When the SARS-CoV-2 virus emerged in 2020, researchers immediately began investigating biological factors that might explain the wide variation in how people responded to the infection. One of the earliest and most intriguing hypotheses centered on an individual’s blood type. Scientific teams across the globe started to analyze patient data to determine if the ABO blood group system influenced a person’s susceptibility to the virus or the ultimate outcome of the resulting COVID-19 illness. Initial population studies quickly suggested a potential association existed, sparking significant public interest in whether a person’s inherited blood group offered protection or increased risk.
Understanding the ABO Blood Group System
The ABO system is the most well-known method for classifying human blood and is based on specific carbohydrate molecules, or antigens, present on the surface of red blood cells. Blood type A individuals possess the A antigen, while those with blood type B possess the B antigen. People with blood type AB carry both the A and B antigens on their red cells, and those with blood type O lack both A and B antigens altogether.
The immune system develops natural antibodies that react against the antigens not present on a person’s own red blood cells. For example, a person with Type O blood has anti-A and anti-B antibodies circulating in their plasma because they lack both antigens. Conversely, a person with Type A blood has anti-B antibodies, and Type AB individuals have neither anti-A nor anti-B antibodies. This fundamental immunological difference is believed to play a role in how the body first encounters and responds to the SARS-CoV-2 virus.
The Link Between Blood Type and COVID-19 Infection Risk
Multiple large-scale population studies conducted during the pandemic’s early phases consistently pointed to a difference in the probability of contracting the SARS-CoV-2 infection based on blood type. Individuals with Type O blood appeared to have a marginally lower risk of testing positive for COVID-19 compared to people with other blood groups. For example, one large study found that Type O individuals were less likely to contract the virus than non-Type O individuals.
Conversely, individuals with Type A blood consistently demonstrated a slightly elevated likelihood of infection. Some research suggested that people with Type A blood were up to 20% more likely to be infected after exposure compared to those with Type O blood. This trend was observed across diverse populations, suggesting a genuine biological relationship. A meta-analysis confirmed that blood Types A, B, and AB were associated with a higher risk of initial infection when compared to Type O blood.
Blood Type’s Influence on Disease Severity
After initial infection, blood type influences the progression and severity of the COVID-19 illness, affecting outcomes like hospitalization and the need for intensive care. Individuals with Type A and AB blood are at a greater risk for severe COVID-19 outcomes than those with Type O or Type B blood. In critically ill patients, those with Type A or AB were more likely to require mechanical ventilation, suggesting a higher rate of severe lung injury.
A large cohort study found that individuals with Type A blood were at an increased risk of death from the disease. The protective effect observed in Type O individuals extended beyond susceptibility, as they were also found to be at a decreased risk for severe illness and death. Furthermore, non-O blood types have been linked to a higher risk of certain long-term complications, such as heart attack and stroke, following a COVID-19 hospitalization.
Scientific Mechanisms Behind the Association
The observed differences in susceptibility and severity stem from two primary biological mechanisms tied to the ABO system. One hypothesis focuses on the role of naturally occurring antibodies in Type O and Type B individuals. These individuals possess anti-A antibodies. Laboratory studies suggest these antibodies might recognize and neutralize SARS-CoV-2 virions that have acquired A-like antigens from infected host cells. This antibody-mediated interference could partially block the virus’s ability to adhere to and infect new host cells, reducing the initial infection risk for Type O and B people.
A second mechanism relates to the blood’s clotting ability and is linked to disease severity. Individuals with non-O blood types, particularly Type A, typically have higher plasma levels of certain coagulation factors, most notably von Willebrand factor (vWF) and Factor VIII. Elevated levels of these factors increase the risk of thrombosis, or blood clotting, which is a common and dangerous complication in severe COVID-19 cases. The blood group antigens themselves are expressed on the endothelial cells lining blood vessels, and their presence may modulate the overall inflammatory response and the propensity for clot formation, contributing to the worse outcomes seen in non-O patients.