Coronavirus disease (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has spread rapidly worldwide (1). Disease severity differs widely due to numerous factors including SARS-CoV-2 variants and viral load, presence of chronic disease comorbidity, genetic factors, and age among others as well as oxidative stress and inflammation (2–6). The ABO gene-derived susceptibility or resistance and ABO blood group antigens have also been associated to COVID-19 disease severity (7). Age has been well described as a risk factor with the 0–4 years age group being 4 times less likely and the 85+ age group being 13 times more likely to be hospitalized compared to the 18–29 age groups (3). Comorbidities including obesity, diabetes, cardiovascular disease, chronic kidney disease, hypertension, tuberculosis, respiratory disease, inflammatory disease, coagulation dysfunction, and cancer have been reported to play a role in disease severity (4). Timing to hospital admission, challenges on health systems and expertise in healthcare delivery are also associated with the risk of increased progression in noncommunicable and communicable diseases including progression to severe COVID-19 presentation requiring hospitalization (8).

The ABO blood typing and the ABO gene variant analysis have been a central component of the immune response in transplantation and transfusion medicine, but has also been of importance in the immune response and progression of viral infections (9). The ABO blood system consists of different molecular determinants leading to different antigenic structures that play varying roles in the immune response. The A and B alleles encode different glycosyltransferases that add N-acetylgalactosamine and D-galactose to the common precursor H determinant side chain, which converts to A or B antigen. As O alleles do not encode a functional enzyme due to a premature stop codon within the glycosyltransferase gene yielding a truncated gene, it results in a complete loss of enzymatic activity and unchanged H-antigen (10, 11).

The genetic basis of the human ABO blood groups is located on chromosome 9 (9q34.1) and is associated with the synthesis of specific ABO glycosyltransferases (12). The ABO blood groups have been included in disease risk and severity of COVID-19, although with mixed results (13). The first link between coronavirus infection and ABO was reported in 2005 following the SARS-CoV-2 pandemic (14). These authors reported that health professionals with type O blood group were less likely to be infected compared to those with other blood types. More recent studies on SARS-CoV-2 reported similar associations between viral infection and the ABO blood groups (15, 16). However, divergent results are still coming forth with respect to the role of the ABO blood groups as a risk marker for COVID-19 severity linked to possible comorbidities and methodological differences as well as population differences in tissue receptors associated with SARS-CoV-2 infection (4, 13, 17–19).

A total of 646 participants were included in the cohort, with 453 participants admitted to hospital for noncritical COVID-19 and 193 participants admitted for critical COVID-19 (Table 1). The average age was 44.70 years (SD ± 15.53) and average BMI was 28.11 kg/m² (SD ± 15.53). Males (78.6%) and Asian nationality (57.0%) made up the majority of the overall population. The noncritical COVID-19 group was significantly younger (p < 0.001) and contained the lower BMI group (p = 0.001). Gender was not associated with a critical presentation of COVID-19 disease (p = 0.640).

Table 2 demonstrates the prevalence and logistic regression analysis of the ABO blood type characteristics of a subset of the participants (n = 527). The multivariate regression analysis adjusted for age, gender, BMI, and presence of comorbid conditions demonstrated that patients with a blood group B [OR: 0.97 (0.57, 1.65); p = 0.975] and blood group AB [OR: 0.61 (0.25, 1.46): p = 0.273] had no association with critical COVID-19. However, there was a protective effect for blood group O [OR: 0.51 (0.31, 0.84); p = 0.008] from developing critical COVID-19, which is consistent with previous findings. After adjustment, circulating anti-A antibodies and anti-B antibodies, as well as A and B antigens, in the plasma had no association to COVID-19 critical phenotype. Given that age and BMI play a significant role on COVID-19 severity, stratification of the blood group was conducted (Supplementary Table 1), demonstrating that age plays a significant role in COVID-19 severity.

To investigate if COVID-19 severity was associated with specific SNPs located on the ABO locus, all the 240 SNPs located in the ABO candidate gene were extracted from a GWAS and analyzed in this study. The ABO candidate gene consists of a 5′-untranslated region (UTR), seven exons, and 3′-UTR. Supplementary Table 1 demonstrates the unadjusted and adjusted OR and p-value of the SNPs at the ABO locus. As blood type frequencies vary across the ancestry groups, the confounding effect of ancestry was evaluated by adjusting for ethnicity. The first ten eigenvectors were used in subsequent analyses for adjustment to population stratification. Following these adjustments, eight SNPs were identified on the ABO gene locus that was associated with COVID-19 severity, all located in the 3'-UTR region (Table 3).

