US companies with investment-grade ratings have been storming the gates to borrow at the fastest clip in history, less than two weeks after the Federal Reserve exercised its crisis-era powers to keep credit flowing to a nation bracing for the worst of the coronavirus outbreak.

The borrowing boom comes before the full contours of the Fed’s new $300bn asset-purchase facilities have taken shape, while businesses have closed, unemployment has soared and more than two-thirds of all Americans have been told to stay home to stop the spread of the virus.