Human trials for a Covid-19 vaccine developed by Oxford University are beginning in South Africa and in Brazil.
Some 2,000 people in South Africa will be involved in what is widely considered to be one of the earliest and most advanced trials for a vaccine to tackle the pandemic.
The first doses of the vaccine are being administered in Johannesburg this week.
South Africa has been chosen, not just for its expertise in this field, but because Covid-19 is now spreading fast there.
That makes it far easier for scientists to find a community at immediate risk of infection, and to then tell whether this British vaccine is effective.
Similar tests are already underway in the UK, but the infection rate there is slowing. Hence the move, not just to South Africa, but to Brazil too, where 5,000 people will be involved in the vaccine trial.