DISCUSSION
A total of 1667 Covid-19 patients were included in our study. In Covid-19 patients, it was observed that the ratio of blood group A and AB was higher than in the healthy control group, and blood group O had a lower ratio. The statistical analysis showed that there was a very significant increase in the number of individuals with blood group A in Covid-19 patients compared to healthy individuals, almost a significant increase in the number of Covid-19 patients with blood group AB, and a very significant decrease in the number of Covid-19 patients with blood group O (P <0.001, P=0,052 and P<0.001, respectively).
The ABO blood group system basically contains A and B antigens and their corresponding antibodies. The gene coding the antigen is located on chromosome 9q34.1-34.2. It consists of A, B and O alleles, and there are 4 phenotypes (A, B, O, and AB blood groups). 11,12Differences in blood group antigen expression may increase or decrease host susceptibility to many infections. Blood group antigens can play a direct role in infection by serving as receptors and / or cofactors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transmission or cell adhesion through the organization of membrane microdomains. Blood group antigens may alter the natural immune response to infection.13 Since the beginning of the Covid-19 pandemic, many studies have been conducted on this subject. Zhao J YY et al. examined the ABO blood group distribution in 2,173 Covid-19 patients and showed that the frequency of blood group A was higher in Covid-19 patients than that of non-A blood groups and found that the blood group O was associated with a lower risk for infection compared to the non-O blood groups.14  Yuqin Wu et al. found similar results in their study on 187 Covid-19 patients and H. Goker et al. in their study on 186 Covid-19 patients.15-16 In the study carried out by E. Arac et al., there was no significant difference between ABO blood groups, but strong significance was found in Rh system.3 We believe that this study had some limitations due to the fact that PCR-negative patients were also included in the study, the number of cases was small, the study was conducted at the beginning of the pandemic, and the prevalence of Covid-19 in some circles, especially among some families, rather than across community. In our study, we found that blood group A was more frequent than in community and statistically significant, blood group O was less frequent than in community and statistically significant, and blood group AB was more frequent than in community, but it was not significant. We think that blood group A is more susceptible to Covid-19 and blood group O is protective.
When Covid-19 patients in need of intensive care and deceased patients were compared, blood groups were shown to have no significant effects on intensive care and mortality. ABO blood groups have been associated with cardiovascular diseases before.17 It is known that thrombotic risks decrease significantly in blood group O compared to non-O blood groups17,18. Studies have shown that microthrombosis that develops in Covid-19 infection in the pulmonary vascular bed contributes significantly to acute respiratory syndrome; therefore, the use of prophylactic anticoagulants is also included in the guidelines.19,20 There are opinions arguing that the protective effect shown in blood group O is based on this phenomenon21. In this study, we found that blood groups were not statistically significant in predicting the patient’s need for intensive care and mortality.
In conclusion, we believe that our study is important because it is a study with the highest number of Covid-19 patients diagnosed by PCR, which examined the blood group and Covid-19 relationship in our country. Our study has found that having blood group O may be protective, that blood group A may have greater susceptibility to the disease, but this does not affect the course of the disease and is not associated with mortality. We want to emphasize that the most important and easiest way to avoid Covid-19, regardless of the risk factor and the facilitating factor, is to maintain social distance, hand hygiene and use of masks. Although there is no significant difference between blood groups and intensive care and mortality, it can be concluded that individuals with blood group A should take stronger and stricter measures and Covid-19 patients with blood group A should be monitored more closely. Larger, multicenter and prospective studies should be conducted to determine the relationship between blood groups and Covid-19 and the protective role of blood group O.