Human challenge studies for COVID-19

Researchers are set to explore a human challenge study with the virus that causes COVID-19, the first such study anywhere in the world.

The Human Challenge Programme is a partnership between Imperial College London, the Department for Business, Energy and Industrial Strategy (BEIS), hVIVO, a leading clinical company with expertise in viral human challenge models, and the Royal Free London NHS Foundation Trust.

The researchers hope that the work will ultimately help to reduce the spread of the coronavirus, mitigate its impact and reduce deaths from COVID-19.

The first stage of the project will explore the feasibility of exposing healthy volunteers to the SARS-CoV-2 coronavirus. The study would recruit volunteers between the ages of 18 and 30 with no previous history or symptoms of COVID-19, no underlying health conditions and no known adverse risk factors for COVID-19, such as heart disease, diabetes or obesity.

In this initial phase, the aim will be to discover the smallest amount of virus it takes to cause a person to develop COVID-19. This is known as a virus characterisation study. Once this first phase is completed, clinical researchers aim to use this human challenge model to study how vaccines work in the body to stop or prevent COVID-19, to look at potential treatments and study the immune response.

Human challenge studies help clinical researchers establish which vaccines are most likely to succeed. As the prevalence of COVID-19 rises and falls in populations, it can make it difficult for traditional vaccine trials to assess if vaccines work, because volunteers receiving the vaccine may not be naturally exposed to the virus. Because a human challenge study deliberately infects the volunteers it should be
possible for scientists to begin to establish efficacy very quickly, by testing if those who have had a vaccine are less likely to become infected with the virus.

Human challenge studies also make it possible for scientists to compare the efficacy of vaccine candidates by testing them side by side to establish which is more effective. At this early stage no specific vaccine candidates for the human challenge trials have been confirmed.


Initially, researchers will assess what amount of virus is needed to cause infection and elicit an immune response by slowly increasing the viral dose to which small groups of volunteers are exposed. The proportion of participants becoming infected and the amount of virus that they subsequently shed will be tracked to better understand the course of infection. As higher viral doses may be linked to more severe
outcomes, the researchers are aiming to infect volunteers with the lowest possible dose to trigger viral replication but minimise symptoms. Before leaving the residential clinical facility, volunteers would be required to test negative for the presence of SARS-CoV-2 in two separate laboratory tests, highly sensitive to the presence of virus.

Dr Chris Chiu, from the Department of Infectious Disease at Imperial College London and lead researcher on the human challenge studies, said: “Human challenge studies can increase our understanding of COVID-19 in unique ways and accelerate development of the many potential new COVID-19 treatments and vaccines.

“Our number one priority is the safety of the volunteers. My team has been safely running human challenge studies with other respiratory viruses for over 10 years. No study is completely risk free, but the Human Challenge Programme partners will be working hard to ensure we make the risks as low as we possibly can.

“The UK’s experience and expertise in human challenge trials as well as in wider COVID-19 science will help us tackle the pandemic, benefiting people in the UK and worldwide.”

The human challenge study will be reviewed by a specially convened ethics committee before any volunteers are enrolled and the Medicines and Healthcare products Regulatory Agency (MHRA), an exemplar globally of medicines and device regulation, will be asked to approve the study before it is conducted.


Since the start of the global pandemic, doctors have learned a great deal about how the SARS-CoV-2 virus spreads and which groups may be at increased risk, as well as gaining clinical insights into treatments which can reduce the severity of disease and its complications.

However, despite these advances, there remain very few therapies for treating patients with COVID-19 and there is no effective vaccine available. Human challenge studies could be a crucial component of progressing research into treating and preventing the disease.

Professor Peter Openshaw, co-investigator on the study and Director of the MRC-funded Human Challenge Consortium (HIC-Vac) at Imperial College London said: “Deliberately infecting volunteers with a known human pathogen is never undertaken lightly. However, such studies are enormously informative about a disease, even one so well studied as COVID-19.

“It is really vital that we move as fast as possible towards getting effective vaccines and other treatments for COVID-19, and challenge studies have the potential to accelerate and de-risk the development of novel drugs and vaccines. These studies form a part of the global effort and play to a unique strength that we have at Imperial and in the UK”.

The study is expected to begin early next year, with further details of volunteer recruitment and the study design to be published in the coming months.

Anyone interested in registering their interest in future COVID-19 human challenge study research should visit

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