If someone were to ask "Has a linkage between blood type and COVID infection (severity) been studied?" A reasonable response would be, see: https://www.acpjournals.org/doi/10.7326/M20-4511 Blood type and outcomes in patients with COVID-19 - PMC and a go-getter would look for more recent publications citing those. Then see where current consensus stands on the matter. Simple and as complicated as that. However if someone were to ask "Why has the linkage between O neg blood type not been studied? What are they trying to hide?" The same reasonable response would perhaps be accompanied by a feeling of sadness. Some maladies, gleefully self-inflicted, are exceedingly difficult to treat.