An upregulation of the following SNPs, located in the same haplotype block (D′ = 1.0; as given in Figure 1), is associated with the critical COVID-19 phenotype: rs199969472 [OR: 1.77 (95% CI: 1.18, 2.66); p = 0.0052], rs10901251 [OR: 1.45 (95% CI: 1.07, 1.96), p = 0.0165], rs34266669 [OR: 1.77 (95% CI: 1.18, 2.66); p = 0.0052], rs76700116 [OR: 1.77 (95% CI: 1.18, 2.66); p = 0.0052], and rs7849280 [OR: 1.77 (95% CI: 1.18, 2.66); p = 0.0052]. Haplotype analyses (Figure 1 and Table 4) of the 5 SNPs were performed to estimate the genetic contribution of haplotypes to critical COVID-19 phenotype, demonstrating a significant association to two haplotype blocks (ACGAA, p = 0.0028; CTACG, p = 0.0079). SNP rs34039247 [OR: 2.35 (95% CI: 1.22, 4.55); p = 0.0104], SNP rs9411475 [OR: 1.55 (95% CI: 1.03, 2.36); p = 0.0377], and SNP rs13291798 [OR: 1.55 (1.03, 2.36); p = 0.0377] have also been identified to be associated with critical COVID-19 phenotype.

To determine which of these alleles contribute to the protective effect found for blood group O in the current cohort, the genotype and allele frequencies of the eight significant SNPs were stratified according to blood group (Table 5). The effect alleles of the eight SNPs associated with higher risk of COVID-19 severity (Table 3) had a significantly lower frequency in patients with blood group O in noncritical vs. critical patients. Of interest is that this is the first report of an allele being identified that is associated with a protective effect of blood group O. For the SNP rs19996947, the A allele was found to be present in 28.8% of critical cases vs. the G allele, which was present in 71.2% of the cases, indicating a significant association (p = 0.002).

Five SNPs are in moderate-strong LD with the SNP associated with the blood type O allele (rs8176719; rs199969472, D′ = 0.62; rs10901251, D′ = 0.53; rs76700116, D′ = 0.62; rs7849280, D′ = 0.62; rs9411475, D′ = 0.55). Only two SNPs are in moderate-strong LD with the SNP associated with the blood type A allele (rs8176746; rs10901251, D′ = 0.66; rs34039247, D′ = 0.96) and blood type B allele (rs8176747; rs10901251, D′ = 0.66; rs34039247, D′ = 0.96). Although our data did not demonstrate significant association to the blood type O allele (rs8176719), the SNPs in moderate and strong LD was associated with a significantly reduced risk of critical phenotype presentation with COVID-19.

This study is the first study to combine genetic and serological evidence of the involvement of the ABO blood groups and ABO gene allelic associations with COVID-19 severity within a Middle Eastern population investigated in the UAE, a country that is characterized by the convergence of multiple cultural and socioeconomic factors.

Patients with blood group O [OR: 0.51 (0.31, 0.84); p = 0.008] were less likely to present with critical COVID-19 to the hospitals included in this study, which is consistent with previous findings. Studies have demonstrated that the adhesion of S protein and angiotensin-converting enzyme 2 (ACE2), a receptor for SARS-CoV-2, can be inhibited by anti-A natural antibody (11). Therefore, the anti-A and anti-B natural antibodies produced by patients with blood group O may decrease the risk of developing critical symptoms by blocking the interaction with ACE2 receptor and prevent viral entry into the lung epithelium. Our data strengthen the evidence for a role for ABO in COVID-19 susceptibility and severity and is notable given the reported links between COVID-19 and blood clotting complications that has now been associated with C5a activity (48).

