Vaccines are a powerful measure to protect the health of individuals and to combat outbreaks such as the COVID-19 pandemic. An ethical dilemma arises when one effective vaccine has been successfully developed against an epidemic disease and researchers seek to test the efficacy of another vaccine for the same pathogen in clinical trials involving human subjects. On the one hand, there are compelling reasons why it would be unethical to trial a novel vaccine when an effective product exists already. First, (...) it is a firm principle of medical ethics that an effective treatment or vaccine should not be withheld from patients if their life may depend on it. Second, since epidemic outbreaks often emerge in settings with less-resourced health systems, there is a pronounced risk that any trial withholding an effective vaccine would disproportionately affect the vulnerable populations that historically have been exploited for biomedical research. Third, clinical trials for novel vaccines may be at odds with efforts to control active outbreaks. On the other hand, it may be justified to conduct a trial for a candidate vaccine if it is expected to have certain advantages compared with the existing product. This essay discusses key factors for comparing vaccines against epidemic pathogens, including immunological, logistical and economic considerations. Alongside a case study of the development of vaccines for Ebola, the essay seeks to establish a general framework that should be expanded and populated by immunologists, epidemiologists, economists and bioethicists, and ultimately could be applied to the case of COVID-19 vaccines. (shrink)