Catenaa, Wednesday, August 19, 2026- Moderna stock surged more than 145% on Wednesday after it announced initial results from a study of an experimental cancer treatment.
Moderna and Merck are developing a treatment using an mRNA-based cancer vaccine called intismeran that is designed to be tailored to individual patients based on unique mutations within their tumors. In current studies, they are pairing it with Keytruda, an immunotherapy drug made by Merck.
The study compared using intismeran and Keytruda together on melanoma patients against using Keytruda alone. The companies said patients who received the combination lived longer without cancer returning or metastasizing than patients who got Keytruda alone.
The companies did not say how long patients survived without cancer returning or whether patients lived longer overall. They said they plan to present the study results at an upcoming international medical meeting.
Biotech peers also gained. Novavax was up 6% while U.S.-listed shares of BioNTech rallied 21%. The S&P 500 healthcare sector rose 2.9% to hit a record high, providing the biggest support to the benchmark index.
Meanwhile, the record surge in Moderna stock delivered a $5 billion blow to short sellers who had bet that the vaccine maker’s years-long slump would continue.
The jump generated paper losses of about $5 billion for short sellers, bringing mark-to-market losses to more than $7 billion this year, according to data from S3 Partners.
“This is an exceptionally painful move for Moderna shorts,” said Matthew Unterman, managing director at S3 Partners. “Today’s move materially changes the risk/reward for anyone maintaining a bearish position.”
Moderna’s stock had been on a tear this year, gaining 114% even before the melanoma trial results, as investors piled into the firm on hopes that its flu vaccine would help the company diversify away from its declining Covid immunization business.
Shares had suffered four straight years of losses as demand for its Covid shot waned, dragging the stock nearly 94% below its 2021 peak.
Wall Street analysts lauded the trial’s results as a huge potential boon for the firm. Needham analyst Joseph Stringer called the melanoma study a “landmark win” for Moderna, saying it could turn the firm’s oncology franchise into the next growth driver.
Meanwhile, William Blair’s Myles Minter upgraded his rating to outperform from market perform, telling investors Moderna now has a clear line of sight to revenue diversification from its Covid business.
Losses for short sellers could have been even worse too. Short interest as a percentage of Moderna’s float, a measure of how many shares are available to be traded, had reached as high as 20% earlier this year before falling to about 14% as traders unwound some of the bearish bets, according to S3 Partners.
“What makes today more consequential is that the short trade had already been unwinding; roughly 20 million shares, or a quarter of positions, have been covered in 2026, so a move of this magnitude will force additional shorts to reassess or reduce positions,” Unterman said.
