Stock Market Today, Oct. 22: Moderna Stock Plummets 18% on Melanoma Vaccine Data Leak Reports

MRNA plunged 18% on Oct. 22 after reports leaked interim data from its personalized melanoma vaccine program, showing the shot fell short of what the market had priced in.

The leaked figures show a 42% reduction in the risk of recurrence or death when the vaccine is paired with MRK's Keytruda. That trails the 49% reduction posted in Phase II. Full Phase 3 results were scheduled for release Oct. 24, so the leak front-ran the official readout by two days.

The selloff hit a stock that had gained 600% in 2026, which left little room for a result weaker than the earlier trial. Interpretation: an 18% single-day drop suggests the market had priced in efficacy at or above the Phase II mark, and a 7-point step-down in benefit was enough to trigger a sharp repricing of that premium. The combination partner, MRK, supplies Keytruda in the regimen, so the readout carries read-through for how investors value the pairing.

Nothing in the supplied reports addresses moves in BNTX or NVAX, so any sympathy trade in vaccine peers remains unconfirmed.