There is double blind studies that tests efficacy of new vaccines for diseases that already had an approved vaccine. And those studies do find differences in ef

There is double blind studies that tests efficacy of new vaccines for diseases that already had an approved vaccine. And those studies do find differences in efficacy (just because the old vaccine was 96% effective doesn't mean the new one can't do better) as well as in occurrence rate of side effects. There have even been studies where the vaccine that you get in the control arm of the study is the vaccine against Meningitis and then after a year or two they do give you what ever vaccine you had not received yet. (Sorry, forgot against which disease the vaccine under test was protecting.) And of course there is double blind studies where you get three injections, one of which is saline, two are active. They are given a few weeks apart, you don't know the order and they check antibody level in the blood every week.