Immunologist Dr Jacques Le Pendu, research director at health research centre l'Institut National de la Santé et de la Recherche Médicale (Inserm), has said that the link should be taken seriously after several studies appeared to support an early Chinese project on the hypothesis.
The Chinese study, which was published on March 16 on the pre-publication website MedRxiv, was completed by scientists at the universities of Wuhan and Shenzhen.
The study considered 2,176 participants by blood type (A, B, AB or O).
It showed that patients with blood type A were at most risk of contracting the virus, while patients with the blood type O had a lower risk of infection.
Since then, eight further studies in Europe have appeared to confirm these initial findings - including one in the British Journal of Haematology.
In an interview with news magazine Marianne, Dr Le Pendu said: “For me, the link between blood group and the risk of contracting the virus is now established. Causality has not yet been shown, but all these studies agree in their results on the fact that the O blood type is less at risk than others.
“Studies on the last coronavirus, (SARS-Cov-1) had already shown the type O effect. Overall, the structure of Covid-19 is the same as that of the coronavirus from 2003, so I am very optimistic about this link.”
Blood group protection
Dr Le Pendu explained that the “most likely hypothesis” about why the blood type O offers a protective effect is due to the normal presence of natural antibodies of blood type A and B in people with the blood type O.
This means that when the virus is spread by someone with the blood type A, B, or AB, someone with the blood type O will already have natural antibodies against the virus. People with blood type O do not have protection from the virus spread by other people with blood group O, but they do from all other blood types.
Those with the blood type A appear to be at highest risk, because they are the most common, and have fewer antibodies against others.
In France, 43% of people are blood type A, followed by 42% O, 10% B, and 4% AB.
People with blood type A produce natural antibodies against B, so will only be protected from 10% of the population.
Yet, in reality, people with the blood type AB are technically the least-protected. This is because the AB blood type has no antibodies against others.
Natural blood type antibodies are distinct from those found in the blood in someone who has previously had the virus. These antibodies can be detected by a blood test in the weeks after a patient first notices symptoms of infection.
Predicting the spread
In France, as government scientific advisory council Le Conseil Scientifique warns that a second wave of the virus is likely, more information on how the virus spreads has been deemed especially pertinent.
Dr Le Pendu explained: “The hypothesis is major when it comes to transmission. If it is verified, it means that there [could be] contacts that do not spread the virus. We know that it is essential to keep the spread of the virus below 1, to stop the epidemic. That was the point of confinement.
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“Natural antibodies help drop this level. The spread would be higher without this. We estimate that 60% of the population would need to be vaccinated or already have caught the virus to block the chain of infection.
“The good news is that with this system of natural antibodies, it means that it's very probable - although we need to remain cautious at this stage - that part of work is already done, that the level of protection among the population is already likely to be at 20%, thanks to these A and B blood antibodies.”
He added: “We could optimise this natural protection system. If we could make it so that individuals could produce a large quantity of antibodies, we could, in theory, protect 30% of the population with nothing but this system, without a vaccine. If you add barrier methods to this, then we should be able to contain the virus.
“The problem is that today, there is no method to increase the number of natural antibodies.”
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