This study demonstrated that age, gender and ABO blood groups might be important risk factors for COVID-19 in Iraqi patients. The mean age of patients approached the fifth decade (48.2 ± 13.8 year), and 48.3% of patients were classified in the age group ≥ 50 years. These findings strongly suggest that individuals aged 50 years and older are at greater risk to develop COVID-19. Previous studies have likewise reported that elderly are more susceptible to COVID-19 than younger adults. Further, severity and outcome of disease largely depend on age of patient. Most of hospitalized COVID-19 cases (80%) were aged 65 years and older and tended to have a higher risk of death (23-fold) compared to younger patients [4, 21,22,23]. Although the elderly are at a greater risk of some comorbidities (cardiovascular diseases, diabetes, obesity and respiratory system diseases), immunological dysfunctional abnormalities have been introduced as risk factors for the evolution of COVID-19 in the elderly population . Two immunological outcomes have been correlated with aging; inflamm-aging and immunosenescence. In inflamm-aging, elevated levels of peripheral pro-inflammatory mediators; for instance, interleukin (IL)-1β, IL-6 and tumor necrosis factor alpha (TNF-α), have been reported. Their elevations may drive the development and maintenance of immunosenescence, which refers to aging-related immunological changes that may have detrimental effects [24, 25]. Probably, most diseases affecting the elderly (including COVID-19) are due to inflamm-aging and immunosenescence, as they contribute to what has been termed a cytokine storm. The cytokine storm is described as life-threatening organ-dysfunction syndrome due to abnormal host immune response against infectious pathogens. Dyspnea, hypoxemia and inflammation in major organs (the lungs, kidneys, heart, liver and brain) are prominent outcomes of the cytokine storm [23, 26]. In severe COVID-19 cases, it has been reported that vascular inflammation is a substantial cause of microvascular injury and thrombosis due to complement-associated activation . Further immunological changes have been associated with aging. They included declined generation of CD3+ T cells, increased CD4/CD8 T cells ratio, increased regulatory T cells (Treg), decreased B lymphocytes and upregulated expression of toll-like receptors (TLRs). These consequences may contribute to the poor outcomes in elderly COVID-19 patients [28,29,30].
Gender can also be considered a predisposing factor for COVID-19, and males may be more susceptible to the disease than women. Among the 1014 confirmed cases of COVID-19, the proportion of males was higher than that of females (60 vs. 40%) and the male:female ratio was 1.5:1.0. Consistent with these findings, a Brazilian study reported that 57.5% of 67,180 cases were males . On the contrary, most of epidemiological reviews and meta-analysis studies reported comparable rates of COVID-19 between males and females, but males tended to have higher fatality and mortality rates than females [5, 6, 32]. Vulnerability of men for worse outcomes of COVID-19 is probably due to gender-based immunological differences between men and women, and this may impact the woman ability to resist infections including COVID-19 . Sex hormones may mediate these variations between men and women in the susceptibility to COVID-19. Experimental data demonstrated that female mice treated with an estrogen receptor antagonist showed increased mortality rates due to SARS-CoV infection. Thus, estrogen receptor signaling has been considered a critical factor in protecting female mice from the infection . The gender disparity in morbidity and mortality rates among COVID-19 patients may also be related to sex-biased differences in the lung expression of angiotensin-converting enzyme 2 (ACE2), which serves as a receptor for COVID-19 entry into cells . The gene encoding ACE2 is mapped to chromosome X, and its expression is also influenced by sex hormones . Collectively, these findings may explain gender drive modification in the COVID-19 outcomes.
