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:
If you feel sick, you should immediately get in touch with our staff to schedule an illness visit. Our medical staff will provide you with care and collect biomedical samples. If you require further care, we will direct you to the appropriate medical facility.
When a 54-year-old civil engineer broke out in a severe rash while in a clinical trial for a brain tumor treatment, Morris Groves, MD, JD, who was leading the trial [...] admitted the patient to the hospital, gave him steroids, and stopped the experimental drug. The man's insurance company paid for his care, and he recovered. This scenario is what researchers hope for if a study participant becomes ill as a result of an experimental intervention.
Federal law recommends, but does not require, that consent forms spell out whether a patient will be compensated if injured as a result of a trial. (See “Federal Consent Guidelines.”)
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.