Several research institutes have already reported that type O blood is more resistant to SARS-CoV-2 and type A blood is less resistant. This suggests that the presence of anti-A antibodies must be related to SARS-CoV-2 infection protection.
A group from Universe libre de Bruxelles (ULB), Brussels, Belgium, etc. has speculated that the presence of anti-A and anti-B antibodies is a true problem and that it is related to the protective effect against SARS-CoV-2.
https://www.ijidonline.com/article/S1201-9712(20)32549-2/fulltext
Type O: IgM anti-A+ anti-B agglutination score: 88.29±33.01 (healthy people), 76.93±34.93 (COVID-19 patients)
Type A: IgM Anti-B agglutination score: 30.40±18.84 (Healthy), 24.93±18.73 (COVID-19 Patients)
Type B: IgM anti-A agglutination score: 36.50±17.41 (healthy persons), 28.56±17.41 (COVID-19 patients)
From these results, it could be said that as the presence of anti-A antibodies and anti-B antibodies is small, the resistance to SARS-CoV-2 weakens and it becomes easy to become infected, and if the amount of presence is not large, the effect of infection protection due to differences in blood types would not be clear.
Anti-A and anti-B antibodies are synthesized by immune stimulation from gastrointestinal microbiota. Both these antibody production and microbiota vary greatly according to an individual’s age and nutritional status. In other words, the promotion of ABO antibody production by the gastrointestinal microbiota will help prevent the infection of the novel coronavirus! ?