Unprecedented global effort is under way to facilitate the testing of countermeasures in infectious disease emergencies. Better understanding of the various options for trial design is needed in advance of outbreaks, as is preliminary global agreement on the most suitable designs for the various scenarios. What would enhance the speed, validity, and ethics of clinical studies of such countermeasures? Focusing on studies of vaccine efficacy and effectiveness in emergencies, we highlight three needs: for formal randomized trials-even in most emergencies; for individually randomized trials-even in many emergencies; and for six areas of innovation in trial methodology. These needs should inform current updates of protocols and roadmaps.