Have you been told that there are no double-blind, placebo-controlled, randomized trials on vaccines? That is untrue. Here is a list of many such studies!
Note: Once there is an effective vaccine, future development of newer vaccines for the same antigen do not get compared to a placebo. They get compared in studies to the prior version of the vaccine. This is so that half of the population in the study does not need to remain exposed to preventable diseases.