COVID-19 has now killed more than five million people worldwide. It is yet another grim milestone in a seemingly endless stream of them. In many countries, including the United States, COVID-19 is now a leading cause of death, alongside heart disease and stroke. And yet experts say the pandemic’s true toll is likely much higher.

“It’s quite possible that the number of deaths is double what we see,” says Amber D’Souza, professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. “But five million is such a staggering number on its own. No country has been able to escape it.”

D'Souza and other experts point out that the official global tally captures only confirmed cases in each country—and