This has always been my objection to the claim that the vaxx makes the symptoms less bad: there is no way to make a controlled experiment to support the claim. What you would have to do: take the exact same person but somehow have one version of him vaxxed and another version unvaxxed. The closest they could come would be identical twins, but even they can have environmental and behavioral differences that can influence different pre-existing health conditions. Next you’d have to infect each of the two versions of the person with the exact same strain of illness with the exact same density of infection, and under the exact same circumstances. Then you’d compare and contrast their symptoms.

Besides the ethical dilemma of intentionally infecting humans, the experiment is practically impossible. So the vaxx hoaxers’ claim isn’t scientifically falsifiable. “Lessens the symptoms” is their last desperate straw to grasp in the face of widespread knowledge of the vaxx’s inefficacy.