The world's first medical trial authorised to deliberately expose participants to the coronavirus is seeking more volunteers as it steps up efforts to help develop better vaccines.
The Oxford University trial was launched last April, three months after Britain became the first country to approve what are known as challenge trials for humans involving Covid-19.
Its first phase, still ongoing, has focused on finding out how much of the virus is needed to trigger an infection while the second will aim to determine the immune response needed to ward one off, the university said in a statement on Tuesday.
Researchers are close to establishing the weakest possible virus infection that assures about half of people exposed to it get asymptomatic or mild Covid-19.
They then plan to expose volunteers - all previously naturally infected or vaccinated - to that dose of the virus's original variant to determine what levels of antibodies or immune T-cells are required to prevent an infection.
"This is the immune response we then need to induce with a new vaccine," said Helen McShane, Oxford University Professor of Vaccinology and the study's chief investigator.