Multiple large studies and meta-analyses have found a modest but consistent statistical link between ABO blood type and the likelihood of catching COVID-19. People with blood type A face a somewhat higher risk of infection, while people with type O appear to have a degree of protection. The effect is real in the statistical sense, but it is small enough that it should not change how anyone thinks about their personal risk profile. The more interesting story is why the connection exists at all, what it means for severity and long-term outcomes, and how it fits into a much older relationship between blood type and infectious disease.
What the Large Studies Found
Several meta-analyses, each pooling data from tens of thousands of patients, converged on a similar pattern during the first waves of the pandemic. One systematic review covering multiple studies found that individuals with blood group A had roughly a third higher odds of testing positive for COVID-19 compared to non-A types, while those with blood group O had about 30% lower odds of infection.1PubMed Central. The impact of ABO blood group on COVID-19 infection risk and mortality: A systematic review and meta-analysis A second meta-analysis that included over 31,000 samples found a very similar pattern, with type A showing increased odds and type O showing decreased odds of infection.2PubMed Central. Association between ABO blood groups and COVID-19 infection, severity and demise: A systematic review and meta-analysis
Genetic evidence pointed in the same direction. An early genome-wide association study published in the New England Journal of Medicine identified a significant association at the ABO blood group locus on chromosome 9. In that cohort, blood group A was linked to about 45% higher odds of severe COVID-19 with respiratory failure compared to other groups, and type O was associated with about 35% lower odds.3PubMed. Genomewide Association Study of Severe Covid-19 with Respiratory Failure Finding the signal in an unbiased genome scan, rather than starting with a hypothesis about blood type, added credibility to the epidemiological observations.
But here is the crucial context those odds ratios can obscure: the absolute difference in risk between blood types is small. One large study estimated absolute risk differences between blood groups ranging from about 0.1% to 8.2% after adjusting for race and ethnicity.4Nature Communications. Associations between blood type and COVID-19 infection, intubation, and death In practical terms, if your baseline risk of infection during a given wave was 10%, being type A might push that to 11% or 12%. That is detectable in large datasets but invisible at the level of any individual person’s experience.
Why Type O Might Be Protective
The leading explanation involves natural antibodies. People with type O blood carry both anti-A and anti-B antibodies in their plasma. People with type B carry anti-A antibodies. Research on the original SARS coronavirus (the 2003 outbreak, not COVID-19) demonstrated that anti-A antibodies could block the interaction between the viral spike protein and its receptor on human cells. In lab experiments, both monoclonal anti-A and natural human anti-A antibodies from type O individuals inhibited the spike protein from latching onto the ACE2 receptor, and the effect was dose-dependent.5PubMed Central. Inhibition of the interaction between the SARS-CoV Spike protein and its cellular receptor by anti-histo-blood group antibodies
The mechanism works like this: when a virus buds off from an infected person’s cell, it can pick up some of the host cell’s surface molecules, including blood group antigens. If a person with type A blood sheds virus, those viral particles may carry A antigens on their envelope. When that virus enters someone with type O or type B blood, the recipient’s anti-A antibodies can recognize those antigens and help neutralize the virus before it establishes infection. It is not a perfect shield, but it adds a small layer of defense that, averaged across millions of encounters, shows up in the data.
Why Type A Faces Higher Risk
The flip side of the antibody story is that type A individuals lack anti-A antibodies entirely, so they cannot mount that particular defense against virus shed by other type A hosts. But there is a more direct mechanism at play as well. Laboratory studies found that the SARS-CoV-2 receptor-binding domain (the part of the spike protein that grabs onto human cells) preferentially recognizes blood group A sugars, specifically the type found on respiratory epithelial cells rather than on red blood cells.6Blood Advances. The SARS-CoV-2 receptor-binding domain preferentially recognizes blood group A Follow-up work showed that this preference held for Delta and Omicron variants as well, and that each variant displayed a preferential ability to infect cells expressing blood group A.7Blood. Blood group A enhances SARS-CoV-2 infection
There may also be a clotting dimension. A review on behalf of the International Society of Blood Transfusion noted that type A individuals tend to have higher levels of von Willebrand factor and factor VIII, both of which promote clotting. Because COVID-19 often caused dangerous blood clots, particularly during severe illness, those naturally elevated clotting factors could amplify the disease’s vascular damage in type A patients.8PubMed Central. ABO blood group and COVID-19: a review on behalf of the ISBT COVID-19 Working Group However, at least one study of critically ill ICU patients found no significant association between ABO blood group and plasma markers of endothelial injury or clotting activation, so this pathway may matter less in practice than in theory.9PLOS ONE. ABO blood group and COVID-19 severity: Associations with endothelial and adipocyte activation in critically ill patients
Does Blood Type Change How Sick You Get?
