Dow futures tumble 550 points as stock-market investors watch rising cases of coronavirus; Fed offers grim outlook of U.S. economy

U.S. stock-index futures on Thursday tumble sharply as investors react to signs of an emergent pickup in cases of coronavirus and digest the Federal Reserve’s downbeat outlook of the domestic economy, even as the central bank held rates steady, as expected on Wednesday, and projected interest rates would be held near subzero levels until at least 2022.