Why Moderna (MRNA) Stock Is Sinking Today

What happened

Shares of Moderna (MRNA), the mRNA vaccine maker, plunged 17.6% on Thursday after results from its late-stage melanoma trial with Merck (MRK) appeared online two days before they were due.

The results were posted on the medical news site Oncology Central around 12:30 p.m. ET and taken down shortly afterward, Investing.com reported. They showed that intismeran autogene, Moderna’s personalized vaccine, given with Merck’s immunotherapy Keytruda, cut the risk of the melanoma returning or death by 42% compared with Keytruda alone in the Phase 3 trial. Merck later confirmed the key data points, according to Investing.com.

The stock fell as much as 26.4% during the session before recovering some ground. The shares closed at $202.63, down 17.6% from the previous close. Merck closed 1.8% lower.

Why it matters

For patients, a 42% cut in relapse risk is a meaningful result. For the stock, the problem was the bar it had to clear. An earlier, smaller Phase 2b study showed a 49% reduction in the risk of recurrence or death with the same combination after five years of follow-up, 24/7 Wall St noted. The two trials enrolled different patients, so the numbers aren’t directly comparable, but investors had been paying for something closer to the first one.

Expectations were high because the shares had risen more than eightfold this year before Thursday. The companies had said in August that the trial met its main goals without giving detailed numbers, so the leak was the market’s first look at the size of the benefit. The full results are due on Saturday, 24 October, at the ESMO cancer congress.

What the market is telling us

Moderna’s shares moved 5% or more on 66 of the last 252 sessions. The last move at least this big in the same direction was on 20 August 2026, when the stock fell 23.6%. The stock is up 587.1% this year, and at $202.63 it sits 17.6% below its 52-week closing high of $245.81 from 21 October 2026.

Our view: the 42% figure still points to a working cancer vaccine, so Saturday’s fuller data, including how long patients stay cancer-free, matters more than one leaked headline for a stock that is still up almost sevenfold this year.