The Importance of Proving There's No Link Between Childhood Vaccines and Autism

Is it more accurate to say that scientists have ruled out a link between autism and the childhood vaccine schedule or that there has been no link found? The burden of proof is always on the person who's making the positive claim. Every first vaccine pretty much is tested in a saline placebo controlled trial. If you want a new vaccine, then you can test it against one that already went through those trials.