Fully-vaccinated people have an around 50 to 60 percent reduced risk of infection from the Delta coronavirus variant, including those who are asymptomatic, a large English coronavirus prevalence study found on Wednesday.

Imperial College London researchers said people who reported receiving two vaccine doses were half as likely to test positive for Covid-19, adjusting for other factors such as age, whether or not the people tested had Covid-19 symptoms.

Focusing on those who had Covid-19 symptoms, effectiveness rose to around 59 percent according to the study, which covered a period when the Delta variant...