Nothing can ever be "completely ruled out". For instance, there are cases where a person wearing a seat-belt gets in a crash and dies, where without the seat-belt, they very likely could have survived. These things happen. Wearing seat-belts is still mandatory, because the benefits far, far out-weigh any risks.
Besides, you have to look at the kind of people who voluntarily refuse vaccinations: They aren't doing it because a specific vaccination may be unsafe. They refuse all vaccinations, because they're evil tools of government control. So, let's say some vaccination gets through testing with some massive flaw that poisons everyone who takes it. Do we really want to be left with a world that ONLY contains the lunatics who refuse vaccinations?