Moderna Sheds About $24B as Leaked Cancer Vaccine Trial Data Triggers Selloff

Moderna $MRNA shares fell 25.2% to $183.88 on Thursday, wiping out about $24B in market value and setting the stock on pace for its steepest single-day percentage drop on record, while Merck $MRK dipped nearly 1%. The selloff began after detailed late-stage trial results for intismeran, an experimental personalized melanoma vaccine co-developed with Merck, leaked on an oncology website ahead of a scheduled Saturday presentation at the European Society for Medical Oncology meeting in Madrid. In the trial of over 1,100 high-risk melanoma patients over the first two years after treatment, combining intismeran with Merck's immunotherapy Keytruda reduced the risk of cancer recurrence or death by 42% and cut the risk of distant metastasis or death by 38% compared to Keytruda alone. Analysts at UBS, William Blair, and TD Cowen praised the clinical figures—which followed positive interim data in August—but noted that Moderna's more than 500% stock rally this year created high expectations and left investors questioning whether the therapy will prove as effective across other cancer indications. Merck and Moderna stated that overall survival data are not yet mature but show a "favorable trend," adding that both companies are engaging with regulatory authorities as competition across oncology vaccine development continues. "Expectations were high given the rally driven by bullish commentary from the companies." — Michael Yee, UBS Analyst.