The Delta variant currently makes up nearly all cases in the UK, with the bulk of those needing hospital treatment not being vaccinated.
The research, published in The Lancet, found the delta strain ( originating in India) is the current threat in the UK, as opposed to the Alpha variant (originating in Kent, UK).
The study, led by Public Health England (PHE) and the Medical Research Council, examined 43,338 Covid cases that occurred between March and May.
The bulk of these infections were in people who had not yet been vaccinated and while most did not require hospital care, a small portion did.
Of those admitted to hospital, 196 (2.3 percent) were infected with Delta and 764 (2.2%) of those had Alpha.
Risk of hospital admission was twice as high for individuals diagnosed with the Delta variant, compared to those with Alpha, after adjusting for differences such as age, sex and ethnicity.
Experts say being vaccinated should bring down this risk. Both doses are needed for maximum protection.
According to PHE, the effectiveness against hospitalisation after two doses is:
- 96% for the Pfizer-BioNTech vaccine
- 92% for the Oxford-AstraZeneca vaccine
There has been growing concern that the vaccination will wear off after some time, resulting in the government discussing booster shots - a third dose of the vaccine.
However, both vaccines still offer good protection against hospitalisations and deaths caused by Delta despite the efficacy rating dropping.
Therefore, only those who are clinically vulnerable, over 70 and those who work on the health and social care frontline will be offered a booster from September.
Vaccination roll out
More than 47.9 million people, or about 88 percent of people aged 16 and over in the UK, have now received a first dose of a vaccine.
Some 42 million people, or about 78 percent of people aged 16 and over, have had a second.
Dr Gavin Dabrera, from PHE, said: "We already know that vaccination offers excellent protection against Delta and, as this variant accounts for over 99% of Covid 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 symptoms, stay home and get a PCR test as soon as possible."
The Lancet study findings are consistent with data recently reported from Scotland comparing Alpha and Delta risks.
Scotland has seen its highest Covid case rates in recent weeks, with most cases coming from people under the age of 25. It is largely attributed to children returning to school in mid-August.
Despite Scotland reporting a huge increase in the 24 hours to Friday, with 6,835 new cases, First minister Nicola Sturgeon has said a national lockdown was not being considered.