Key Highlights

  • Moderna shares closed down 17.57% at $202.63, the worst performer in the S&P 500 that day.
  • Melanoma-vaccine data leaked two days before Moderna's planned ESMO presentation.
  • The intismeran-Keytruda combination cut recurrence or death risk by 42%, below the Phase 2b result.
  • The stock had risen roughly 600% year-to-date before Thursday's decline.

Moderna (NASDAQ:MRNA) plunged on Thursday, 22 October 2026, closing down 17.57% at $202.63, the worst performer in the S&P 500, after shares fell as much as 26% intraday on volume well above average. The move followed the leak of personalized melanoma-vaccine data on a cancer news site, two days before Moderna's planned presentation at the European Society for Medical Oncology (ESMO) congress.

What the Leaked Data Showed

Moderna and partner Merck subsequently released full Phase 3 results: the combination of intismeran and Merck's Keytruda cut the risk of recurrence or death by 42%, reflected in a hazard ratio of 0.58. That result came in below the 49% risk reduction seen in the smaller Phase 2b study that had driven earlier optimism about the programme.

A 'Sell the News' Reaction

Analysts at UBS, Bernstein and William Blair called the data solid but below the elevated expectations that had built up, with the sharp drop widely characterised as a 'sell the news' reaction following a roughly 600% year-to-date run in the shares. TD Cowen said the results were compelling but noted that several price targets sat far below where the stock had been trading, including $45 from Bernstein, $90 from TD Cowen and $80 from Citi.

A Company-Specific Move

Moderna's decline stood apart from the day's broader selloff, which was concentrated in technology and AI-linked names. Merck shares also fell, reflecting its stake in the trial's outcome, but Moderna's drop was by far the larger of the two and points to the leaked, below-expectations data as the dominant factor. Shares slipped a further roughly 3.7% in after-hours trading.

What Comes Next

Investors now await the full ESMO presentation on Saturday, which is expected to include overall survival data and read-through to other tumour types beyond melanoma, as well as Moderna's third-quarter earnings report on 6 November. Those two events will offer the next opportunities for the company to address how the market is weighing the trial's results against prior expectations.