Moderna Shares Drop Up to 25% as Leaked Melanoma Vaccine Data Show 42% Risk Cut

Moderna shares fell as much as 25% on Thursday, Oct. 22, after detailed Phase 3 results for its personalized melanoma vaccine, developed with Merck, surfaced online two days before their scheduled debut. The data showed the shot, intismeran autogene, plus Merck's Keytruda cut the risk of melanoma returning or death by 42% compared with Keytruda alone.

That is a clear win for patients in the trial. But investors had bid the stock up so far that a solid result still triggered a steep selloff.

The full results will be presented at the European Society for Medical Oncology (ESMO) Congress on Saturday, Oct. 24. Until then, the market is digesting a strange mix of strong science and a punishing share price reaction.

Key facts at a glance

• The result: intismeran plus Keytruda reduced the risk of recurrence or death by 42% versus Keytruda alone (hazard ratio 0.58) after a median follow-up of 24.6 months, according to Merck and Moderna's announcement.
• Two-year outcome: 82.2% of patients on the combination were alive and cancer-free at two years, against 70.0% on Keytruda alone.
• The trial: INTerpath-001 enrolled 1,137 patients with high-risk melanoma that had been fully removed by surgery.
• The stock: Moderna dropped as much as 25% intraday, while Merck slipped about 1%, CNBC reported.
• Next: the data will be presented at ESMO on Oct. 24 and has been accepted by The New England Journal of Medicine.

How we got here

Melanoma is one of the deadliest forms of skin cancer. Merck estimates about 112,000 new US cases and more than 8,500 US deaths from the disease in 2026, according to its August trial announcement.

For patients whose tumour has been removed but who face a high risk of it returning, a standard option is about a year of Keytruda. It is a "PD-1 inhibitor", a drug that releases a brake on the immune system.

Keytruda works, but it leaves a big gap. Even with it, one-third or more of people diagnosed with late-stage melanoma will die or see their disease return within five years, BioPharma Dive reported.

Moderna and Merck's answer is an individualized neoantigen therapy. Doctors sequence a patient's own tumour, find the protein flags unique to that cancer, and then build an mRNA shot coding for up to 34 of them. The goal is to teach T cells, the immune system's hunters, to find and kill any cancer cells left behind.

On Aug. 19, 2026, the companies said the trial had hit its main goal at a planned interim check, but gave no numbers. Moderna's stock more than quadrupled afterwards, according to BioPharma Dive, setting up Thursday's release as one of the most closely watched events in biotech this year.

A leak forced the data out early

The numbers were not supposed to be public until the weekend. According to Yahoo Finance, the results appeared on the medical news site Oncology Central around 12:30 p.m. ET on Thursday and were taken down shortly afterwards. Moderna's shares were halted for volatility minutes later.

Screenshots spread on social media, which prompted ESMO to release the closely guarded study details immediately rather than wait for the meeting, BioPharma Dive reported. Merck and Moderna then put out their full announcement the same day.

The trading reaction was severe. CNBC put the intraday fall at as much as 25%, and Yahoo Finance reported the stock down about 18% by mid-afternoon.

The leak also raises an uncomfortable question for the conference circuit. Medical meetings rely on embargoes to keep market-moving data under wraps until doctors can review it in full. Yahoo Finance noted that a statement from Moderna or Merck on the leak could come before Saturday.

What the trial actually showed

Strip out the stock chart and the trial data look strong. Here are the headline figures from the companies' release and CancerNetwork's summary:

Source: Merck and Moderna via Business Wire; CancerNetwork

Beyond the main result, the combination cut the risk of cancer spreading to distant organs, or death, by 38% (hazard ratio 0.62).

The most interesting detail is timing. In the first six months, the two groups did about the same. After the vaccine course was finished, a pre-specified analysis showed the combination cut the risk of recurrence or death by 58%. That fits the idea that the immune system needs time to learn its targets.

Overall survival data are not yet mature, though the companies said they have seen a favourable trend. Side effects were mostly fever and injection-site pain, each reported by roughly half of patients on the combination. About 7% of vaccine recipients stopped treatment because of side effects, against 5% in the comparison group, BioPharma Dive reported.

