• In medical research, human challenge trials are controlled studies that involve deliberately infecting participants with a pathogen or a bug to study the effects of that infection.
• "Challenge studies tell us things that other studies cannot because, unlike natural infection, they are tightly controlled," said Helen McShane, chief investigator of the study and professor of vaccinology at the Department of Paediatrics, University of Oxford.
• The full length of the study will be 12 months, including a minimum of eight follow-up appointments after being discharged.

LONDON — Researchers at the University of Oxford on Monday announced the launch of a human challenge trial to better understand what happens when people who have already contracted the coronavirus are infected for a second time.

Researchers will examine what kind of immune response could prevent people from becoming reinfected with Covid-19 and investigate how the immune system reacts to the virus a second time round.

At present, little is known about what happens to people who have already had the virus when they are infected for a second time.

The trial will take place in two phases, with different participants in each phase. The first phase is scheduled to get underway this month and the second phase is due to start in the summer.

In medical research, human challenge trials are controlled studies that involve deliberately exposing participants with a pathogen or a bug to study the effects.

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

"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," McShane said.

It is hoped the study will help to improve scientists' basic understanding of the virus and help to design tests that can reliably predict whether people are protected.

What happens in each phase?

For phase one, up to 64 volunteers aged between 18 to 30-years-old who have previously been naturally infected will be re-exposed to the virus in controlled conditions.

Researchers will oversee the care of the participants as they undergo CT scans of the lungs and MRI scans of the heart while isolating in a specially designed suite for a minimum of 17 days.

All of those who take part are required to be fit and well and must have completely recovered from their first infection of Covid to minimize risk.

The trial participants will only be discharged from the quarantine unit when they are no longer infected and at risk of spreading the disease.

The second phase of the trial will explore two different areas.

"First, we will define very carefully the baseline immune response in the volunteers, before we infect them. We will then infect them with the dose of virus chosen from the first study and measure how much virus we can detect after infection. We will then be able to understand what kind of immune responses protect against re-infection," McShane said.

"Second, we will measure the immune response at several time points after infection so we can understand what immune response is generated by the virus," she added.

The full length of the study will be 12 months, including a minimum of eight follow-up appointments after being discharged.

"This study has the potential to transform our understanding by providing high-quality data on how our immune system responds to a second infection with this virus," Shobana Balasingam, vaccines senior research advisor at Wellcome, a charitable foundation that is funding the study.

"The findings could have important implications for how we handle COVID-19 in the future, and inform not just vaccine development but also research into the range of effective treatments that are also urgently needed," Balasingam said.