Ongoing and anticipated COVID-19 human challenge studies in the UK may advance our understanding of COVID-19 and facilitate the licensure of safe, effective, and easily deployable next-generation COVID-19 vaccines and boosters. We argue that international volunteer recruitment for COVID-19 human challenge trials can help promote diversity in these trials and ensure a sufficient number of eligible volunteers, both of which will increase the benefits of challenge research. We explore the ethical ramifications of dealing with unfair background conditions of global vaccine (...) injustice to expand medical research, and conclude that international recruitment for COVID-19 human challenge trials can be conducted ethically provided that several robust protections are in place for volunteers. (shrink)