They will be paid up to £4,500 for taking part
Scientists are currently growing the Delta Covid variant in laboratories with the aim of intentionally infecting volunteers.
British researchers are developing the samples to use in challenge trials — which see participants paid up to £4,500 for taking part.
Two trials by Imperial College London and the University of Oxford started in London in March with the goal of developing new vaccines and treatments.
Forty healthy, young volunteers have already been exposed to the original Wuhan strain, under careful supervision.
But the next stage of trials will infect participants with Delta, the more transmissible strain which is now dominant across the UK.