People who contract the delta variant of COVID-19 are more than twice as likely to be hospitalized as those infected with the alpha strain, according to a U.K. study, raising the prospect of a greater burden on health services this winter.

The review of more than 43,000 COVID-19 cases in England, most of whom were unvaccinated or partially vaccinated, was published in The Lancet Infectious Diseases journal Friday, and highlighted the protection shots provide against hospitalization from both variants.

The study assessed positive cases that occurred between March 29 and May 23, analyzing samples of the virus using whole-genome sequencing to determine which variant a person was infected with. Nearly three-quarters of the patients were unvaccinated, 24% were partially vaccinated and 2% had received both shots.