People are twice as likely to need hospital care if they are sick with the Delta Covid variant, rather than the Alpha variant that was once prevalent in the UK, data from England suggests.
Experts say the big study, published in The Lancet, reinforces why it is important that people get fully jabbed.
Vaccination cuts the risk of serious illness from either variant, although Delta is the current threat.
Delta accounts for almost all UK cases currently.
A further 32,406 new coronavirus cases were recorded in the UK on Saturday, alongside another 133 deaths within 28 days of a positive test.
The study, led by Public Health England (PHE) and the Medical Research Council, looked at 43,338 Covid cases that occurred between March and May - when both Alpha, also known as Kent, variant, and Delta were circulating in the UK.
The bulk of these infections were in people who had not yet been vaccinated.
Most did not need hospital care, but a small proportion - 196 (2.3%) of the people infected with Delta and 764 (2.2%) of those with Alpha - did.
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.
A recent analysis by PHE suggests that effectiveness against hospitalisation after two doses is:
- 96% for the Pfizer-BioNTech vaccine
- 92% for the Oxford-AstraZeneca vaccine
Other research suggests that while protection against symptomatic infections may wane somewhat over time, both vaccines still offer good protection against hospitalisations and deaths caused by Delta.
More than 47.9 million people, or about 88% 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% of people aged 16 and over, have had a second.
It takes a couple of weeks from receiving your second dose to get the fullest protection against Covid.
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.
