Typically clinical trials enroll the healthiest of the potentially ill because they don’t want outside factors to “confuse” the data.
If for example someone is already coming down with Covid, or may be at extreme risk of adverse consequences if they have Covid, they probably don’t want them in the trial because it will make the adverse effects or poor outcomes look worse.
For example, if someone is at very high risk of a lethal heart attack before they enroll in the trial, if they die of a heart attack during the trial, that will be counted as an adverse event due to the trial, not as something that would have happened regardless of their enrollment.
