4. Adverse events: grade 3 or 4.

Study	Number of participants	Grade 3 or 4 adverse eventsa
Ahn 2020	2	0
Duan 2020b	10 (convalescent plasma group)	0
Jin 2020	6	0
Li 2020	52 (convalescent plasma group)	3 (in 2 participants) 1  possible severe transfusion‐associated dyspnoea (participant had "shortness of breath, cyanosis, and severe dyspnoea within 6 hours of transfusion. The  patient was given dexamethasone, aminophylline, and other supportive care immediately and gradually improved after 2 hours").    1 non‐severe allergic transfusion reaction and 1 probable non‐severe febrile non‐haemolytic transfusion reaction (participant developed chills and rashes within 2 hours of transfusion but recovered fully after treatment with dexamethasone and promethazine).
Liu 2020	39 (convalescent plasma group)	0
Pei 2020	3	1 (anaphylactic shock)
Perotti 2020	46	5 (in 4 participants) chills and fever during transfusion (relation likely) urticaria (relation likely) anaphylaxis/hypersensitivity (relation possible) transfusion‐related acute lung injury (relation possible) subsegmental pulmonary embolism (relation unlikely/excluded)
Salazar 2020c	25	0
Tan 2020	1	1 (fever)
Ye 2020	6	0
Zeng 2020	6 (convalescent plasma group)	0
Zhang 2020ad	4	0
Zhang 2020b	1	0

^aWe assume that these adverse events were grade 3 or 4, but the studies did not specify the degree of severity.
^bOne participant with evanescent red face (grade unclear).
^cOne participant with morbilliform rash one day post‐transfusion that lasted for several days (grade unclear).
^dAssessment of adverse events only reported for one individual. Unclear information provided for the other three participants.