However all these studies are relative. You test a new trial vaccine against your selected placebo and you determine whether over the trial period the new trial vaccine has statistically significant efficacy and whether it has the same, less or more adverse consequences than your placebo. If your placebo happens to be another vaccine you know its historical safety performance so can ascertain your vaccines safety against that. Logically all you would need to do is test the first vaccine against a fully inert placebo and you know how safe or unsafe any subsequent vaccine tested against a vaccine placebo would be relative to that fully inert placebo.
There is a wealth of historical record with which to compare the incidence/frequency of rare conditions that sometimes occur temporally with vaccination with what happens in the general population - that's why broad and credible studies examining these issues can't find a connection between autism and mercury/aluminium in vaccines etc.
So you're saying we have to test everything against a saline placebo and I'm saying we don't have to because we can determine safety against any relative base and know its true safety.
There are probably a whole host of reasons why it's preferable to test a vaccine against a placebo other than saline. The obvious one is the ethical one of where a vaccine already exists and you could cause increased harm if you used a saline placebo. Menta posted in one of the other threads that the UK Covid-19 vaccine trial will use a vaccine and the reason quoted was that it would be too easy to guess if you received the vaccine candidate or not and potentially bias the result - if their assessment is reasonable and correct, that's fine by me.
