3. 'Risk of bias' assessment criteria for observational studies.

Heading	Internal validity	External validity
Study group	Selection bias (representative: yes/no) if the described study group consisted of > 80% of individuals with COVID‐19 treated with convalescent plasma therapy or hyperimmune globulin in the original cohort or if it was a random sample with respect to the treatment and important prognostic factors	Reporting bias (well defined: yes/no) if the study population was well described (e.g. severity of disease, age, risk factors) and the intervention was well described (e.g. number of doses, volume)
Follow‐up	Attrition bias (adequate: yes/no) if the outcome was assessed for > 90% of the study group of interest (++) or if the outcome was assessed for 60% to 90% of the study group of interest (+)	Reporting bias (well defined: yes/no) if the length of follow‐up was mentioned
Outcome	Detection bias (blind: yes/no) if the outcome assessors were blinded to the investigated determinant	Reporting bias (well defined: yes/no) if the outcome definition was objective and precise, and the method of detection was provided
Risk estimation	Confounding (adjustment for other factors: yes/no) if important prognostic factors (i.e. age, co‐treatment, comorbidities) or follow‐up were taken adequately into account	Analyses (well defined: yes/no) if a risk ratio, odds ratio, attributable risk, linear or logistic regression model, mean difference or Chi2 statistic was calculated