So from what I've heard is that sometimes very rarely the vaccine doesn't actually work as intended for a number of reasons. Could be that the person had multiple infections and the vaccine got bombarded too quickly etc. I've even heard of an instance where the vaccine was accidentally administered into a tumor and the person died later because of the mistake. Anyways I think the booster itself is mainly just to make sure everything is good. Its basically like antibiotics, doc wants to make sure you took them all so you don't come in later with something more serious that they can't fix. Last thing anyone wants is a lab result coming back showing you have massive holes in your brain, lock jaw and a very painful last few months to live. But your medical treatment is entirely up to you so if your fine with not getting it that's entirely your choice.