know how their child reacts to a particular vaccine? NO POSSIBLE WAY. It could've food. It could've been a spider bite. It could've been lack of sleep. It could've been an inversion. It could've been an unknown medical problem. Good grief, even if a child gets sick the day after a vaccine that's ZERO evidence the vaccine had anything to do with it. And these parents, to be honest, don't do anything to help the problem.
A couple years ago a teenager died the day after he got a flu shot. The doctors believe that there was some sort of brain swelling, but they weren't sure. The Mom was convinced it was the flu shot. When questioned how, her response: "I can just feel it." Yeah, that's not weird (or unreliable and ridiculous) at all. Her son's doctor really wanted to do a autopsy to see if they could figure out if there was some relationship to the vaccine, or what it was that caused her son's death. She refused. So a potential opportunity to learn something lost. That's what the anti-vax crowd gives us. A bunch of anecdotal evidence, misinterpretation of the primary literature, putting enormous stock in fabricated publications, and a complete refusal to contribute to real science and discovery when offered the opportunity.