People are twice as likely to need hospital care if they are sick with Delta Covid, rather than the Alpha, or Kent, variant that was once common in the UK, data from England suggests.
Vaccination cuts the risk of serious illness from either variant, although Delta is the current threat, reports BBC.
Delta accounts for almost all UK cases currently.
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 and Delta were circulating in the UK.
The bulk of these infections were in people who had not yet been vaccinated.