This is where the picture gets murkier, and the distinction between catching the virus and becoming seriously ill matters. Most of the stronger evidence ties blood type to susceptibility to infection. The relationship with severity and death is less consistent.
A large multicenter study that followed patients across three dominant variant waves found that the proportion of severe disease was essentially identical across all four blood types, ranging from 8.6% to 8.9%. Even when the researchers compared type O against all non-O types combined, there was no meaningful difference in severe outcomes.10PubMed Central. Association between ABO blood type and coronavirus disease 2019 severe outcomes across dominant variant strains That finding held across different variant periods, suggesting the null result was not just a fluke of one wave.
Other studies, though, did report severity differences. One analysis found that type AB was at increased risk of both intubation and death relative to type O, while type A was actually at decreased risk of those outcomes, a finding that runs against the infection-susceptibility pattern.4Nature Communications. Associations between blood type and COVID-19 infection, intubation, and death The contradictions across studies likely reflect differences in how patients were enrolled, what counted as “severe,” and the demographics of each study population. A thoughtful review of the literature noted that between-study variation could be explained by differences in study settings and that multiple mechanisms were probably acting at once.11PubMed Central. ABO Blood Types and COVID-19: Spurious, Anecdotal, or Truly Important Relationships? A Reasoned Review of Available Data
The honest reading of the evidence is that blood type plays a role in whether you catch the virus, but once infected, other factors like age, obesity, diabetes, and immune status overwhelm whatever modest influence blood type exerts on disease progression.
The Rh Factor Angle
ABO type gets most of the attention, but several studies examined Rh status as well. A population-based cohort study in Canada found that Rh-negative individuals were about 21% less likely to test positive for SARS-CoV-2 compared to Rh-positive individuals, and the combination of O-negative appeared especially protective, with a 26% lower adjusted risk of infection.12PubMed Central. Association Between ABO and Rh Blood Groups and SARS-CoV-2 Infection or Severe COVID-19 Illness That same study found a lower risk of severe illness or death among both type O and Rh-negative individuals, though the absolute risk reductions were tiny.
Multiple research groups confirmed the infection-susceptibility finding for Rh-negative status, though the link to severity was less consistent. Some studies found Rh-negative individuals had lower rates of severe illness and death, while others did not.13PubMed Central. Relationship between blood type and outcomes following COVID-19 infection A review of the evidence characterized the connection as a “consistent theme” for susceptibility but acknowledged controversy around severity.14PubMed Central. Effects of ABO blood groups and RH-factor on COVID-19 transmission, course and outcome: A review The biological explanation for why Rh status would matter is less developed than for ABO. The Rh protein does not sit on epithelial cells the way ABO antigens do, and the mechanisms that would connect it to viral susceptibility remain speculative.
Blood Type and Long COVID
Given the early interest in blood type as a risk factor for acute infection, researchers naturally asked whether it mattered for lingering symptoms. The short answer: it does not appear to. A large observational study that adjusted for age, sex, ethnicity, and severity of the initial infection found no significant association between any ABO subgroup and developing long COVID. The Rh factor also showed no significant effect.15PLOS ONE. ABO blood group as a determinant of COVID-19 and Long COVID: An observational, longitudinal, large study A cross-sectional study likewise found no association between blood group and long COVID prevalence.16PubMed Central. Prevalence of Long COVID Syndrome and its Association With Blood Group: A Cross-Sectional Study
One study did find that blood group AB patients had higher rates of specific long-term symptoms like palpitations and dizziness compared to other groups.17PubMed Central. Association of ABO blood group with in-hospital adverse outcome and long term persistent symptoms of COVID-19 infection But that finding comes from a single-center study and has not been replicated in larger cohorts. The weight of evidence suggests that whatever role blood type plays in catching COVID, it does not meaningfully determine whether symptoms persist months later.
