Introduction
The number of SARS-CoV-2 (Covid-19) cases, which was recognized as a pandemic by the World Health Organization (WHO) in March, continues to increase rapidly worldwide.1 As of July 2020, the number of Covid-19 patients reported by WHO has exceeded 13 million and the number of deaths 580 thousand. People most commonly affected by Covid-19 are those with chronic lung or cardiovascular diseases, men with hypertension and diabetes.2 There are some parameters such as CRP, ferritin and LDH that can predict the clinical severity of Covid-19, but there are no biomarkers indicating a predisposition to Covid-19.3
The general blood group system called ABO discovered by Landsteiner is a kind of cell identity system determined by the antigenic structure located on the erythrocyte surface. The Rh system, known as whether there is an antigenic structure or not, is another grouping.4 Blood groups are a qualitative characteristic in which there are no environmental effects, with all of the phenotypic variation being a reflection of the genetic structure. Blood group antigens are genetically encoded, and these antigens may be a predisposing factor for some diseases and a protective factor for some others. Studies have shown that some rheumatological diseases, viral infections such as Norwalk virus and Hepatitis B, and some types of cancer are associated with the ABO blood group.5-8 In addition, it has been found that the rate of infection with SARS coronavirus is lower in blood group O.9
By examining the relationship of the human blood group with virus infection, it is possible to determine the susceptibility of people with different blood types to viruses.  There are some studies examining the relationship between Covid-19 and blood groups in the literature. The aim of this study was to investigate the ABO and Rh blood group distribution and clinical characteristics in patients with Covid-19.