U.S. stocks rose modestly Wednesday and long-term Treasury yields pulled back sharply after the government said it would buy back more longer-dated bonds, easing pressure on a market that had pushed borrowing costs to their highest levels in nearly two decades. A landmark cancer vaccine result sent Moderna shares soaring.

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. According to preliminary data, there were 1755 advancing issues and 996 declining issues on the NYSE.

The Treasury Department said it would double its 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 move was read on Wall Street 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, however, 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. The WSJ Dollar Index fell to its lowest level since May as lower yields reduced the appeal of U.S. debt to international investors.

Moderna more than doubled, surging 177%, after the company and Merck announced that their experimental mRNA-based melanoma vaccine succeeded in a late-stage study, preventing cancer from returning in high-risk patients. Merck rose 13%. Analysts said the result carried symbolic weight beyond the two companies, offering what some read as a broader vindication of mRNA technology after years of post-pandemic setbacks. The SPDR S&P Biotech ETF jumped 5.9% and the broader healthcare sector rallied sharply.