The U.S. economy grew by 5.7 percent in 2021, the fastest full-year clip since 1984, roaring back in the pandemic’s second year despite two new virus variants that rocked the country. The growth came in fits and starts, with a burst of government spending helping propel a fast start, even as a surge in new cases and deaths in the second half of the year created new pressures. The economy grew at a 6.9 percent annual rate from October to December, the Bureau of Economic Analysis said Thursday, a sharp acceleration from 2.3 percent in the previous quarter. In a powerful rebound from 2020, when the economy contracted by 3.4 percent — its worst result since 1946 — 2021′s strong growth created a record 6.4 million jobs. But it also brought a host of complications, helping fuel the highest inflation in 40 years and creating supply chain snarls as consumers hungry for products overwhelmed the global delivery system. To beat back rising prices, the Federal Reserve is now shifting its strategy and preparing for interest rate hikes this year, convinced it has given enough support to help the labor market and now must keep the economy from overheating further. Earlier in the year, economists worried that global supply chain problems would keep businesses from being able to fully stock shelves. But a rush by companies in the final months of 2021 to bolster their inventories ultimately drove GDP much higher.