What are human infection studies and why do we need them?

Human infection studies have the power to rapidly accelerate the development of much-needed vaccines and treatments. In this Q&A, we explain what they are, how they work and why they are important.

Animation showing a doctor administering a vaccine to a volunteer

Human infection studies can give an indication that a vaccine is safe and effective far more quickly than would be possible through large-scale population trials.

What are human infection studies?

Human infection studies are clinical trials that involve deliberately exposing volunteers to infectious diseases. 

The results can help researchers to:

  • understand more about how the body’s immune system responds to a disease
  • work out how to prevent the disease or improve its treatment 
  • test the effectiveness of potential new vaccines and treatments.

How do human infection studies work?

Researchers recruit a group of healthy adult volunteers to take part. They explain the process and the risks involved, and they monitor the volunteers’ health throughout the study.

The study could involve testing:

  • vaccines – doctors give the volunteers a dose of the vaccine, then administer a controlled dose of the relevant infectious agent
  • treatments – doctors administer the infectious agent, then give volunteers a treatment.

If volunteers get ill, help is close at hand. Doctors can administer existing treatments that are known to work, and provide other care as needed. If they don’t become ill, it means the vaccine or treatment works.

There are strict guidelines in place to protect the people taking part, and they are financially reimbursed for their time.

Can animals be used instead?

The process of developing and testing new vaccines involves preclinical stages (carried out in lab assays and on animals) and clinical trials (on human subjects). It’s a slow process that can take as long as ten years. 

Few test vaccines make the transition from the preclinical stages to clinical trials. And even for those that get to the first round of clinical trials, a test vaccine that performs well in animals doesn’t always predict protection in humans. Using animals to reflect human disease can be unreliable, because their biology is different from ours and many infectious organisms are specific to humans. 

What are the benefits of human infection studies?

Because the studies use humans in preliminary testing, they give a much earlier indication of whether a vaccine or treatment will work. If it’s successful, it can be made available quickly. 

Human infection studies can take place anywhere there are people. They can focus on a particular population where a disease is endemic, helping to test and develop effective vaccines and treatments for the communities most at risk.   

Another benefit is that the studies produce highly-detailed information. They are performed under controlled conditions, so the timing of infection is known. The volunteer’s health is monitored to see how the disease is progressing and how their body is responding to the treatment or vaccine being tested.  

Our animation explains what human infection studies are and why they are important.

What successful human infection studies have there been?

There have been many successes using human infection studies. A key achievement is the development of the new typhoid conjugate vaccine Vi-TT (Vi-tetanus toxoid conjugate vaccine). 

During testing in humans, the vaccine showed efficacy of 87.1%. As a result, the World Health Organization recommended it as the preferred vaccine for all ages. 

Can the studies be used to develop vaccines and treatments for all diseases?

No, but they have a very wide application. Human infection studies are currently being used to develop vaccines and treatments for diseases such as malaria, typhoid, influenza and dengue. 

They are most appropriate for diseases where no suitable animal models exist. They are only used when there’s already some form of treatment for the disease. 

The health of participants is paramount, so human infection studies are not suitable for diseases such as Ebola or HIV where no cure is currently available. 

What support and compensation do volunteers receive during a study?

Before taking part, volunteers are given an in-depth explanation of what will be involved. Recruiting and screening volunteers often takes weeks, so that they have time to fully consider their decision to take part. The volunteers are required to give their written and spoken consent repeatedly during the study.

Throughout the testing, volunteers have access to doctors and support workers who can answer any questions and provide necessary treatment. 

Wellcome and other major funders of human infection studies are committed to making sure that studies of this kind are conducted ethically and we have a set of principles that all the studies we fund must follow. 

More information