Moderna Shares Plunge 25% Following Apparent Early Data Leak
A steep sell-off hit the biotech firm after details regarding an experimental melanoma vaccine developed with Merck appeared online ahead of a scheduled medical conference presentation.
Moderna stock experienced a severe 25% decline on Thursday afternoon following the reported early publication of highly anticipated Phase 3 data for an experimental cancer vaccine. The treatment, known as intismeran or intismeran autogene, is being developed in partnership with Merck to prevent the recurrence of melanoma by using a custom approach tailored to a patient's own tumor.
The data was briefly published on a cancer news website operated by Informa before being quickly removed. The information predated a formal presentation scheduled for Saturday at the European Society for Medical Oncology annual conference in Madrid, an event that typically draws around 30,000 attendees and features studies with the potential to alter standard medical care.
While the leaked report indicated that the therapeutic vaccine combined with Merck's immunotherapy drug Keytruda lowered the risk of melanoma recurrence or death compared to Keytruda alone, the premature disclosure triggered a sharp market reaction. Moderna shares fell to $184.87, reflecting elevated expectations and a massive year-to-date run-up that had previously seen the stock surge significantly.
Merck shares also dipped, sliding 1.4% following the news. Representatives for Moderna declined to comment on the incident, while spokespeople for Merck and the European Society for Medical Oncology did not immediately respond to inquiries.
The companies initially announced in August that the late-stage trial had successfully met its primary and secondary endpoints, though specific numerical details were withheld at that time. Earlier Phase 2b findings presented in 2026 demonstrated a 49% reduction in the risk of recurrence or death when utilizing the combination therapy compared to the standalone immunotherapy.
Broader sector movement remained relatively contained during the sell-off. The iShares Biotechnology ETF declined by about 2%, and the broader S&P 500 dropped approximately 1.32%. Meanwhile, messenger RNA rival BioNTech dipped 2% while Pfizer edged up 0.9%, highlighting that market pressure was largely concentrated on Moderna rather than the broader industry.
Intismeran autogene represents a critical cornerstone for Moderna as the company attempts to expand its commercial oncology footprint beyond traditional respiratory vaccines. The therapeutic approach is currently being evaluated across multiple Phase 2 and Phase 3 trials targeting various forms of cancer, including non-small cell lung cancer, bladder cancer, and renal cell carcinoma.
Market participants and oncologists now await the official full data presentation at the upcoming medical symposium to determine whether the complete clinical findings validate the optimism previously priced into the biotechnology firm's valuation.