150 vaccines in development for Covid-19

There are now 150 vaccines in development for Covid-19 (National Geography, 9//3/2020). The U.S. Food & Drug Administration (FDA) has hired you to oversee the research of three companies that are working on a vaccine against the SARS-CoV-2 virus. The FDA knows you are an expert in applying the scientific method and wants you to be sure the Phase III trials are done properly. Phase III trials determine if a vaccine is safe and effective for a wide variety (array) of the human population. Pfizer, BionTech, and Moderna Company have the same approach for their vaccines: Use the viral messenger RNA (mRNA) to cause human cells to produce viral S proteins (S=spike proteins), those S proteins should stimulate the human immune system to create antibodies against this virus. The proposed new antibody would thereby prevent the viral S-protein from attaching to human cells thus stopping infection of vaccinated humans. How would you investigate which company has the best vaccine using the scientific method?