Human challenge trials have never been more popular
The ethics of clinical trials that deliberately infect people with a disease aren't clear-cut – but there's a strong case for doing more of them
Leader and Health
The ethics of clinical trials that deliberately infect people with a disease aren't clear-cut – but there's a strong case for doing more of them
3 December 2025
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Would you answer an ad like that? What about one offering the promise of violent diarrhoea? How many stars worth of service would tempt you to add a sexually transmitted infection to your stay? Perhaps a nice monetary payment would help?
Welcome to the bizarre world of human challenge trials – coming soon to a biosecure quarantine facility near you.
Boosted by the collective nightmare of the covid-19 pandemic, an increasing number of researchers are asking healthy people to sign up for experiments that deliberately make them sick. And from dysentery and cholera to gonorrhoea, volunteers are agreeing to catch diseases in greater numbers than ever before.
As we discuss on page 38, the trials are a quick and relatively cheap way to test vaccines and treatments and track the progress of infections. They also aren’t as dangerous as they might sound. The trials only get the go-ahead under strict medical supervision and if the symptoms can be quickly doused with effective therapies.
“ Deliberately infecting healthy volunteers isn’t risk-free, nor are the ethics clear-cut “
But they aren’t risk-free, nor are the ethics clear-cut. Unlike people with an illness agreeing to try an experimental drug that might cure them, challenge trials aim to make people feel worse, even if just for a short time, with little or no direct medical benefit in return.
And it isn’t always possible to guarantee there will be no enduring effects. Some ethicists, for example, are still unhappy that UK scientists carried out challenge trials with covid-19 during the pandemic, because of the risks of the chronic symptoms of long covid.
But the pandemic also highlighted the enormous positive value and impact of vaccines. Data gathered so far suggest human challenge trials are safe, especially for the young and healthy. As these studies could speed the development of new protections against long-standing scourges, including malaria, Zika and norovirus, the only real challenge should be: how can we do more?