Clinical trials take long because they are evaluating long term side effects. They didn't deem it unsafe to give to smaller groups of people, in many cases the company withdrew it from consideration because earlier uses were centered on oncology - taking pieces of patients' cancer cells and attempting to get the body to fight them immunologically - and they weren't able to obtain a clinically significant impact. Running a long term clinical trial is an expensive proposition.

1. https://www.clinicaltrialsarena.com/comment/evolution-mrna-v...