'Risk of bias' assessment of Zeng 2020
Domain	Assessed outcomes	Authors' judgement	Support for judgement
Bias due to confounding	Mortality Clinical improvement	Serious	Not adjusted for confounding factors
	Adverse events	Not applicable	Paper only reports adverse events after plasma transfusion for intervention group
Bias in selection of participants into the study	Mortality Clinical improvement	Moderate	Allocation to intervention and control group based on donor‐availability Quote: "A total of 21 contemporaneous critically ill patients with COVID‐19 were enrolled in the current study (Table  1), and all of them required intensive care unit admission. Six of the patients received convalescent plasma treatment based on the limited availability of convalescent plasma and ABO compatibility. Five of 6 patients in the convalescent plasma treatment group and 11 of 15 in the non–convalescent plasma treatment (control) group were male."
	Adverse events	Not applicable	Paper only reports adverse events after plasma transfusion for intervention group
Bias in classification of interventions	Mortality Clinical improvement	Critical	Retrospective study design. Despite missingness of donors, unclear how control group was selected. Treatment details of control group are provided, but knowledge of participants' outcomes at the time of assignment to the control group could have had a major impact on the selection.
	Adverse events	Not applicable	Paper only reports adverse events after plasma transfusion for intervention group
Bias due to deviations from intended interventions	Mortality Clinical improvement	Low	All participants received intended intervention. Co‐interventions (e.g. antiviral therapy, traditional Chinese medicine, etc.) seem to be balanced across treatment groups.
	Adverse events	Low	All assessed participants received intended intervention
Bias due to missing data	Mortality	Low	Data were reasonably complete
	Clinical improvement	Low	Living participants discharged
	Adverse events	Critical	No safety data for control group available
Bias in measurement of outcomes	Mortality Clinical improvement	Low	Follow‐up until death or discharge
	Adverse events	Critical	Only adverse events after plasma transfusion reported
Bias in selection of the reported results	Mortality Clinical improvement	Critical	Retrospective study; selection of all reported results are likely biased
	Adverse events	Critical	Retrospective study; selection of all reported results are likely biased; only transfusion‐related adverse events reported; no safety data for control group available
Overall bias	Mortality	Critical	The study is too problematic to provide any useful evidence, however better evidence is yet insufficient.
	Clinical improvement	Critical	The study is too problematic to provide any useful evidence, however better evidence is yet insufficient.
	Adverse events	Critical	The study is too problematic to provide any useful evidence, however better evidence is yet insufficient.