Impact of Blood Group
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A study from Wuhan showed that blood group A had a higher risk for COVID-19 compared to other blood groups.​
What this data shows is that there is potential for correlation between blood type and risk of COVID-19, with people of A blood type more susceptible to the disease, and O being less at risk. These percentage distribution differences were relatively consistent in Shenzhen as well.
Although these could be coincidences, it could be that blood type does make people more susceptible as other viruses are blood group dependent, for example people with blood type A are more at risk form Hepatitis B and HIV.
There are two main blood groupings in humans called ABO and rhesus. Both are genetically determined. Both are independent of each other so somebody who is B blood group can be rhesus positive or rhesus negative for example. Blood types are expressed as molecules on the surface of red blood cells.
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A few weeks later a team at Columbia University in New York found a similar pattern to the Chinese results but only among patients whose blood type were rhesus positive. Earlier work on SARS had shown that the protection that those with blood group O came from the fact that they already had protective antibodies which may have been created in response to immunogenic molecules or antigens from other pathogens. These antibodies inhibited the SARS virus from latching onto the ACE2 cell receptor that the virus used to get into the cells.
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The antibodies seen in blood group O seem to be elicited by antigens which are very similar to those that are on blood group A cells. Thus the reason that blood group A people don't have these antibodies could be because their immune systems recognise the antigens as 'self' and don't produce the antibodies even if they are exposed to the same pathogens as the blood group O people. Given the similarity to SARS scientists believe that the same mechanism is behind the protective effect for blood group 0.
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However, the other question is does blood group affect the severity of the disease. In June, researchers scanned the genomes of approximately 1600 patients from Italy and Spain who had gone into respiratory failure looking for variants associated with the severity of COVID-19 and found two. One was a cluster of 6 genes with possible links to COVID-19 inlcuding genes that regulate ACE2 and the other was the ABO blood group system. However a study by Anahita Due at Massachusetts General Hospital in a study of thousands of people from the Boston area with COVID-19 found that there wasn't any association between blood type, severe disease and death.
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In conclusion the studies seem to agree on the link between blood group and susceptibility but more research is needed to come to a conclusion about the link between blood group and severity of disease with respect to COVID-19.