COVID-19: vaccine development - human challenge trials

As the race to find treatments and ultimately a vaccine for COVID-19 intensifies, governments and policymakers are under increased pressure to consider unprecedented methods to address the urgency of the pandemic as quickly as possible. One potential avenue to increase the speed of development in this area is through human challenge trials (HCTs), which involve the deliberate infection of human volunteers in order to gain valuable insight into the disease/organism and into the production of a vaccine.

Unknown  risks vs societal benefit

HCTs are thought to streamline or accelerate vaccine development, which may otherwise be limited by imprecise animal trial models.

The World Health Organisation cautions that the value of the information to be gained from HCTs should clearly justify the risks to human subjects. In order to analyse the benefit of such trials, one would first need sufficient understanding of the virus to assess if the HCT is a reasonable risk.

To qualify as a reasonable risk, HCTs would need to offer sufficient potential for societal benefit.

By way of example, a HCT was proposed in 2017 in relation to the development of a Zika virus vaccine. However, ethicists decided the risk was too high due to bystander risk, that is the risk to non-participants who may become infected through sexual activity or pregnancy, not to mention, potential injury to participants including but not limited to chronic paralysis or death.

The societal benefit in the case of COVID-19 is clear. Yet, as there is still so much we do not know about the virus, the risk associated with HCTs may be perceived as unreasonable in a risk/benefit analysis. Although there is a long history of human challenge trials, and they have successfully contributed to advances in the development of drugs and vaccines over the years, it is clear that such research can appear to be in conflict with the guiding principle in medicine to do no harm, and therefore must be carefully considered.

Ethical considerations

Participation in HCTs raises many ethical questions, not least whether competent adults are legally able to consent to an uncertain degree of risk or harm. What is more, doubt has been cast over whether regulators such as the FDA would accept results from COVID-19 HCTs or indeed if pharmaceutical companies would utilise them given the level of risk involved and the absence of any treatment for those who become seriously ill.

Current position

Regardless of this, young, healthy individuals have been signing up as volunteers for HCTs, such as the one run by US-based vaccine advocacy group 1DaySooner. This site encourages people to either sign up for any potential HCTs or advocate on behalf of their use. Whilst no such studies are yet being conducted, more than 24,000 people from 102 countries have signed up to express their interest on the 1DaySooner website.

As the infection levels around the world increase, scientific and political support is growing for the use of HCTs. However, as regulators have yet to clarify their position on HCTs in relation to COVID-19, it may be that traditional clinical trials could be just as effective and efficient in assisting with a vaccine development.


With speed being a crucial factor in the fight against COVID-19, regulators and lawmakers will come under increased pressure to consider unprecedented measures relatively quickly. To do so will require , balancing legal, medical and ethical considerations that may have potential long term and global implications.

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