Besides age and gender, this study indicates that ABO blood group determinants can be considered biomarkers of susceptibility to COVID-19 infection, and groups A, A + B, and A + B + AB have been associated with a significantly increased risk. In line with these results, Chinese and American studies reported that group A frequency significantly elevated in COVID-19 patients compared to controls while group O frequency significantly decreased [9, 10, 12, 14, 15].. Further, a genome-wide association study was conducted at seven hospitals in the Italian and Spanish centers of the SARS-CoV-2 epidemic in Europe, and the analysis confirmed that group A was associated with a high risk of developing COVID-19, while group O had a protective effect compared to other blood groups .. In a study from China (Wuhan city), the frequency of group A was significantly increased in COVID-19 patients compared to controls, while groups B, AB, and O frequencies showed no significant differences .. A Spanish study suggested a lower susceptibility to COVID-19 for group O, while a higher risk of complications was found in group B patients . Canadian data indicated that group A or AB was associated with critical illness of COVID-19 compared to group O or B . In a previous Iraqi study conducted by our group, susceptibility to COVID-19 was associated with group AB in patients from Baghdad, while group A was associated with an increased risk of death . In an Iranian study, no association was found with group A, but a significant decrease in group O frequency was recorded . On the contrary, no association between ABO blood groups and COVID-19 was found in French and Spanish patients [8, 13]. However, in a meta-analysis study, it was emphasized that individuals of group A are at greater risk of developing COVID-19 while those of group O were at lower risk . Further meta-analysis suggested that individuals of group A may be more susceptible to COVID-19, while individuals of group O may have a lower risk of developing the disease. However, the analysis also showed no relationship between ABO blood groups and severity of COVID-19 .
Although some inconsistent results have been reported, most studies agree that ABO blood groups are of particular significance with regard to their association with susceptibility to COVID-19, but the molecular mechanism underlying this association has not been well described. It has been hypothesized that the blood group impact on susceptibility to COVID-19 may depend on a differential clustering of the virus glycoprotein receptors on host cell surface, induced by ABO(H) determinants through interactions (carbohydrate-carbohydrate) with the glycan motif of these receptors, and this may interfere with the binding of virus and its entrance to target cells . The carbohydrate structures of ABO(H) blood groups are not restricted to the surface of red blood cells, and other cells and tissues express these structures; for instance, lymphocytes, endothelial cells, platelets, gastric mucosa and bone marrow. Further, blood group antigens are present in secretions (i.e. saliva) of about 80% of individuals (ABO secretors) . Therefore, their involvement in physiological and pathological processes may be expected during viral, bacterial and parasitic infections. Further evidence depicts that covalently-linked ABO(H) structures are found in some plasma glycoproteins; for instance, von Willebrand factor (VWF), and factor VIII (FVIII). It has been demonstrated that non-O individuals have significantly higher expression of endothelial cell-associated VWF protein compared to individuals of group O. The VWF expression was associated with pulmonary vascular endothelial cells and this expression was influenced by the ABO determinants . In this context, elevated circulating levels of VWF and FVIII have been demonstrated in patients with severe COVID-19 pneumonia, and this may indirectly link ABO blood groups with susceptibility to COVID-19 . Similar to COVID-19, group A has been associated with severe malaria, while individuals with group O are less susceptible to the infection. Further, group A individuals are more likely to have debilitated aging than those of group O .
To gain further understanding of the ABO-COVID-19 association, ABO blood groups were analyzed in COVID-19 patients and controls after stratification according to age group (< 30, 30-39, 40-49 and 49 50 years) and gender. In the analysis of age groups, frequencies of groups A, B, AB, and O showed no significant differences between age groups of patients or controls. Therefore, the study suggests that the ABO-COVID-19 association may not be influenced by age. A similar conclusion was reached through a Chinese study, in which the COVID-19 cases were divided into two age groups (< 40 and ≥ 40 years), and ABO blood group frequencies showed no significant differences between the two groups . However, a further Chinese study reported a significantly increased frequency of group A and a significantly decreased frequency of group O in patients aged ≥ 60 years compared to patients in the age groups < 40 and 40-59 year .
With respect to gender, ABO blood group frequencies showed significant differences between male and female COVID-19 cases, while no significant differences were recorded between male and female controls. A re-analysis of gender-based association revealed that males of group A were more susceptible to COVID-19 than females of group A. In fact, female patients showed no significant association with ABO blood groups. These findings propose a predisposing role for group A to develop COVID-19 in males. In line with these results, a Chinese study reported that group A was encountered more frequently in male patients than in female patients . On the contrary, another Chinese study reported that group A is a risk factor for COVID-19 in females but not in males. However, the study was based on low sample size of patients and controls (105 and 103, respectively) . Also, no gender-related differences were seen in ABO blood groups among COVID-19 patients in two studies from China and Spain [12, 13].