Twins Kimberly, left and Kelly Standard in Rochester, Mich., on Wednesday, Jan. 13, 2021. Both sisters had COVID-19 last spring, but their experiences differed significantly: Kelly was hospitalised for less than a week while Kimberly spent a month in critical condition. (Emily Rose Bennett/The New York Times)

Twins help untangle COVID’s genetic roots

Why does one twin have the disease worse than the other? This is one of the questions that spur scientific projects around the globe