People who get the Delta variant of the coronavirus are twice as likely to be hospitalised as those who were infected by the Alpha variant which was first detected in England last year, a study showed on Friday.

The study, based on more than 43,000 COVID-19 cases of mostly unvaccinated people in England, compared the risk of hospitalisation for people infected with Delta, which was first detected in India, with people who caught Alpha.

"Our analysis highlights that in the absence of vaccination, any Delta outbreaks will impose a greater burden on healthcare than an Alpha epidemic," Anne Presanis, one of the study's lead authors and a University of Cambridge statistician, said.

The study was based on cases between March and May during the early stages of Britain's COVID-19 vaccination campaign, so it was not able to assess the extra risk of hospital admission for unvaccinated or partially vaccinated people.

The study, published in the Lancet Infectious Diseases journal, was the largest so far to analyse COVID-19 cases confirmed by virus genome sequencing.