'Risk of bias' assessment of Duan 2020
Domain	Assessed outcomes	Authors' judgement	Support for judgement
Bias due to confounding	Mortality Clinical improvement	Serious	Quote: "Historic control group was formed by random selection of 10 patients from the cohort treated in the same hospitals and matched by age, gender, and severity of the diseases to the 10 cases in our trial." Not adjusted for co‐morbidities, previous treatments, time of disease onset, etc.
	Adverse events	Not applicable	Paper only reports transfusion‐related adverse events for intervention group
Bias in selection of participants into the study	Mortality Clinical improvement	Critical	Small sample size, unclear how participants were selected into intervention group, unclear how long participants of historical control group were followed
	Adverse events	Not applicable	Paper only reports transfusion‐related adverse events for intervention group
Bias in classification of interventions	Mortality Clinical improvement	Critical	Assignment to control group was done retrospectively. Treatment details of control group are not provided, and it is unclear whether patients were treated during the same period or at a time when less was known about the disease and the treatment options. Knowledge of patients' 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 transfusion‐related adverse events for intervention group
Bias due to deviations from intended interventions	Mortality Clinical improvement Adverse events	Low	All participants received the intended intervention.
Bias due to missing data	Mortality	Serious	Mortality is reported for participants in intervention group until day 3 of follow‐up. Unclear how long control group was followed and how clinical status was assessed
	Clinical improvement	Critical	Unclear how long control group was followed and clinical status in terms of respiratory support was not assessed
	Adverse events	Critical	No safety data for control group reported
Bias in measurement of outcomes	Mortality	Critical	Unclear whether follow‐up was comparable between groups
	Clinical improvement	Critical	Clinical course is reported for participants in intervention group until day 3 of follow‐up
	Adverse events	Critical	Only transfusion‐related adverse events reported
Bias in selection of the reported results	Mortality Clinical improvement	Critical	Study was registered as single‐arm trial and control group was retrospectively selected
	Adverse events	Critical	Observation period unclear; only transfusion‐related adverse events assessed and reported
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.