Does Blood Type Affect Vaccine Response?
A population-based study of over 3,000 people examined whether ABO or Rh type influenced antibody levels after booster vaccination. It found no association between blood type and antibody levels four weeks after the booster, regardless of whether they measured binding antibodies or neutralizing antibodies against wild-type or Omicron.18PubMed Central. Impact of ABO Blood System on Immunogenicity and Vaccine Efficacy of COVID‐19 Booster Vaccination—A Population‐Based Study of 3066 Individuals Interestingly, that same study found that type AB individuals had significantly lower rates of breakthrough infections compared to type O, even after adjustment for covariates. That is a flip from the pre-vaccination pattern, where O was the “lucky” blood type.
A separate study looking at different vaccine platforms found some variation: type A recipients had a higher antibody rise after certain AstraZeneca-based regimens, while type O recipients showed better antibody persistence after Sinopharm vaccination.19PubMed. The impact of ABO blood types on humoral immunity responses and antibody persistency after different COVID-19 vaccine regimens These findings are platform-specific and difficult to generalize, but they suggest that whatever mechanism connects blood type to susceptibility, vaccination largely levels the playing field in terms of immune protection.
What This Means Practically
The answer to whether you should worry about your blood type in the context of COVID can be stated plainly: no. The associations are statistically meaningful but clinically marginal. Age, vaccination status, chronic disease, and immune health all dwarf blood type as predictors of who gets sick and how badly. Early in the pandemic, some ethicists discussed whether blood type data could help prioritize scarce vaccines or interventions.20PubMed Central. Roles of genetics and blood type in clinical responses to COVID-19: ethical and policy concerns That idea never gained traction, in part because the effect sizes were too small to justify stratifying public health responses by blood group.
If you are type A and reading this with a pang of anxiety, consider the scale. Your blood type might nudge your infection risk by a few percentage points in one direction. Wearing a good mask during a surge nudges it by far more. Getting vaccinated nudges it enormously more. Blood type is a background variable, not an actionable one. You cannot change it, and even if you could, the benefit would be barely perceptible.
Blood Type and Other Viruses
The relationship between ABO type and COVID-19 is not an isolated curiosity. Blood group antigens have been implicated in susceptibility to a range of infections. During the 2003 SARS outbreak, an epidemiological analysis of a hospital cluster in Hong Kong found that blood group O was associated with lower risk of infection, a pattern that foreshadowed the COVID-19 findings by nearly two decades.21Glycobiology. Inhibition of the interaction between the SARS-CoV Spike protein and its cellular receptor by anti-histo-blood group antibodies
Noroviruses, the leading cause of viral gastroenteritis, use blood group antigens as receptors to attach to gut cells, and different norovirus strains show distinct binding patterns depending on the host’s ABO and secretor status.22PubMed Central. Norovirus and histo-blood group antigens: demonstration of a wide spectrum of strain specificities and classification of two major binding groups among multiple binding patterns Rotaviruses similarly recognize blood group antigens as ligands for attachment.23PubMed Central. Histo-blood group antigens: a common niche for norovirus and rotavirus
This broader pattern helps explain why ABO diversity persists in human populations at all. Evolutionary modeling suggests that two selective pressures maintain the polymorphism: bacteria that adapt to common blood group phenotypes create selection favoring rarer types, while viruses that acquire host blood group antigens generate asymmetric transmission that favors different types depending on the pathogen landscape.24PubMed Central. Evolution of the human ABO polymorphism by two complementary selective pressures The fact that no single blood type has swept to fixation in any human population suggests that, over evolutionary time, each type carries both advantages and vulnerabilities depending on which pathogens are circulating. COVID-19 is the latest chapter in that story, not an exception to it.