The experimental antibody drug given to President Donald Trump has been called one of the most promising approaches to preventing serious illness from a COVID-19 infection.

Its maker, Regeneron Pharmaceuticals Inc., said the company agreed to supply a single dose, given through an IV, for Trump at the request of his physician under "compassionate use" provisions, when an experimental medicine is provided on a case-by-case emergency basis, while studies of it continue.

The new drug is in late-stage testing and its safety and effectiveness are not yet known. No treatment has yet proved able to p...