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Additional evidence continued to suggest blood type may not only play a role in COVID-19 susceptibility, but also severity of infection, according to two retrospective studies.

In Denmark, blood type O was associated with reduced risk of developing COVID-19 (RR 0.87, 95% CI 0.83-0.91), based on the proportion of those with type O blood who tested positive for SARS-CoV-2 compared with a reference population, reported Torben Barington, MD, of Odense University Hospital, and colleagues.

However, there was no increased risk for COVID-19 hospitalization or death associated with blood type, the authors wrote in

Limitations to the data include that ABO blood group information was only available for 62% of individuals, and that the sex of the testing population was skewed, with women accounting for 71% who tested negative and 67% who tested positive.

They pointed to the recent research that blood type plays a role in infection, noting the lower than expected prevalence of blood group O individuals among COVID-19 patients. Researchers also observed how blood groups are "increasingly recognized to influence susceptibility to certain viruses," among them SARS-CoV-1 and norovirus, adding that individuals with A, B, and AB blood types may be at "increased risk for thrombosis and cardiovascular diseases," which are important comorbidities among patients hospitalized with COVID-19.

ABO and RhD blood group information was available for 473,654 individuals who were tested for SARS-CoV-2 from February 27 to July 30, as well as for 2,204,742 individuals not tested for SARS-CoV-2 as a reference.

Of the individuals tested, 7,422 tested positive for SARS-CoV-2. About a third of both those who tested positive and negative were men, and those with positive tests were slightly older (52 vs 50, respectively).

Among individuals testing positive for SARS-CoV-2, about 38% (95% CI 37.5-39.5%) belonged to blood group O versus about 42% of those in the reference population. There were significantly more group A and AB individuals in the positive testing group versus the reference population, though the difference was non-significant for group B. When group O individuals were removed, there was no difference between the remaining groups.

Blood Type Linked to COVID-19 Severity?

Meanwhile, a in Blood Advances did report a connection between blood type and COVID-19 severity.

Blood types A or AB in COVID-19 patients were associated with increased risk for mechanical ventilation, continuous renal replacement therapy, and prolonged ICU admission versus patients with blood type O or B, according to Mypinder Sekhon, MD, of the University of British Columbia in Vancouver, and colleagues. Inflammatory cytokines did not differ between groups, however.

These authors also cited research that found that blood groups were linked to virus susceptibility, but that the relationship between SARS-CoV-2 infection severity and blood groups remains "unresolved." However, COVID-19 appears to be a multisystem disease with renal and hepatic manifestations.

"If ABO blood groups play a role in determining disease severity, these differences would be expected to manifest within multiple organ systems and hold relevance for multiple resource-intensive treatments, such as mechanical ventilation and continuous renal replacement therapy," Sekhon and colleagues wrote.

They collected data from six metropolitan Vancouver hospitals from Feb. 21 to April 28, identifying 95 COVID-19 patients admitted to an ICU with known ABO blood type.

Among these patients, 57 were group O or B, while 38 were group A or AB. A significantly higher proportion of A/AB patients required mechanical ventilation versus O/B patients (84% vs 61%, respectively, P=0.02). Similar figures were seen for patients requiring continuous renal replacement therapy (32% vs 9%, P=0.04). Median ICU stay length was also longer for A or AB patients compared with O or B patients (13.5 days vs 9 days, P=0.03).

There was no difference in probability of ICU discharge, and eight patients died in the O/B group versus nine patients in the A/AB group. Not surprisingly, biomarkers of renal and hepatic dysfunction were higher in the A/AB group, as well.

"The unique part of our study is our focus on the severity effect of blood type on COVID-19. We observed this lung and kidney damage, and in future studies, we will want to tease out the effect of blood group and COVID-19 on other vital organs," Sekhon said in a statement.

About 25% of patients were missing data on blood group, and the nature of the study makes it impossible to infer causality, the authors acknowledged. Ethnic ancestry and outcomes in patients with COVID-19 could be an unaddressed confounder. Additionally, anti-A antibody titers may affect COVID-19 severity, and these were not measured.

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