The slippery slope fallacy is an argument that utilizes the false assumption that once a first step is taken, one can predict the next step, which then leads to the next step until some conclusion is reached that leads from the initial step. The arguer then concludes that we therefore shouldn’t do the first thing because it will lead to that conclusion.
There are several issues with this type of fallacy, specifically, it makes a prediction that simply cannot be tested. It also assumes that there isn’t a restraint placed on subsequent steps that modify the conclusion.
Once we start mandatory vaccinations, it’s a slippery slope to a world where our government will take our children away from us to indoctrinate them into socialism.