hVIVO, a clinical research group in London, has attracted more than 20,000 volunteers willing to be infected with tamer relatives of the virus that causes Covid-19 in exchange for a fee of £3,500 ($4,480). It says such experiments could play an important role in the development of a vaccine against the new coronavirus, for which there are no proven treatments or vaccines.

Unlike drugs, which are tested on people who already have a particular illness, vaccines have to be given to healthy people who are later exposed to a disease. Typically, this is done by giving an experimental vaccine to thousands of people in an area where an infection is circulating, and then tracking them for months or even years. The vaccine is considered successful if those who got the shot avoid infection.

One way of getting a quicker read on a vaccine’s effectiveness is by giving it to people who are then deliberately infected with the bug in question. Such challenge studies are routinely used in the development of vaccines for the flu, common cold and other respiratory illnesses.