U.S. stocks rose modestly Wednesday and long-term Treasury yields pulled back sharply after the Treasury Department said it would double buyback operations for bonds maturing in 10 years or more, to at least $4 billion per operation from $2 billion currently, effective Sept. 9. The Dow Jones Industrial Average rose 119.65 points, or 0.22%, to 53463.05. The S&P 500 added 16.22 points, or 0.21%, to 7707.98, while the Nasdaq Composite advanced 41.38 points, or 0.16%, to 26331.09.

The move was read as a signal that Treasury Secretary Scott Bessent had grown alarmed by the bond selloff. "I've been saying 'bond traders can stop panicking when the Fed starts panicking,'" said Jim Bianco, president at Bianco Research. "I guess I should have said, 'bond traders can stop panicking when Scott Bessent starts panicking.'" Some analysts were skeptical the intervention would have a lasting effect, noting the buyback size is miniscule relative to the $432 billion deficit run in July alone.

Shares of Moderna soared 177% after the company and Merck said their experimental mRNA-based melanoma vaccine succeeded in a late-stage study, preventing cancer from returning in high-risk patients. Merck rose 13%. The SPDR S&P Biotech ETF jumped 5.9% and the broader healthcare sector rallied sharply.