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A new study uncovered a baffling new risk factor for severe or fatal coronavirus cases: how briskly people typically walk.

Looking at data from more than 400,000 middle-aged and older coronavirus patients, scientists from Leicester General Hospital in the UK identified several risk factors for more severe cases of COVID-19 and a heightened risk of dying from the disease. Some were unsurprising, but the study, published last month in The International Journal of Obesity, also found that walking speed seems to be a significant predictor of mortality.

According to the data, self-reported “slow” walkers were 88 percent more likely to become seriously ill and 83 percent more likely to die from the coronavirus than the “brisk” group.

“We know already obesity and frailty are key risk factors for COVID-19 outcomes,” lead author and Leicester diabetes expert Thomas Yates told Yahoo News. “This is the first study to show slow walkers have a much higher risk of contracting severe COVID-19 outcomes, irrespective of their weight.”

This was a retroactive study, so the researchers will need to do more work to nail down just how much walk speed matters for disease severity. But if it ends up being an important factor, it could help doctors better identify high-risk patients in advance.

The study doesn’t necessarily identify why the self-reported “slow” walkers had more severe cases of COVID-19 and died at a higher rate than their “average,” “steady,” and “brisk” counterparts. But given that the study primarily analyzed elderly patients, walking speed likely served as a proxy for overall physical fitness and cardiovascular health.

It’s also worth noting that the patients themselves described their walking speeds. What one patient thought of as slow might have been moderate to another, so questions remain about just how standardized the data was in the first place.

“It is my view ongoing public health and research surveillance studies should consider incorporating simple measures of physical fitness such as self-reported walking pace in addition to BMI as potential risk predictors of COVID-19 outcomes,” Yates told Yahoo, “that could ultimately enable better prevention methods that save lives.”