Blood Type May Have Minimal Effect On Covid-19 Health Risk

It was a catchy idea: Your blood type might affect your risk of contracting Covid-19 or of developing a serious case of the disease.

Numerous news outlets reported the story a few weeks ago, including CNN and NBC News. NBC's website summarized the results as follows: "Overall, the findings indicate that people with Type O blood seem to be more protected and that those with Type A appear more vulnerable." NBC was also careful to note this was only an "association," not causation. And in the original paper published in the New England Journal of Medicine, the authors noted several limitations to their study, including the fact that "adjustments for all potential sources of bias (e.g., underlying cardiovascular and metabolic factors relevant to Covid-19) could not be performed."

But the idea spread around the world, reported by health and science journalists.

More recently, further studies have shown that these associations may not be as strong as initially reported.

A major multi-institutional study published by researchers based at the Massachusetts General Hospital showed "there is no reason to believe being a certain ABO blood type will lead to increased disease severity, which we defined as requiring intubation or leading to death."

Another paper from Columbia Presbyterian Hospital in New York found that having type A blood did not increase a person's risk of becoming infected, and that having type O blood might slightly decrease one's risk. But lead author Nicholas Tatonetti noted, "[T]he effect is so small that people shouldn't count on it."

This is not the first time that an initial intriguing scientific result has been modified or overturned by further research. This has happened repeatedly in nutrition science or drug trials. Spurious correlations based on small data sets are a well-known phenomena, as nicely illustrated by this classic xkcd cartoon.

Good scientists know this can happen, which is why they welcome further research that either confirms or disproves their initial reports. Our best understanding of the truth is always subject to further refinement (or occasional upheaval) based on new evidence and better methods of analysis. But when we're dealing with emotionally charged issues such as the Covid-19 pandemic, it's easy to forget this.

So when we read stories about various drugs or vaccines that might (or might not) work, it's probably wise to temper our initial exuberance (or disappointment) at early results. And be careful when reading mainstream news reports that might not capture all the nuances of the original scientific papers. The truth is out there. We just need to exercise patience and good judgment.

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