There's always some criteria for the test group. An example might be:
Age 20-65, BMI below 2, with no long term medical conditions
You find enough people that fit that group and then randomly divide them into two groups, one of which is the test group and one the control. It is possible but unlikely that the criteria includes "has not previously had Covid 19". Assuming the criteria didn't include that, then some of the participants and some of the control group will have previously had Covid, and so we now have some evidence as to whether a cytokine storm occurs.
Doesn't comparing to a control group account for that?