Researchers stunned the world in July when they announced success with a trial of an Ebola vaccine in Guinea. But the unusual design of the study and the decision to abandon the control group because of strong evidence that the vaccine prevented disease means there may not be enough data to win approval from regulatory agencies with traditional efficacy results. The vaccine, which is made by Merck, and a second one developed by GlaxoSmithKline that also has been tested during the recent Ebola epidemic in West Africa, may still come to market through alternative pathways that do not require as rigorous a level of proof that they work. Both vaccines have worked extremely well in monkey studies, and the U.S. Food and Drug Administration may grant approval based on what’s known as the “animal rule” or through the more demanding “accelerated approval” pathway. Neither company has yet filed for licensure and both are continuing to collect more human and animal data to support their applications for approval.