WASHINGTON—A measure of U.S. manufacturing activity soared to its highest level in more than 37 years in March, driven by strong growth in new orders, the clearest sign yet that a much anticipated economic boom was probably underway. The Institute for Supply Management (ISM) said on Thursday its index of national factory activity jumped to a reading of 64.7 last month from 60.8 in February. That was the highest level since December 1983. A reading above 50 indicates expansion in manufacturing, which accounts for 11.9 percent of the U.S. economy. Economists polled by Reuters had forecast the index rising to 61.3 in March. The year-long COVID-19 pandemic has boosted demand for goods. Economic growth is expected take off this year, juiced up by the White House’s massive $1.9 trillion pandemic relief package and the reopening of nonessential businesses as more Americans are vaccinated against the coronavirus. The relief package passed …