Recent media articles linking blood type to outcomes in COVID-19 infections have led some of you to ask about your blood (ABO) type. These studies looked at populations in New York City, China and Europe and noted worse outcomes for those with type A blood and better outcomes for those with type O blood. Given the language used, you might worry that someone with type A blood had “almost a 50% higher chance of getting severe COVID-19.”
A better understanding of how to measure risk should alleviate concern by demonstrating a much smaller increased risk–in the single digits or less (depending on age)—of the outcome for an individual.
What is a blood type? The blood type ABO was discovered almost 120 years ago. There are four (A, B, AB and O) distinct types, based on the specific configuration of “antigens” (protein and sugar molecules) that coat the surface of your red blood cells. The blood types are inherited, but we are still not sure what functions these antigens perform. Even before COVID-19, we recognized that different blood types are associated with different susceptibilities to various infections and blood clotting disorders. Therefore, different outcomes with COVID-19 according to blood type follow a pattern from earlier research.
The recent studies on blood type-associated COVID-19 risk strive to increase our understanding of how the disease attacks the body by looking for structures and mechanisms associated with worse outcomes. However, the press picked up and amplified potential fears based on an individual’s risk based on blood type. For example, a recent news story alarmingly reported:
“The risk for severe COVID-19 was 45% higher for people with type A blood than those with other blood types. It appeared to be 35% lower for people with type O.”
But what does this really mean?
To understand the magnitude of an individual’s risk, we need to return to Epidemiology 101 and explore the difference between the concepts of relative risk and absolute risk. Imagine a hypothetical illness that infected 6% of blue-eyed people and 4% of non-blue-eyed people. The relative risk of the “blue-eyed” risk factor is defined as the ratio of percentage of blue-eyed people infected divided by the percentage of non-blue-eyed people infected (i.e. 6%/4 % = 1.5). That is, a blue-eyed person would be 50% more likely to be infected than a non-blue-eyed person. A 50% increased risk certainly sounds alarming!
However, the absolute risk is more relevant. It describes how many people are affected in each group. Thus, a blue-eyed person would have a (6%-4%=) 2% greater risk of acquiring the illness. The relative risk will stay constant independent of the number of people affected, but the absolute risk will vary based on how many people have the illness. For example, an illness affecting 60% of blue-eyed and 40% of non-blue eyed people would still yield a relative risk of 50% (60%/40% = 1.5). However, the difference in absolute risk in this new example would be 60%-40% = 20%. An increase in your absolute risk from 4% to 6% is very different from a 40% to 60% increase.
The articles about blood type only describe relative risk. To understand your risk as an individual with type A blood (myself included), you would need to understand your absolute risk.
As an example, let’s consider what these new studies mean for me. Due to my type A blood, I have a 50% elevated relative risk. As a 50-59 year old, my absolute risk of dying from COVID-19 is approximately 1%. A 50% increase in relative risk would raise my absolute risk to 1.5%, a difference that would not change my level of fear or motivate any difference in behavior. Even among the oldest age group (above 80), the estimated mortality of 16% would increase to 24%, frightening numbers but not appreciably different in terms of modifying behavior.
In conclusion, I would not recommend getting your blood type tested because the results would not change your absolute risk appreciably and should not change your behavior. You need to be careful regardless of blood type.
Please contact me with any questions or concerns.
Barry Rotman, MD