Why Is Merck & Co. (MRK) Stock Price Up 12.60% Today, August 19? Breakthrough mRNA Cancer Vaccine Data Fuels Mega-Cap Rally

Merck's shares surged after Phase 3 INTerpath-001 data showed intismeran plus Keytruda met recurrence-free and distant-metastasis-free survival endpoints in resected melanoma, reinforcing confidence in Merck's immuno-oncology franchise extension. The move is largely catalyst-driven rather than tied to earnings or guidance changes, with sector-wide spillover evident via Moderna's sharp rally. Near-term focus shifts to full dataset details, regulatory pathway, and pending overall-survival readout.