Every number tells a story. So far, the story of coronavirus has largely been told in death counts. Assembled by epidemiologists and public health leaders, those figures paint a picture of devastating loss: Out of more than 9 million cases worldwide, nearly half a million people have died.
But there’s another number that tells a slightly different story. Some public health wonks use another measure of mortality you may not be familiar with: years of life lost, or YLLs. Based on early averages, Covid-19 cuts 12 to 14 years off the lifespan of its victims.
Both YLLs and total fatalities reflect the astounding mortal cost of coronavirus. But they highlight different aspects of the challenge facing every country in the world. And it’s important to understand what each of them can and can’t tell us about the progression of the virus through society.
YLLs have been used in academic circles since the 1940s, but scientists didn’t widely use them until the 1990s. That’s when the first Global Burden of Disease study was published, led by researchers at the World Health Organization and the Institute for Health Metrics and Evaluation at the University of Washington. It was the first to try to quantify not just how many people across the globe were dying, but when they were dying, and how that may effect the country’s development overall.
YLLs put extra weight on people who die at a younger age. If you’d expect that people in a population of 1,000 would live to 85, but 100 of them died at 75 because of an illness, the total fatalities would be 100, but the YLLs would be 1,000 (100 lives multiplied by 10 years fewer each). If that same group of people died at 65, fatalities would remain at 100—but YLLs would suddenly be 2,000. Suddenly, the problem would seem much graver.
Economists in particular appreciate the YLL because it can expose the financial toll of a disease, in addition to the incalculable loss of human life. The earlier a disease kills, the more earning power it eliminates, allowing economists to calculate a rough estimate of the economic devastation of a disease.
This approach may be useful for public health officials trying to choose a course of action with a limited budget. If a large group of children or younger adults were dying of diarrheal diseases in one country, the YLLs would be higher than the same number of older adults dying of heart attacks. You could save more years of life—and potentially dollars—by implementing better sanitation systems than you would if you invested more in machines that could restart a person’s heart, even though the fatalities would be the same.
But using YLL to inform those kinds of decisions is far from simple. For one, YLL calculations can have built-in assumptions that make them less relevant for some populations. Life expectancy is a moving target; when researchers the first Global Burden of Disease report in 1993, they used women from Japan, with an average life expectancy of 82.5 years, as their baseline (pdf p. 5); for men, they assumed 80 years. In recent years, the WHO has switched to nearly 92 years for all genders, based on the predicted life expectancy for women living in South Korea in 2050.
Not everyone will reach their 10th decade, of course, so a more precise YLL could be calibrated by country. In the United States, for example, the average life expectancy is 78.6 years for someone born in 2016. But even that doesn’t cover all the variation; systemic racism has reduced the average life expectancy for Black Americans born in 2015 to around 75, compared to 79 for white Americans. Those complications mean that people have to be very careful about how YLLs are applied.
There are ethical complications, too. Early in the Covid-19 pandemic, some countries were slow to implement lockdowns and other measures to mitigate the virus’s spread. They argued that because the virus was most fatal in the elderly, it wasn’t a serious threat. Whether or not they knew it, they were making an argument based on YLLs.
At the time, Tedros Adhanom Ghebreyesus, the director-general of the WHO—the popularizer of the YLL—decried this response as the age discrimination it is. “I can see why you’d be a little more worried of younger people getting knocked down in the prime of their life,” says Jeffrey Hammer, a health and development economist formerly at the World Bank and Princeton University. We expect older people to die, even if it’s the result of an unexpected illness; it’s more shocking when a younger person does. But that doesn’t mean YLLs should be used to justify decisions that devalue older adults.
As a result of this faulty logic, some countries may have facilitated the spread in their entire populations—and put their particularly vulnerable populations at higher risk. In the story of coronavirus, the YLL can add shades of meaning. But it can’t dominate the narrative.