Healthy young volunteers who have previously had Covid-19 will be deliberately exposed to the virus for a second time to see how the immune system reacts as part of a new UK study.

Researchers at the University of Oxford have launched what is known as a “human challenge” trial to look at what happens when someone who has recovered from Covid-19 infection is then re-exposed to the virus.

They will aim to determine what dose of virus is needed to re-infect after natural infection, how the immune system responds, and what this may mean for developing protective immunity against the disease.

The study, which is funded by the Wellcome Trust and is expected to start this month after receiving ethics approval, will recruit people aged 18-30 who have previously been naturally infected with Covid-19.

They will be re-exposed to the virus in a safe and controlled environment while a team of researchers monitor their health.

Human challenge studies have played a key role in furthering the development of treatments for diseases such as malaria, TB, typhoid, cholera and flu.

A similar study is ongoing in the UK where volunteers are being infected with coronavirus to test vaccines and treatments.

Helen McShane, professor of vaccinology at the department of paediatrics, University of Oxford and chief investigator on the study, said: “Challenge studies tell us things that other studies cannot because, unlike natural infection, they are tightly controlled.

“When we re-infect these participants, we will know exactly how their immune system has reacted to the first Covid infection, exactly when the second infection occurs, and exactly how much virus they got.

“As well as enhancing our basic understanding, this may help us to design tests that can accurately predict whether people are protected.”

While Covid-19 infections have been rare, recent research suggests prior infection may not fully protect young people against reinfection.

The observational study, published in the Lancet involving US Marine Corps members mostly aged 18-20, showed that between May and November 2020, around 10% of participants who had previously caught coronavirus became re-infected.

The Oxford study will take phase in two phases.

The first phase, involving 64 healthy volunteers, will aim to establish the lowest dose of virus which can take hold and start replicating.

Once the dosing amount is established, it will be used to infect participants in the second phase of the study, which is expected to start in the summer.

Prof McShane said: “We will measure the immune response at several time points after infection so we can understand what immune response is generated by the virus.

“A challenge study allows us to make these measurements very precisely because we know exactly when someone is infected.

“The information from this work will allow us to design better vaccines and treatments, and also to understand if people are protected after having Covid, and for how long.”

Prof McShane virus used in the study will be the original strain from Wuhan, China, “because that is the strain that we have most clinical, immunological (and) virological data on” but added discussions are under way to include one of the new coronavirus variants.

The participants will be quarantined for 17 days under and cared for by the research team at a hospital until they are no longer at risk of infecting others.

Those who develop symptoms will be given a monoclonal antibody treatment developed by Regeneron, which contains laboratory-made antibodies that have shown to reduced the risk of disease progression in clinical trials.

The full length of the study will be 12 months, which will include eight follow-up appointments after discharge.

Prof McShane said those taking part will be reimbursed for their efforts, which will be just under £5,000 for each participant.