The vaccine is what it is. There was no way to know how good it could be before it was used. It is certainly better than not taking it.
What, then, is the reason behind the FDA approval process, normally spanning two to three years, not counting the trials? Look at the Pfizer FDA approval - read the entire letter. There is a list of recommended trials and tests that normally would have to be completed but were bypassed in this case.
You don't need to release it to test the effects, just like you don't need to pass a bill to find out what's in it.