The paper, by MIT economics professor and physician Jeffrey Harris, points to a parallel between high ridership “and the rapid, exponential surge in infections” in the first two weeks of March — when the subways were still packed with up to 5 million riders per day — as well as between turnstile entries and virus hotspots.

“New York City’s multitentacled subway system was a major disseminator — if not the principal transmission vehicle — of coronavirus infection during the initial takeoff of the massive epidemic,” argues Harris, who works as a physician in Massachusetts.

While the study concedes that the data “cannot by itself answer question of causation,” Harris says the conditions of a typical subway car or bus match up with the current understanding of how the virus spreads.