I'm looking for a solution to the liability issue.
Do you mean an explanation of how liability works?
From what I've read it probably varies from trial to trial. To know what Pfizer offered/required we'd have to see one of their consent forms.
My guess - based on reading about other trials - is that they would be liable if they had done something wrong but probably, outside of providing basic treatment in the immediate event, they don't need to do anything.
This is from an FAQ talking about Covid-19 vaccine trials:
To be honest, I did some drug studies when I lived in New York in my early-twenties - as an illegal alien! - and I know I didn't read the consent form, didn't think for one second that something might go wrong, and had no idea about healthcare. All I thought about was the nice bed, the unlimited sorbets, and the check at the end of it. I'm sure there are still people like that today.