ABO and deep vein thrombosis: Both genetic and non-genetic studies [25, 27, 78] have showed the involvement of ABO gene in COVID-19 susceptibility, while the ABO gene encodes protein that is relevant to the ABO blood group system, and it was also notably associated with several thrombotic and coagulation-related traits including deep vein thrombosis and pulmonary heart disease, which have been reported to be risk factors and sequalae to severe COVID-19 [79, 80].