It’s this third phase that the Russian government is skipping and what’s giving researchers pause. A key component to Phase 3 testing is making sure that the drug doesn’t cause any harmful side effects. As reported, rare yet harmful side effects don’t always become evident until an experimental medicine has been administered to thousands of people, and vaccines are no exception. One hypothetical scenario, as Patricia Winokur, the executive dean of the University of Iowa Carver College of Medicine in Iowa City, previously told Popular Science, is that a vaccine could contain ingredients that look like molecular structures in our own bodies, and trigger a dangerous autoimmune reaction. A rare side effect like this wouldn’t necessarily show up if you give the drugs to hundreds of people, but would become clear when given to thousands.