Researchers looked at more than 43,000 cases in England between March and May with their findings published in medical journal The Lancet Infectious Diseases.
Nearly three-quarters of coronavirus cases in the study were in people who were unvaccinated, with 1.8% coming in those who had received both inoculations.
The study said around one in 50 patients were admitted to hospital within two weeks of their first positive test, or 2.3% of Delta cases and 2.2% of those with the Alpha variant, first identified in Kent.
Researchers then adjusted these figures to take into account other factors to determine the risk of being admitted to hospital was more than doubled with the Delta variant compared with the Alpha variant.
These included someone’s age, ethnicity and vaccination status.
Dr Gavin Dabrera, one of the study’s lead authors and a consultant epidemiologist at the National Infection Service, Public Health England, said: “This study confirms previous findings that people infected with Delta are significantly more likely to require hospitalisation than those with Alpha, although most cases included in the analysis were unvaccinated.
“We already know that vaccination offers excellent protection against Delta and as this variant accounts for over 98% of Covid-19 cases in the UK, it is vital that those who have not received two doses of vaccine do so as soon as possible.
“It is still important that if you have Covid-19 symptoms, stay home and get a PCR test as soon as possible.”
The authors said it is not possible to draw conclusions about risk among those who have been vaccinated and go on to develop infections, and studies have shown a link between vaccination and prevention of serious illness from coronavirus.
Data from the UK Government shows 88.2% of people aged 16 and over have had at least one dose of a vaccine, and 90.2 million doses given overall.
Dr Anne Presanis, one of the study’s lead authors and Senior Statistician at the MRC Biostatistics Unit, University of Cambridge, said: “Our analysis highlights that in the absence of vaccination, any Delta outbreaks will impose a greater burden on healthcare than an Alpha epidemic.
“Getting fully vaccinated is crucial for reducing an individual’s risk of symptomatic infection with Delta in the first place, and, importantly, of reducing a Delta patient’s risk of severe illness and hospital admission.”