The standard way to judge the efficacy of a vaccine is to conduct a trial in which those who could benefit are randomly assigned to two groups, one of which receives the potentially beneficial vaccine, while the other receives a substance with no active ingredients, known as a placebo. To avoid possible confounding factors, such as greater risk-taking by those who know they have been vaccinated, or the desire of those conducting the trial to show that the vaccine works, the study is “double blind”: Neither the subjects nor those administering the trial and collecting the data know who got the vaccine and who got the placebo.