A lot of things, but just the top two would be enough:
1) Being able to sue. If you can sue J&J for baby powder, should be able to sue Pfizer for vaccines if anything goes wrong.
2) Falsifiability in government actions. Many rights have been taken away (some permanently) by saying "we just need to do this and then it will all be over" like "a few weeks to flatten the curve", "the last lockdown", and "x% vaccinated and we'll be back to normal".
But when they don't work, the people are blamed, more rights are taken and more restrictions imposed. It's never that the actions were wrong.
I expect something like if "70% of people are vaccinated then we expect no more than 5000 cases per day" and if the statement turns out to be false there will be no more vaccine impositions because it clearly will have turned out to be wrong.
If something doesn't work you shouldn't double down.