"We have tried really hard. I have led clinical trials. None of them has improved over PD-1," Hussein Tawbi, a professor of medicine at MD Anderson Cancer Center and a trial investigator, told BioPharma Dive. "And now we see we can do better."

Why good news sank the stock

Wall Street wanted more than good. Investors had expected anywhere from a 40% to 50% reduction in risk, according to BioPharma Dive. Moderna itself had said 35% would count as clinically significant.

The problem was the comparison with earlier data. The smaller Phase 2b trial initially showed a 44% reduction and, after five years of follow-up, a 49% reduction. Thursday's 42% landed at the low end of hopes, even though the two studies differ in size and follow-up time.

Valuation magnified the move. Moderna had climbed more than 530% this year as of Thursday afternoon, CNBC reported. Going into the release, it traded at about 52 times forward sales, against a five-year average of 7.7 times, according to Yahoo Finance.

That premium exists because the rest of the business has shrunk. Moderna's revenue fell from $6.85 billion in 2023 to $1.94 billion in 2025, and analysts expect only $3.40 billion in 2028, Yahoo Finance reported. Much of the company's value now rests on the cancer pipeline.

The other side of the argument

Not every analyst saw the drop as a verdict on the science. "Such a reaction likely reflects inflated expectations into the full data release, rather than a true indictment on the efficacy of the combo therapy," BMO Capital Markets analyst Evan Seigerman wrote, according to CNBC. UBS called it "overall a strong data set."

TD Cowen's Tyler Van Buren said the findings "substantially clear the [physician]-defined bar for a practice-changing dataset in adjuvant melanoma." William Blair's Myles Minter described a "sell-the-news reaction."

The sharpest caution came from RBC Capital Markets analyst Trung Huynh. He said the results were on the "low end" of expectations and showed a "deterioration" from earlier testing. "While approval seems likely, commercial success is far less certain," Huynh wrote, as quoted by BioPharma Dive.

There are practical hurdles too. Because each shot is custom-made, manufacturing can take about six weeks, CNBC reported. Merck CEO Robert Davis has said the company believes it can shorten that.

What it means for you

If you or a family member has melanoma: this treatment is not approved anywhere yet. The companies say they are "engaging with regulatory authorities," which typically means filings with agencies such as the US Food and Drug Administration. Patients with resected stage IIB to IV melanoma can ask their oncologist whether related trials are recruiting.

For readers in Australia: the study's principal investigator, Georgina Long, is medical director of Melanoma Institute Australia. She called the results "transformative" and said they support "a fundamentally new and individualized approach" to treating melanoma after surgery.

If you own Moderna shares or a fund holding them: Thursday showed how sharply a richly valued stock can move on a single data point. Saturday's full presentation is the next catalyst, and Yahoo Finance's analysis noted the stock still trades at roughly 38 times forward sales even after the drop.

If you follow mRNA technology: this is the first positive Phase 3 readout for an mRNA-based cancer therapy, according to the companies' August announcement. That matters for the wider field, though BioNTech's personalized vaccine recently failed a mid-stage colorectal cancer trial.

What happens next

• Saturday, Oct. 24: full results in ESMO's Presidential Symposium I (presentation LBA1), with publication in The New England Journal of Medicine to follow.
• Regulators: Merck and Moderna say they are engaging with authorities based on these data; no filing date has been announced.
• Other cancers: the companies are testing intismeran in nine Phase 2 and Phase 3 trials, including three Phase 3 lung cancer studies. RBC's Huynh expects interim kidney cancer results later this year or in the first half of 2027.
• Survival data: the trial continues so researchers can measure whether the combination helps patients live longer.

The bottom line

Thursday's selloff says more about expectations than about the vaccine. The trial delivered a 42% cut in recurrence risk and a 58% cut after the vaccine course ended, but a stock priced for perfection fell anyway.

Practical takeaways:

1. Patients and carers should treat this as promising but not yet available, and ask their oncologist about eligible clinical trials.
2. Investors should watch Saturday's ESMO presentation and any overall survival update before drawing conclusions, and size positions for large swings.
3. Anyone tracking the field should watch the kidney, bladder and lung cancer readouts, which will show whether personalized mRNA vaccines work beyond melanoma.
4. Everyone should be wary of trading on leaked screenshots; the official release came hours later with far more detail.