Human Challenge Trials Overview

Exposing healthy volunteers to a virus or bacteria to learn more about disease – and better protect those we care about

A closer look at human challenge trials, why they’re important, and what it’s like to be a participant.

In a human challenge trial, healthy, informed, consenting participants are intentionally exposed to a bacteria or virus in a safe environment. These trials can provide important insights on how to improve a vaccine or treatment.

Human challenge trials could help us learn how to better protect people around the world from severe infections.

Vaccines are a vital part of protecting people of all ages from infectious diseases and have been among the greatest and most effective public health achievements for decades. Every year, immunization prevents 3.5-5 million deaths from diseases like diphtheria, tetanus, pertussis (whooping cough), influenza and measles.

Vaccines are held to high standards and must be carefully tested and studied through a series of clinical trials before approval. In these trials, researchers give the vaccine to volunteers and evaluate how safe the vaccine is, how and if the immune system responds to the vaccine, and what the most effective dose is.

“Despite the thoroughness of these trials, there are some questions they can’t answer. ”

These multi-phase vaccine trials are seen as the gold standard for evaluating new vaccines – they give us the highest level of information about a vaccine’s safety and effectiveness, they are monitored and regulated to ensure high standards are met, and they allow regulators to make decisions about a vaccine with confidence. But, despite the thoroughness of these trials, there are some questions they can’t answer.

Another type of study, known as a human challenge trial or a controlled human infection model, may give us a better idea of how well vaccines protect us.

Human challenge trials usually take place as inpatient studies. The participants stay in a specialized facility with healthcare support available around the clock.

What is a human challenge trial?

In human challenge trials/studies, healthy, informed, and consenting adults are intentionally exposed to a pathogen (virus or bacteria). This is carefully planned to keep participants safe and takes place in a controlled environment. The participants are monitored for symptoms and their immune responses are studied with healthcare providers present. They are given a treatment to completely clear the infection before symptoms become severe and are scheduled for check-ups to continue monitoring them once they’ve fully recovered.

Why are human challenge trials conducted?

Human challenge trials are considered when questions about an infectious disease have gone unanswered for a long time, and the illness has significant impacts on society. A challenge trial in these cases can give researchers a clearer picture of a vaccine’s ability to reduce or prevent disease.

Human challenge trials provide important insights on how well vaccines protect us, our immune responses, and how to improve a vaccine or treatment.

The studies are usually inpatient, which means there are fewer outside influences on the study and the results are clearer. Because the exposure is carefully planned, researchers are also able to closely monitor each stage of the illness in relation to the timing of exposure. This can help researchers learn how the immune system responds to the illness itself.

Read more: why human challenge trials are conducted

The insights gained from human challenge trials can help us improve existing, or develop new, vaccines or treatments. These advancements can prolong lives around the world and have a significant impact on those who may be most vulnerable to disease.

Read more: the impact of human challenge trials

How is participant safety ensured?

Informed consent is crucial to an ethical and safe clinical study. Volunteers must understand what they are being asked to do, before agreeing to take part.

Since participants are guaranteed to be exposed to an illness, human challenge trials must meet additional requirements to ensure the study will be ethical and safe. No study involving people can be completely risk-free, but there are measures that human challenge trials put in place to minimize risk and make the study as safe as possible. Because of the way these trials are designed and regulated, the level of risk is comparable to a standard clinical trial.

Volunteers are fully informed of the study details, potential risks, and what they are being asked to do before consenting to taking part.

Human challenge trials are only considered for diseases with highly effective treatments that can be given before symptoms become severe. Researchers make sure that participants only develop mild symptoms and are as comfortable as possible. Healthcare staff are on site around the clock. To further minimize risks, the volunteers in a trial can’t be anyone who might be particularly vulnerable to serious illness from the disease.

Challenge trial participants stay for days to weeks in a specialized facility. Participants can use the time between scheduled tests during the day for remote work or to relax. Despite a short period of feeling under the weather, some past volunteers have even described their experience as a kind of vacation.

Read more about participant safety and experiences

Volunteers are Vital

Volunteers are a vital part of every clinical study, and human challenge trials are no different.

Thousands of people have volunteered for human challenge trials around the world. Their participation has made a difference – and yours can, too. To learn more about participation in a challenge trial at CCfV, visit our participation page.