Researchers then showed the participants two reassuring statements that vaccines pose little risk. Half the participants read: “There is only sporadic evidence that repeated vaccinations overwhelm the immune system.” The other half read: “There is no evidence that repeated vaccinations overwhelm the immune system.”That makes sense to me. If someone tells me that some medical treatment has no known adverse reaction, that tells me that either he is lying or the treatment has not been studied. The pro-vaccine lobbyists insist on giving dumbed-down propaganda that is contrary to common sense.
Those who were told there was no evidence for risk reported greater concern about vaccination and less intention to vaccinate their child than those who read the moderate messaging.
"Evidence is mounting that research is riddled with positive bias. Left unchecked, the problem could erode public trust, argues Dan Sarewitz, a science policy expert, in a comment piece in Nature. ...These are strange ideas about science. Of course it is possible to positively show that measles vaccine is reasonably safe and prevents epidemics. That is why the vaccine is recommended. The package insert also warns of various adverse effects that were reported in studies.
There's a brilliant line on this in "The science of discworld" - I won't pretend I can quote it 100% accurately off the top of my head but it goes something like this: "In the media you will often read that a certain scientist is trying to prove a theory. Maybe it's because journalists are trained in journalism and don't know how science works or maybe it's because journalists are trained in journalism and don't care how science works - but a good scientist never tries to prove her theory, a good scientist tries her best to disprove her theory before somebody else does it for her, failing to disprove it is what makes a theory trustworthy." ...
Actually, you need to read "The Black Swan" by N. N. Taleb. Science that tries to confirm a theory is already infected with confirmation bias. There are a pile of examples that demonstrate the fallacy of confirmatory inference. Taleb uses a variant of Bertrand Russell's -- a turkey might reasonably infer, based on his daily experience, that humans exist for the sole purpose of feeding him, caring for him, providing for his every need. This might go on for day after day, increasing the turkey's degree of belief in his hypothesis of a good and beneficent humanity filled with love of turkeys, right up to the day that -- ulp -- something unexpected happens.