Shares of drugmakers Moderna MRNA.O and Merck MRK.N fall over 8% to $160.3 and 1.3% to $150.3, respectively, in premarket trading.
Companies said on Wednesday a personalized mRNA cancer vaccine together with Merck's immunotherapy Keytruda reduced the risk of recurrence and spread of melanoma in a late-stage trial, a major success in a new field of cancer treatment.
Moderna shares more than doubled on Wednesday, adding over $44 billion to its market value.
"While we were thrilled to see a positive market reaction for the definitive success of the therapeutic cancer vaccine, intismeran autogene, the reception was overly optimistic and creates expectations that we believe will be difficult to meet," Leerink Partners analysts said.
Up to last day's close, MRNA's stock had risen 491% YTD.
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