With respect to the ABO locus, eight SNPs showed an association signal in a candidate gene level and play a role in dysregulating the antibody, natural killer cell, and immune mediator profiles: rs199969472 (p = 0.0052), rs34266669 (p = 0.0052), rs76700116 (p = 0.0052), rs7849280 (p = 0.0052), rs34039247 (p = 0.0104), rs10901251 (p = 0.0165), rs9411475 (p = 0.0377), and rs13291798 (p = 0.0377). Stratification of the SNPs with respect to the ABO blood group indicated significant associations between blood group O and the allelic polymorphism that provided protection to the current cohort for critical presentation of COVID-19. All the SNPs that have showed an association signal are located at 3'-UTR, which play an important role in the ABO gene expression and transcriptional signaling in association with COVID-19 symptom presentation (49).

The eight SNPs identified in this study, however, did not include rs657152 and rs8176719, which have been previously observed to be associated with COVID-19 (2). Several reasons can be proposed for this finding including a yet unidentified interaction between different genome variants with minor allele frequencies below 5% and very weak effects (50). Geographical distribution of ancestral risk alleles and population characteristics such as consanguinity, environmental pressures, and incorrect reporting of phenotype may also have contributed to the differences to previous observed results (51). However, the differences in association between the ABO blood group and SNPs may also be explained by the study of Valenti et al. (48) who reported that the association between impact of rs11385949 and the ABO blood group on complement activation disappeared at 30 days after admission during remission of symptoms in surviving patients.

Conflicting findings are present in studies that have investigated the association of the ABO blood group and COVID-19 susceptibility and severity (16, 41, 52–58). Most systematic reviews and meta-analysis studies have demonstrated a decreased risk in susceptibility and severity among patients with COVID-19 in O group (10, 13, 28, 54, 59), whereas other reviews and manuscripts demonstrated no significant association (39, 53, 60–62). These differences may be due to ascertainment bias, multiple confounding effects, diverse studies populations and their geographics locations, presence and adjustment to comorbidities, and case and control selection criteria. These factors may lead to inaccurate estimation of the relative risk. Therefore, appropriately designed observational case–control studies would be more suited to minimize bias and investigate the association of the ABO blood group and COVID-19 severity. In addition, given that the population ancestries in Asian and Middle Eastern countries are relatively homogeneous, relative frequencies of ABO phenotypes may affect the findings of this study. In this study, even with the adjustment of stratification for ancestry, 8 SNPs were identified on the ABO gene locus to be significantly associated with COVID-19 severity, with a lower frequency (protective factor) of the effect allele in patients with blood group O.

Direct susceptibility or resistance of disease may be a function of disease severity, as shown by a meta-analysis reported by Pourali et al. (28). In addition, host protein levels may play a role in disease severity in terms of virulence, as blood group O is associated with lower ACE levels and the converse has been found for blood group A (27). Associated with this finding is the binding probability of SARS-CoV-2. ACE2, being the main receptor, binds to cells expressing blood group moieties most reported in the mucous membrane of the respiratory tract. Thus, blood group AB has the most contact and blood group O the least contact with SARS-CoV-2 (27, 63). This is further demonstrated in this study, where patients with blood type O were less likely to present to hospital with critical COVID-19. The association between COVID-19 severity and the ABO group may be due to the development of neutralizing antibodies against N-linked glycans that extensively cover SARS-CoV-2 spike protein or due to the stabilization of the von Willebrand factor including a one nucleotide deletion in exon 6 (codon 87) determining O allele. Four SNPs at residues 176, 235, 266, and 268, which switch enzyme function from A transferase to B transferase activity, may also contribute to the current findings (12, 40, 64).

Blood groups are also linked to several other diseases, including cardiovascular diseases and pulmonary thromboembolism, which have been identified as one of the main complications in COVID-19 disease progression (21, 65). The ABO blood type trait reflects polymorphisms within the ABO gene. The ABO gene is associated with a number of other traits including risk factors for COVID-19 morbidity and mortality. The genetic locus located within the ABO gene plays a role in hemoglobin concentration, hematocrit (66, 67), von Willebrand factor (68), myocardial infection (69), coronary artery disease (70), ischemic stroke (71), type 2 diabetes (72), and venous thromboembolism (73). In fact, one study recently reported an overall 24% cumulative incidence of pulmonary embolism in patients with COVID-19 pneumonia, of which 50% were the ICU patients (74, 75). Hence, this study does not limit itself to identify ABO links with COVID-19, but investigated comorbidities reported in a population sample that were admitted to hospitals in the UAE with confirmed SARS-CoV-2 infection. However, a similar finding has been demonstrated in this study, where specific risk alleles of SNPs were upregulated in the critical COVID-19 group.

