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Can blood type influence COVID risk?
International studies suggest those with blood type A are more at risk, but experts say further evidence is needed.
Various studies have been conducted to explore the association between blood groups and COVID-19 risk and severity.
People with type A blood are associated with having a 45% higher risk of contracting the novel coronavirus that causes COVID-19, according to a study published in the New England Journal of Medicine (NEJM).
The study tracked 1900 people in Spain and Italy who were seriously ill with COVID-19 and compared their results with 2000 well people.
‘Blood-group specific analysis showed a higher risk in blood group A than in other blood groups … and a protective effect in blood group O as compared with other blood groups,’ the study authors wrote.
Although an early study with limitations and yet to be peer reviewed, findings from Chinese researchers also indicated that people with type A blood might be more susceptible to contracting the virus, while those with type O blood may be more protected.
The ABO blood-group distribution of 2173 patients diagnosed with COVID-19 from three hospitals in Wuhan and Shenzhen, China, was compared with the blood types of the general population in the same regions.
Further findings published in the British Journal of Haemotology (BJH) demonstrated that, from a cohort of patients in Wuhan, those from A blood groups were at higher risk of hospitalisation following SARS‐CoV‐2 infection. Patients from O blood groups had lower risk, suggesting that ‘the ABO blood type could be used as a biomarker to predict the risk of SARS‐CoV‐2 infection’.
Professor Anthony Kelleher, Director of the Kirby Institute and Head of its Immunovirology and Pathogenesis Program, told newsGP that while these studies raise some intriguing issues, further research is needed.
‘The control or comparator groups for these studies are not ideal and may lead to biases in the interpretation,’ he said.
‘Further, if you really want to look at the association you need to incorporate all those with infection, not just those who present to hospital.
‘Remember, the majority of people do well with this disease and may never go near a hospital, so just relying on hospital recruitment for a study may give an ascertainment bias, especially when the major comparator group are healthy volunteers.’
Professor Kelleher believes that if further studies are to be conducted into the influence a person’s blood type can have on their COVID-19 risk, the patient cohort should be expanded.
‘A study that incorporates community recruitment with controls from the same community may be more ideal than these [existing] studies,’ he said.
‘Any protection by blood group status of either infection or disease severity is partial. Further, none of these elucidate the mechanism.
‘So better studies to consolidate the robustness of the observation and some studies of biological mechanism are required.’
However, Dr James Daly, Medical Director of Pathology Services at the Australian Red Cross Lifeblood, is more convinced.
He agrees the studies do suggest an ‘interesting association’ with ABO blood groups and risk of COVID-19 infection, as well as severe infection.
‘This association has now been demonstrated by several studies from several countries – including the [NEJM] genomewide association study – so I think it is likely to be a real association,’ he told newsGP.
Antibodies to the ABO blood groups may have some protective role for COVID-19, according to Dr James Daly.
Dr Daly cites other examples of blood group associations with susceptibility to infectious diseases, including malaria, Helicobacter pylori, hepatitis B, and the previous SARS-CoV virus.
‘Apart from the ABO blood-group system that many people are aware of, there are over 37 other systems with over 300 different blood-group variants [antigens],’ he said.
‘Differences in blood-group antigens reflect slight genetic differences in the structure or function of sugars, glycoproteins or proteins on the surface of our blood and other cells, or an enzyme that modifies those structures.
‘These blood-group structures … perform some role in the function of the cells, and in some cases they are also the structures that infectious agents use to gain access to our cells.’
In addition to the potential role of blood-group structures in susceptibility to infection, Dr Daly believes that for COVID-19 there is the possibility that antibodies to the ABO blood groups may have some protective role.
‘For the ABO blood-group system, from a very early age people develop antibodies or an immune response to the blood groups that they do not possess,’ he said.
‘There is some evidence that the antibody against the A blood group (anti-A), which is found in all group O and B individuals, has some neutralising effect against the previous SARS-CoV outbreak – meaning that it inhibits the ability of the virus to infect our cells.
‘Now similar evidence is emerging for SARS-CoV-2.’
While Dr Daly agrees that the association of blood types appears to be real, it doesn’t necessarily confirm causation, and investigations on the mechanism of the different susceptibility to COVID-19 between blood group A and O individuals would be of value.
‘I think further studies to determine whether the anti-A in non-group A people inhibits SARS-CoV-2 virus will be important, and whether this inhibition depends on the blood group of the person transmitting the virus,’ he said.
‘For example, maybe group O individuals are only more protected from transmission of SARS-CoV-2 from an infective person who is group A, because the blood group of the infected person may influence the surface structures of the virus particles – the ABO blood-group genes actually code for an enzyme that modifies particular surface sugars.
‘It is possible there are other mechanisms that may contribute to an apparent association between ABO blood groups and COVID-19 severity that may be indirect.’
In the BJH study, the authors concluded that although people with blood group A had a significantly higher risk of SARS‐CoV‐2 infection than those from blood group O, people with blood type O should ‘not take the virus lightly’ and must still take precautions to avoid increasing the risk of infection.
‘It’s important to recognise that there were still plenty of blood group O people with COVID, including those with severe disease and those who died of COVID,’ Dr Daly said.
And those with blood type A should strengthen protection to reduce the risk of infection.
Despite these pieces of emerging evidence, Dr Daly said the overall message needs to remain that individuals should practice standard protection measures methods, regardless of their blood type.
‘While there appears to be a reduced risk of being hospitalised with COVID if you are blood group O, the odds ratio is modest – around 0.7 – so being blood group O is by no means guaranteed protection against COVID-19,’ he said.
‘The type of reduction in risk achieved with appropriate social distancing and hand hygiene are significantly better than depending on your blood group for protection, so there is absolutely no place for group O people to be complacent about general public health advice.’
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