When could human challenge trials be deployed to combat emerging infectious diseases? Lessons from the case of a Zika virus human challenge trial.

Human challenge trials (HCTs) deliberately infect participants in order to test vaccines and treatments in a controlled setting, rather than enrolling individuals with natural exposure to a disease. HCTs are therefore potentially powerful tools to prepare for future outbreaks of emerging infectious diseases. Yet when an infectious disease is emerging, there is often substantial risk and uncertainty about its complications, and few available interventions, making an HCT ethically complex. In light of the need to consider ethical issues proactively as a part of epidemic preparedness, we use the case of a Zika virus HCT to explore whether and when HCTs might be ethically justified to combat emerging infectious diseases. We conclude that emerging infectious diseases could be appropriate candidates for HCTs and we identify relevant considerations and provide a case example to illustrate when they might be ethically acceptable.