The ABO blood group system, which determines a person’s blood type, has long been recognized as a factor influencing susceptibility to various diseases, including certain types of ulcers and infectious agents like malaria. Early in the COVID-19 pandemic, researchers began investigating whether this genetic factor might also play a role in how individuals respond to the SARS-CoV-2 virus. This inquiry was prompted by the observation that some people experienced mild symptoms while others developed severe, life-threatening illness. Initial findings suggested a possible connection between an individual’s blood type and their likelihood of contracting the infection or developing a more serious form of the disease.
Association Between Blood Type and COVID-19 Risk
Large-scale genetic studies and cohort analyses conducted worldwide have established a consistent, yet modest, association between ABO blood type and SARS-CoV-2 infection risk. Specifically, individuals with Type A blood have been shown to have a slightly higher risk of contracting the virus compared to those with other blood types. Meta-analyses suggest that Type A individuals have increased susceptibility to infection, with one large review calculating an odds ratio of approximately 1.06 compared to non-A types.
Conversely, the data strongly indicate a slight protective effect for Type O blood against initial infection. Individuals with Type O blood have been consistently associated with a lower likelihood of testing positive for SARS-CoV-2. This protective effect is small but statistically significant.
For disease severity, the association is less uniform across studies, although a trend exists. Type A blood has been linked to a greater risk of developing severe COVID-19 outcomes, such as the need for mechanical ventilation. Blood types B and AB generally fall in the middle of this spectrum of risk for both susceptibility and severity. The ABO blood group is primarily a factor in initial infection susceptibility, with a secondary, less pronounced influence on progression to severe disease.
Potential Biological Mechanisms of Influence
The observed differences in risk are thought to stem from two distinct biological mechanisms related to the function of ABO antigens. The first theory focuses on the initial interaction between the virus and host cells during infection. ABO blood group antigens are not only present on red blood cells but also on the surface of epithelial cells lining the respiratory tract.
The presence of the Type A antigen on these respiratory cells may facilitate the binding of the SARS-CoV-2 spike protein to the ACE2 receptor, which is the primary entry point for the virus. Laboratory experiments have demonstrated that the virus shows a preferential ability to infect Type A cells compared to Type O cells. Furthermore, individuals with Type O and Type B blood naturally produce anti-A antibodies, which may offer a protective mechanism by neutralizing or interfering with the virus’s ability to attach to the Type A antigens, effectively blocking the entry pathway.
The second mechanism relates to the risk of severe disease, which often involves dangerous blood clotting events. Individuals with Type O blood naturally have lower baseline levels of key coagulation factors, specifically von Willebrand factor (vWF) and Factor VIII. These proteins are central to the blood clotting cascade, and non-O blood types can have vWF levels that are 25 to 35% higher.
Severe COVID-19 is characterized by a hypercoagulable state where the body’s inflammatory response leads to extremely high levels of vWF and Factor VIII, contributing to widespread micro-clotting. Because Type O individuals start with a lower baseline of these clotting factors, this pre-existing condition may offer a minor biological advantage against the severe thrombotic complications common in advanced COVID-19.
Significance and Context of the Findings
While the association between blood type and COVID-19 risk is statistically significant, it is important to understand its relative importance within the full spectrum of risk factors. The influence of blood type is considered minor when compared to factors such as age, vaccination status, and the presence of underlying health conditions. Older age and comorbidities like obesity, diabetes, and cardiovascular disease rank substantially higher as determinants of severe illness and mortality.
Furthermore, the initial studies that identified this link have faced limitations, including potential confounding by population genetics and differences in study design. Not all subsequent research has perfectly replicated the findings, particularly concerning the risk of severe outcomes, suggesting that the ABO system is just one small variable in a multifactorial disease process.
The practical takeaway is that an individual’s blood type should not be a primary driver of their behavior or precautions against SARS-CoV-2 infection. The minimal difference in risk associated with blood type does not outweigh the proven protective benefits of public health measures. Preventive strategies, such as remaining up-to-date on vaccinations and boosters, along with other non-pharmaceutical interventions, remain the most effective determinants of personal risk.