what we want is a period of time where people can actually maximise their immune responses.
“I think any sort of increase in the window of opportunity before lockdown measures are completely brought to an end, will be helpful, because that will help us to control community transmission.”
“So, overall, I’d be very supportive of any delays that might be announced.”
The researchers said their findings had been passed on to Sage and scientists advising the Government about the relaxation of measures on June 21.
Referring to the study findings, Dr Jim McMenamin, Covid-19 National Incident Director for Public Health Scotland said: “These results provide early encouragement that two doses of either Pfizer/BioNTech or Oxford?AstraZeneca vaccines significantly reduce the risk of infection against both the Alpha or new Delta variants.
“They also show the vaccines offer protection against the risk of hospitalisation with the new Delta variant.
“Though no vaccine can be 100% protective, they provide the best protection against Covid-19 and it remains important to get both doses when offered.”
The findings are published as a research letter in The Lancet.
The researchers say to increase confidence in these early findings, the research needs to be repeated in other countries and settings.
Professor Chris Robertson, Strathclyde University, said “It is important to recognise that these are preliminary results using rapidly accessible data.
“A fuller understanding will come when the results presented here are combined with similar analyses from other data sets in the UK.”
The team say that because of the observational nature of the study, data about vaccine effectiveness should be interpreted with caution and it is not possible to make a direct comparison between both vaccines.
The study was funded by the Medical Research Council, UK Research and Innovation Industrial Strategy Challenge Fund and Health Data Research UK (HDR UK), and was supported by the Scottish Government.