Three volunteers are reportedly in good health after being intentionally exposed to the novel coronavirus during the first phase of a controversial United Kingdom COVID-19 trial.

The ultimate goal of the so-called human challenge trial is to test vaccines on infected individuals. In this initial stage, researchers are establishing the lowest viral dose needed to cause infection, by administering nasal droplets containing the virus in a range of concentrations.

The three participants, whose identities are being kept confidential, have recently been released after two weeks in quarantine at the Royal Free Hospital in London, which is hosting the trial in collaboration with biotechnology company hVIVO and Imperial College London.

Researchers are set to perform further viral dosage tests on new groups of volunteers, before entering the vaccine testing stage of the study.

"We're pleased to confirm the first group of three healthy volunteers has now successfully completed the first stage of the trial, with no unexpected issues," said Chris Chiu, who is an infectious disease specialist at Imperial College London, and chief investigator on the trial."The volunteers are in good health. It would be premature to discuss further details at this early stage."

Human challenge trials, which generally involve around 100 volunteers, offer a swift and efficient way to test whether or not a vaccine works, as researchers can guarantee that participants in the study have come into contact with a pathogen. Traditional field trials for vaccines have to be much larger, usually involving tens of thousands of people, and can be delayed if public health measures, including lockdowns that interrupt the spread of disease through a population, are in place.

Challenge trials do however pose a number of ethical concerns, and are usually reserved for non-lethal diseases such as the common cold, or those for which highly effective treatments exist, such as malaria.

Some experts have questioned if enough is known about COVID-19 and its long-term effects in order to conduct challenge trials safely. Meanwhile, others argue that the risk of medical complications among young and healthy volunteers is low, and that such trials would markedly benefit the wider population by speeding up vaccine development.

"This is vital research to understand the course of disease in those with mild infections and to investigate the detailed natural history of SARS-CoV-2 infection," said Peter Openshaw, co-investigator on the study and a professor of experimental medicine at Imperial College London.

"Once this investigational human challenge model is established, we will be able to go on to test vaccine efficacy, immune defense and the benefits of new medicines that become available in the future."