The conduct of COVID-19 vaccine trials in the context of a candidate vaccine being issued with Emergency Use Designation raises challenging ethical questions, including in relation to the use of placebo controls and unblinding of trial participants in current and future COVID-19 vaccine trials. This policy brief was developed by the WHO Access to COVID-19 Tools (ACT) Accelerator Ethics & Governance Working Group, to provide guidance for researchers, sponsors, regulators, research ethics committees, and policy-makers, on these and related issues.