In a study published June 3 in the Annals of Internal Medicine, researchers at the Scripps Research Translational Institute reviewed data from 16 different groups of COVID-19 patients from around the world to get a better idea of how many cases of coronavirus can likely be traced to people who spread the virus without ever knowing they were infected. Their conclusion: at minimum, 30%, and more likely 40% to 45%.

Such so-called asymptomatic spread is unique for a respiratory virus; most cause symptoms and disease once they infect their hosts. SARS-CoV-2, the virus responsible for COVID-19, is, however, particularly wily because it can also infect hosts silently and use them as unwitting pawns in its infectious campaign. “The range we found is extraordinarily high,” says Dr. Eric Topol, director and founder of the Institute and one of the authors of the paper.