I'm reminded of this story I heard not too long ago: http://lesswrong.com/lw/37k/rationality_quotes_december_2010...

Note that in this case of circumcision the possible negative effects are well known, but a vaccine might have complications in X years that would not be worth the decreased contraction of AIDS. In hypothetical example, perhaps it increases the chances and severity of heart attacks by some ridiculous percentage.

In general, with medicine, it sucks to be on the wrong side of an experiment, but it can be difficult to know which side that is.