A surprise pledge from the Federal Reserve to buy recently downgraded corporate bonds boosted some of the biggest ETFs tracking the securities.

The $14.8 billion iShares iBoxx High Yield Corporate Bond ETF, or HYG, surged about 7.5% -- the most since January 2009 -- after the Fed said Thursday that it will expand its corporate bond buying program to include debt from companies that recently lost their investment-grade rating. The announcement also gave a boost of the same magnitude to the $8.5 billion SPDR Bloomberg Barclays High Yield Bond ETF, or JNK.

Concerns about the economic fallout from the coronavirus outbreak had reignited fears about “fallen angels,” as companies that are downgraded to speculative grade are known. Fidelity estimated that roughly $215 billion of U.S. debt could lose its investment-grade status this year, with the energy sector most at risk.

“Fears about how the high-yield market would absorb the likely wave of oncoming ‘fallen angels’ has been weighing heavily on the market,” said Seema Shah, chief strategist at Principal Global Investors. “The announcement is a significant relief, as reflected in the HY market’s response.”

Thursday’s move expands the size and scope of the Fed’s previously announced Secondary Market Corporate Credit Facilities, which enabled the central bank to purchase of up to 10% of an issuer’s outstanding bonds and up to 20% of the assets of eligible ETFs. While the majority of the Fed’s purchases will be aimed at ETFs whose primary objective is U.S. high-grade bond exposure, “the remainder will be in ETFs whose primary investment objective is exposure to U.S. high-yield corporate bonds,” according to a primer.