Potential limitations of this study also need to be considered. Due to the pandemic associated lockdown over several months, we were unable to collect data on a large number of patients. Selection bias is a fundamental limitation of this study, so the association estimates are conditional on presentation to the hospital. Nonetheless, the methodology of the cross-sectional study minimized additional selection bias, with respect to the outcome of interest. Due to the geographical location of the Middle East region, there is substantial genetic admixture present within the population. However, to limit population stratification, 10 eigenvectors were adjusted for. An important limitation was the use of a GWAS arrays that were manufactured based on the Caucasian population and this limits the possibility of including targeted SNPs in the genome. To limit this error, imputation of genotypes for genetic variants that are untyped in the arrays increases the information provided by each microarray by accurately evaluating the evidence for association at genetic markers that are not directly genotyped. However, the imputed SNPs from previous studies did not reach significance suggesting that there may be specific population characteristics associated with COVID-19 severity. The inclusion of multiple data collection sites from across the country may have included cases that were not entirely homogeneous. It is possible that the criteria for hospitalization of patients with COVID-19 are different across the collection sites, thus measurement errors may exist in this study. Misclassification and ascertainment biases in the control group were limited by only selecting patients with COVID positive with the noncritical or critical cases.

There is much to be done to understand the role of ABO and its association to COVID-19 severe phenotypes. The combination of genetic and serological evidence of the involvement of the ABO blood groups and ABO gene allelic associations with COVID-19 severity provides a unique opportunity to study host genomics to the interindividual phenotypic variability. By conducting a GWAS and extracting the SNPs in the ABO gene, we have provided further insight into the genetic mechanisms associated with COVID-19 disease severity and the possible link to allelic variants and COVID-19 critical phenotypes. Future study will address linking genomic data to electronic health records that can be leveraged to improve clinical management and lead to better patient outcomes.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

This study was approved by the Abu Dhabi Health COVID-19 Research Ethics Committee (DOH/DQD/2020/538), Dubai Scientific Research Ethics Committee (DSREC-04/2020_09), and SEHA Research Ethics Committee (SEHA-IRB-005). The patients/participants provided their written informed consent to participate in this study.

Juan Acuna, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Eman Alefishat, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Ernesto Damiani, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Samuel F. Feng, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Andreas Henschel, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Abdulrahim Sajini, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Ahmed Yousef, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates; Bassam Ali, United Arab Emirates University, Al Ain, United Arab Emirates; Hiba Alhumaidan, Sheikh Khalifa Medical City and SEHA, Abu Dhabi; Hala Imambabaccus, Sheikh Khalifa Medical City and SEHA, Abu Dhabi, United Arab Emirates; Amirtharaj Francis, Sheikh Khalifa Medical City and SEHA, Abu Dhabi, United Arab Emirates; Stefan Weber, Sheikh Khalifa Medical City and SEHA, Abu Dhabi, United Arab Emirates; Mohammad Tahseen Al Bataineh, University of Sharjah, Sharjah, United Arab Emirates; Rabih Halwani, University of Sharjah, Sharjah, United Arab Emirates; Rifat Akram Hamoudi, University of Sharjah, Sharjah, United Arab Emirates; Abdulmajeed Al Khajeh, Dubai Health Authority, Dubai, United Arab Emirates; Laila Salameh, Dubai Health Authority, Dubai, United Arab Emirates.

HAls and GT conceived the project to study the role of the virus and host in COVID-19 in the United Arab Emirates. HJ and HAlH conceived the central research questions for the ABO data and initiated the first draft of the manuscript. MM, GD, and HJ analyzed and constructed the figures. BM, FA, MU, and NA were responsible for the recruitment of the patients and collecting data for the study. HJ, GT, EA, and HAls provided critical review during manuscript preparation. All authors on the primary list contributed to the data interpretation and critically reviewed the manuscript and approved the final manuscript for submission.

This study was commissioned as part of a project to study the host cell receptors of coronaviruses funded by Khalifa University's CPRA grant (Reference Number 2020-004). The project was funded by internal funds provided by Khalifa University awarded to HAls.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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