The United Kingdom has become the first country to green-light a medical trial that would expose volunteers to the coronavirus. Up to 90 healthy volunteers - aged 18 to 30 -- will be compensated for their participation. The research aims to first establish the smallest amount of virus needed to cause infection. It will then study vaccine efficacy against infection, where volunteers could be given a COVID-19 shot before exposure to the virus. The medical trial is set to begin within a month.