On August 19, Merck (MRK) said its personalized cancer vaccine had proved effective in a large, late-stage melanoma study, and Wall Street responded fast.
Merck shares closed up about 12.5% that day, one of the biggest single-day moves the stock has ever posted.
The rally added roughly $43 billion to Merck’s market value in hours.
The reason the market cared so much goes beyond one skin-cancer result.
Merck faces a problem in 2028, when its top-selling drug loses patent protection. This new vaccine gives the company a way to replace some of that revenue.
For investors, the question now is whether the jump reflects real long-term value or a one-day reaction that fades.
What Merck’s Phase 3 melanoma result actually showed
Merck and partner Moderna (MRNA) said their Phase 3 INTerpath-001 trial hit its main goal.
The study tested a vaccine called Intismeran alongside Merck’s blockbuster immunotherapy Keytruda in patients whose melanoma had been surgically removed.
Intismeran is a personalized treatment. It reads the specific mutations in each patient’s tumor, then trains the immune system to recognize and attack cancer cells that carry them.
The trial enrolled 1,137 people with high-risk, stage 2B to stage 4 melanoma, according to Moderna‘s release.
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The combination of the two drugs beat Keytruda alone on two measures: how long patients went without their cancer returning, and how long before it spread to distant parts of the body.
The companies called it the first positive Phase 3 result for an individualized mRNA cancer therapy.
Why the $43 billion jump surprised even Wall Street
The rally looks big compared to the actual melanoma sales at stake.
Goldman Sachs analysts, in a research note shared with TheStreet, estimated global peak sales for Intismeran for treatment of melanoma at about $4.3 billion a year.
Merck and Moderna will split the profits 50/50, though Merck books all the global sales.
So why did Merck add roughly $43 billion in market value over a program worth a few billion in annual sales?
Investors were pricing in more than melanoma.
The result validated the underlying technology, opening the door to using the same approach against other cancers.
Goldman noted that Merck shares carried lower expectations for this program going in, which left more room for the stock to move once the data landed.
The Keytruda cliff that makes this vaccine matter
Keytruda is Merck’s best-selling drug and made up close to half of the company’s 2025 sales.
That drug loses key U.S. patent protection in 2028, which means cheaper competition can enter, and revenue could drop sharply.
Investors have worried for years about what replaces that income. Intismeran gives Merck a credible answer.
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A vaccine like Intismeran, tailored to each patient, is hard to copy, so it faces limited generic competition and could produce a longer, steadier revenue stream than a typical drug.
That durability is what strengthened Wall Street confidence in Merck’s future progress, after 2028, when Keytruda loses exclusivity.
How analysts reset their Merck price targets
The data triggered a wave of higher price targets across Wall Street.
Goldman Sachs raised its 12-month target on Merck to $160 from $140 and applied a higher earnings multiple, of 16 times, up from 14 times.
Other firms moved in the same direction:
- UBS lifted its target to $175 from $145 and kept a Buy rating.
- Bank of America raised its target to $166 from $141.
- JPMorgan moved to $150.
Not every analyst turned more bullish.
RBC Capital downgraded Merck to Sector Perform, even while raising its target to $150, a reminder that a strong result can still leave a stock fully valued after a double-digit jump.
What still has to happen for Merck
The melanoma win is one indication. The bigger prize depends on results in other cancers, and those are still pending.
For example, Merck is focusing the vaccine on tumors that respond well to immune-based treatment, such as melanoma and non-small cell lung cancer.
Here is the near-term timeline investors are watching:
- Renal cell carcinoma (a kidney cancer): Phase 2 data expected in 2026 or early 2027
- Muscle-invasive bladder cancer: Phase 2 data in 2027
- Non-small cell lung cancer: Phase 3 data in 2027
A win in kidney cancer would suggest the approach can work in the same settings where Keytruda already helps patients.
The companies also plan to present full melanoma data at an international medical meeting and file submissions with regulators, according to Merck.
Who else moved on the news
The result lifted a group of related health care stocks in what traders call a sympathy move.
BioNTech (BNTX), which runs its own personalized cancer vaccine programs, rose about 24% as investors saw the data as broad support for the approach.
Companies tied to manufacturing and cancer diagnostics also jumped, including Maravai LifeSciences (MRVI), up about 28%, and Tempus AI (TEM), up about 24%.
Investors watched Bristol-Myers Squibb (BMY), a major player in melanoma treatment, for risk.
Goldman estimated a long-term risk of up to $1 billion to Bristol’s melanoma franchise, but expects the impact to be limited and gradual.
This is because Intismeran targets earlier-stage patients, while much of Bristol’s melanoma revenue comes from later-stage, metastatic cases.
What Merck’s move means for your portfolio
Merck’s rally reflects a real change in the company’s long-term outlook.
For investors, a few things are worth keeping in mind:
- The stock already climbed sharply, so much of the good news is now in the price.
- The melanoma result is strong, but survival data is still early, and full results have not been published.
- The larger payoff depends on kidney, bladder, and lung cancer trials that report in 2026 and 2027.
If those readouts succeed, Merck’s case for replacing Keytruda revenue gets much stronger.
If they disappoint, the stock could give back some of this gain, which is why the coming trial dates matter more than the one-day rally.
For long-term investors, Merck now offers a clearer story beyond its patent cliff, backed by a technology that just proved it can work.
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