Moderna Stock Crashes 25% in Its Worst Day Ever: Leaked Cancer Vaccine Data Wipes Out $24 Billion

Moderna stock fell about 25% — its worst day on record

Moderna (NASDAQ: MRNA) shares sank more than 25% on Thursday, October 22, 2026 — last down 25.2% at $183.88 in Reuters' reporting, on pace for the steepest one-day fall in the company's history — after detailed results for its personalized melanoma cancer vaccine with Merck leaked early and landed below the market's sky-high expectations. The sell-off wiped out about $24 billion of market value and made Moderna the worst-performing stock in the S&P 500 on the day, even though the data itself showed the vaccine cut the risk of cancer returning or death by 42%.

What happened

The results were not supposed to arrive this way. Intismeran is a personalized mRNA treatment given alongside Merck's blockbuster immunotherapy Keytruda to patients with high-risk melanoma. The full Phase 3 details were scheduled for Saturday at the European Society for Medical Oncology conference. Instead, OncologyCentral appears to have published some results early on Thursday; the page was taken down, screenshots spread on X, and the companies released the data after the leak sparked the fall. Both declined to comment on the leak itself.

The detailed numbers were positive. In over 1,100 patients, intismeran plus Keytruda cut recurrence or death risk by 42% versus Keytruda alone, and distant metastasis or death by 38%. Merck shares were flat to down nearly 1% — the reaction was aimed squarely at Moderna.

Why it's blowing up

This was an expectations story, not a failed trial. In August, interim data showing the vaccine prevented melanoma returning or spreading sent Moderna's stock more than doubling; the shares had risen over 500% this year, pricing in a best case. Against that, the 38% metastasis figure looked like a step down from an earlier study's 49% recurrence and 59% metastasis reductions.

Analysts agreed on why the stock fell. TD Cowen's Tyler Van Buren called the 38% figure favorable but short of elevated expectations, and noted softening results between trials could raise questions about harder cancers — important because the vaccine is being tested in several cancer types. UBS's Michael Yee called the data "solid" and the pullback an expected "sell-the-news" event; William Blair's Myles Minter called the data "great" but questioned whether other cancers can justify the valuation. Volume ran at roughly double its average, per Investor's Business Daily.

Key facts

• The move: Moderna fell about 23–25% on October 22 — down 25.2% at $183.88 at last check by Reuters — its steepest one-day drop on record, wiping out about $24 billion in market value.
• The trigger: Phase 3 melanoma vaccine results appeared early on OncologyCentral, were taken down, and circulated as screenshots on X; the companies then released the data ahead of Saturday's scheduled ESMO presentation.
• The efficacy: intismeran plus Keytruda cut recurrence or death risk by 42% and distant metastasis or death risk by 38% versus Keytruda alone, in more than 1,100 high-risk melanoma patients.
• The context: Moderna had risen over 500% in 2026 and more than doubled in August on interim data; earlier results had shown 49% and 59% risk reductions, setting a higher bar.
• The partner: Merck (MRK), which co-develops the vaccine and sells Keytruda, was flat to down nearly 1% — a fraction of Moderna's fall.
• Analyst verdict: "solid" and "great" data (UBS, William Blair) that still missed inflated expectations (TD Cowen) — a classic sell-the-news reaction.

What's next

The full dataset is presented at the ESMO conference on Saturday, including any details the screenshots did not show. Beyond melanoma, the bigger question is whether intismeran works in other, harder cancers — a large part of why the stock ran so far. A plaintiff law firm also announced an investigation into whether investors had complete information before the early disclosure — common after a drop this size, and not a finding of wrongdoing. Thursday's data was still the first positive Phase 3 result for a personalized neoantigen therapy and an mRNA cancer treatment; what changed was the price investors would pay for it. This report describes what happened and why markets reacted; it is not a recommendation to buy, sell, or hold any security.

Sources

• Reuters — Moderna sheds about $24 billion as cancer vaccine data triggers 'sell-the-news' retreat
• Wall Street Journal — Moderna's Stock Plummets After Cancer-Vaccine Data Shared on Social Media
• Investor's Business Daily — Stock Market Today: Moderna collapsed on leaked melanoma vaccine data
• MarketWatch — Moderna tumbles to become the worst-performing stock in S&P 500
• Business Wire — Moderna shares dropped 23.07% after October 22 disclosure

FAQ

Why did Moderna stock fall 25% if the vaccine data was positive?

The stock had risen over 500% in 2026. The data was positive but below the earlier 49% and 59% figures investors had anchored on, triggering a sell-the-news reaction.

What is intismeran, Moderna's cancer vaccine?

Intismeran is a personalized mRNA cancer vaccine developed with Merck. In the Phase 3 trial it was given with Merck's immunotherapy Keytruda to more than 1,100 patients with high-risk melanoma after surgery.

How did the Moderna vaccine data leak?

OncologyCentral briefly published results on Thursday, October 22, ahead of Saturday's ESMO presentation. Screenshots circulated on X, and the companies then released the data.

What happens next for Moderna's vaccine?

The full dataset is presented at the ESMO conference on Saturday as planned. Investors are also watching trials of intismeran in other cancer types, which analysts say will help determine whether the treatment supports Moderna's valuation.