They’ll figure it out as they go along and gather more test data.
And the entire population of the USA are the lab rats. This is why I wouldn't take it. It is "gene therapy", and even the makers, Pfizer, Moderna, admit that it is gene therapy. There are no studies of the long-term affects of doing this. We will learn, eventually.
Hubby and I got our vaccines, we took the J&J (Jansen) vaccine. Don't yell at me, I don't see any good option here. This one, however, (as I heard someone explain it) is made the way all other vaccines have been made up to this point. The 2-shot Pfizer/Moderna one is a first of its kind. So, we will just have to see what, if any, are the long term effects.