I caught on at the start of these 'clinical trials'. When people dropped out because of bad side effects or even death, they were not included in the rest of the study obviously. They only counted those who completed the study (so, of course there were no deaths) and complaints of side effects were minimal.

Simply put, if 1000 people participated in the study, 900 died, they based findings on the 100 who lived through and completed the study.