The COVID-19 vaccines will soon be available to the general public, but most jurisdictions are prioritizing certain groups for a least a little while longer. Age, occupation, or a qualifying medical condition will get you the jab, and in most places one of those conditions is a high body mass index, or BMI—essentially a simple ratio of height to weight.
This crude metric has increasingly come to be seen as outdated—it makes no attempt to account for muscle mass, or the location of body fat, or any other more precise measures of overall health. (Social media is full of athletic guys who are big from lifting weights getting the shot.) But for many people whose weight qualifies them to get the vaccine, it’s a frustrating situation: a number that’s plagued their medical care for years is finally, for once, helping them get a leg up on their health—but only because of how unwell the famously-uninformative number makes them seem.
(We should say right here that whether BMI is a good measurement of health is a completely separate question of whether any individual person should get vaccinated—if you qualify to get the shot in any way, please just go get it!)
The BMI, at its simplest, is a shortcut. All that’s needed is two measurements—height and weight—that can be taken quickly with minimal equipment. The metric is a barely-evolved interpretation of the studies of a 19th-century sociologist named Adolphe Quetelet. He collected data, based solely on height and weight, on what he believed was the average and therefore ideal human. He exclusively studied white European men, but his findings are still the foundation of what physicians use today to determine good health. But even he warned against using the number to assess an individual patient—it was intended only as a simple snapshot of an entire population.
In the 20th century insurance companies often relied on inconsistent weight tables to determine the health risks of their customers, which managed to filter back to actual physicians trying to determine what the “ideal weight” was. Quetelet himself didn’t pop back up until the 1970s, when a group of researchers, studying exclusively men, tentatively determined that Quetelet’s ratio, renamed as the body mass index, was the best indicator of obesity they could find.
Health care facilities rely on BMI partially because our medical system pressures them to move quickly through as many patients as they can and to avoid sending tests to labs or using expensive equipment to assess someone’s health. Determining what tests someone needs based on their self-reported symptoms takes time and attention, two things doctors and nurses are notoriously short on.
On top of that, they seem to pick up biases against high BMIs throughout their medical education, causing them to spend even less time listening to a patient the second they see that number. Dozens of studies have show patients with obesity get significantly worse medical care, with their symptoms often chalked up to weight and the real root overlooked for years.
Some observers see an even darker underbelly to our reliance on the BMI, though: there’s a lot of money to be made off of it, and it sure isn’t going towards helping caregivers find better solutions. Take the federal government’s 1998 decision to lower the “healthy” BMI by three points, effectively turning 29 million Americans “fat” overnight. Adrienne Bitar, a postdoctoral associate at Cornell who studies American health and culture, says that “BMI became shorthand for understanding the rise in the obesity epidemic.” That perceived uptick played a massive factor in the next decade’s panic that we were all too fat—a panic that made the diet industry what it is today.