The new study also confirmed two doses of vaccine still offers the best protection against the variant, which was first identified in India, and accounts for more than 90 per cent of all new cases in the UK.
Although the vaccines reduced the risk of people needing to be admitted to hospital, it could take up to 28 days after the first dose to have sufficient immunity.
The research, published in The Lancet, by a team from the Universities of Edinburgh and Strathclyde, as well as Public Health Scotland, examined 19,543 community infections and 377 hospitalisations with a specific variant of Covid between 1 April and 6 June.
Approximately 7,700 cases and 134 hospitalisations were found to have the Delta variant.
The researchers found the risk of being hospitalised with the Delta variant was 85 per cent higher than the Alpha strain after adjusting for age, sex, deprivation and other comorbidities. People with underlying conditions were more at risk of needing hospital treatment.
The team defined a hospital admission as an admission 14 days after testing positive for the virus as well as any patient testing positive within two days of being admitted.
In community cases at least two weeks after the second dose, the Pfizer/BioNTech vaccine was found to provide 79 per cent protection against infection from the Delta variant, compared with 92 per cent against the Alpha variant.
For the same scenario, the Oxford-AstraZeneca vaccine offered 60 per cent protection against infection with the Delta variant compared with 73 per cent for the Alpha variant. This lower vaccine effect may reflect that it takes longer to develop immunity with Oxford-AstraZeneca, experts say.
The Delta variant is able to be identified through genetic testing, because it has a gene that is present and can be detected, compared to the Alpha variant which lacks the same gene.
The experts say the research, which is only observational, needs to be repeated in other countries and some uncertainty around the vaccine effectiveness remains.
Professor Aziz Sheikh, director of the University of Edinburgh’s Usher Institute, 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.”
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 per cent protective, they provide the best protection against Covid-19 and it remains important to get both doses when offered.”