WASHINGTON (Reuters) - The shuttering of the U.S. economy due to the coronavirus pandemic is a shock of historic proportions that likely will push the national unemployment rate to 16 or higher this month and require more stimulus to ensure a strong rebound, a White House economic adviser said on Sunday. “It’s a really grave situation,” President Donald Trump’s adviser Kevin Hassett told the ABC program “This Week.” “This is the biggest negative shock that our economy, I think, has ever seen.