The researchers used blood group A as a reference for how likely people were to catch the coronavirus. 

After taking into consideration all the other risk factors for Covid-19, scientists said those with type AB blood were 15 per cent more likely to test positive for the virus compared with type A.

And those in type B group were 21 per cent more likely than type A to get the virus.

People in A and AB blood groups appear to be at a greater risk of infection and the worst Covid-19 outcomes.

Infection is five per cent less likely in type O than A. However, when this group was compared with all blood groups, this grows to 12 per cent. 

The researchers also found those who have a negative blood type (O-, A-, B- or AB-) are 21 per cent less likely to be struck with the disease, on average.

Taking the findings into account, people who were O- were 16 per cent less likely than all others to be infected with the coronavirus.

But the study didn’t give a breakdown for the other groups. 

Worse outcomes

In the study cohort, there were 1,328 cases of severe Covid-19 illness or death.

Those who were in AB (the rarest) and B blood groups were more likely to suffer the worst outcomes of the disease – 21 per cent more likely than those in group A.

Meanwhile, those in group O were six per cent less likely than group A to deteriorate, jumping up to 13 per cent less likely compared to all blood groups.

Negative types were also 18 per cent less likely than positive types to get severe Covid-19 or die. 

The study suggests that people with AB+ or B+ blood are the most at risk of severe Covid-19 outcomes, while those with O- are the least.

But the study did not confirm this due to the way it was designed. 

 

Mounting evidence

The Canadian researchers wrote in their paper: “Taken together, the current body of evidence suggests that O and Rh− (negative) blood types may protect against SARS-CoV-2 infection and, possibly, severe COVID-19 illness.

“Whether this information can influence COVID-19 prevention or treatment strategies remains to be determined.”

The findings, published in the journal Annals of Internal Medicine, chime with a Danish study which found fewer positive test results among those with blood type O and more in peers with A, B and AB.

The researchers had compared registry data of over 473,000 individuals tested for Covid-19 to a control group of more than 2.2 million from the general population.

Another study of 95 critically ill Covid-19 patients in hospital in Vancouver found the A and AB blood types were at higher risk of severe symptoms than those with O or B.

They were more likely to require mechanical ventilation – suggesting they had greater rates of lung injury from the virus – and dialysis for kidney failure.

It suggests these two blood groups have an increased risk of organ dysfunction or failure due to Covid-19.

Another study of Italian and Spanish people found the risk of needing ventilation for Covid-19 was 35 per cent lower in people with O blood compared to other groups.

Similarly an Iranian hospital found O blood had a 32 per cent lower risk of being hospitalised with Covid-19.

But not all the studies done so far have taken variables such as pre-existing illnesses into account, which may skew the results. And some have not been peer-reviewed by other scientists to check for mistakes.