Moderna Inc. shares experienced a record surge of up to 160% on Wednesday, August 19, 2026, primarily driven by the positive results of a late-stage trial for its personalized cancer vaccine, developed in partnership with Merck & Co. This dramatic increase resulted in an estimated $5 billion in paper losses for short sellers on that day alone, pushing their year-to-date mark-to-market losses to over $7 billion, according to data from S3 Partners LLC.
The significant stock rally has made a bearish stance on Moderna exceptionally painful for short sellers, with S3 Partners managing director Matthew Unterman noting that the move materially changes the risk/reward calculus for those maintaining short positions. The company's stock had already gained 114% earlier in the year as investors hoped its flu vaccine would diversify revenue streams away from its declining COVID-19 immunization business, which had led to four consecutive years of losses and a 94% drop from its 2021 peak.
Wall Street analysts reacted positively to the trial results. Joseph Stringer from Needham hailed the melanoma study as a "landmark win," predicting it could establish Moderna's oncology franchise as a major growth driver. Myles Minter of William Blair upgraded his rating to outperform, citing a clear path for Moderna to diversify its revenue beyond its COVID business. The losses for short sellers could have been even greater, as short interest had been as high as 20% of Moderna's float earlier in the year before falling to about 14% as some bearish bets were unwound.
Approximately 20 million shares, or a quarter of short positions, had been covered in 2026 prior to this surge. This means that the magnitude of Wednesday's move is likely to compel additional short sellers to reassess or reduce their positions, potentially fueling further upward pressure on the stock as they scramble to cover their bets.
