Because HPV vaccination is relatively modern, there's trail where maybe the trial for the vaccine you can get today A, was A vs B, but then the previous B has a trial B v Placebo, or it might go back twice, A v B then B v C and C v placebo. There were placebo trials for the early HPV vaccines decades ago.

I had Hodgkins Lymphoma (many years ago now when I was young) and we've been treating that for so long that the earliest curative treatments pre-date scientific medicine. So the trail dries up - we have no proof that the cures are better than nothing except, you know, circumstantially it sure does seem like the freaks who reject treatment just drop dead and I didn't so...

They're still doing research, as with the HPV vaccines they do A v B trials as a placebo would be unethical - it's basically a death sentence. I actually wasn't selected for a trial comparing Stanford V (a new potential therapy from Stanford) against the current gold standard (which I was getting) ABVD. That was a long time ago, but I think the gold standard today is still ABVD.