The study, published in The Lancet Infectious Diseases on Friday, is an analysis of more than 40,000 coronavirus infections in England. It adds to evidence suggesting that Delta may cause more severe illness than other variants do.

Fewer than 2 percent of the infections occurred in fully vaccinated people, and there was not enough data to draw firm conclusions about hospitalization risks in that group specifically, the researchers said.

“The main takeaway is that if you have an unvaccinated or only partially vaccinated population, then an outbreak of Delta can lead to a higher burden on hospitals, on health care, than an Alpha outbreak would,” said Anne Presanis, a senior statistician at the University of Cambridge and one of the study’s lead authors.