Table 2.

Studies specifically analyzing an effect on disease severity ^a.

Country	Type of Study	ABO Effect b				Ref.
		O	A	B	AB
Italy	Analysis of clinical and biological criteria among hypertensive COVID-19 patients	<risk				[30]
Canada	Analysis of clinical and biological criteria among critically ill COVID-19 patients	<risk	>risk	<risk	>risk	[29]
Spain	Analysis of the risk of death among COVID-19 patients requiring transfusion	<risk		ns	ns	[12]
Canada	Separate analyses of patients with or without severe illness	<risk	ns	>risk	>risk	[15]
France	Comparison between patients who underwent transcatheter aortic valve replacement	ns	>risk	ns	ns	[31]

^a Studies including patients of various severity degrees, not specifying clinical criteria and comparing with non-COVID-19 control groups were not taken into account; ^b Significant associations are marked in green to indicate a decreased risk relative to the other blood types or in red to indicate an increased risk; ns: non-significant.