What this boils down to is a set of observable factors being multiplied. Probably millions of people got the shot in August. From them, probably tens of thousan

What this boils down to is a set of observable factors being multiplied. Probably millions of people got the shot in August. From them, probably tens of thousands were overweight. From them, hundreds had diabetes. Add a few other factors like rough age and gender and you get to that one person along with all of their medical history. If we are not talking about the entire US but can know the state, for example, the task becomes so much easier.