Leaked data from Moderna Inc. and Merck & Co.’s highly anticipated melanoma vaccine trial has led to a rare disconnect between doctors, who celebrated the result, and Wall Street, which sold off the stocks.

Normally results of such pivotal drug studies are binary, with doctors, patients and investors all learning whether a novel medicine will make a meaningful difference in people’s lives and forming the same conclusion. Instead, as reports leaked out on social media about the findings of the melanoma shot — days before the carefully scripted presentation planned for thousands of oncologists gathered in Madrid for the European Society for Medical Oncology meeting — realities diverged.

The personalized cancer vaccine developed by Moderna and Merck clearly helped patients, reducing the likelihood that their tumors would return after being surgically removed by 42% and preventing their distant spread by 38%. But shares of both companies fell, with Moderna’s plunge hitting record levels during Thursday’s trading.

The results are a breakthrough for melanoma, one of the most common types of cancer, and potentially a powerful new tool for treating many operable forms of the disease. The vaccine could help many thousands of people with melanoma that are a high risk of recurrence. Such results typically form the basis for new drug approvals, and doctors welcomed the findings.

“I am quite impressed,” said Michael Postow, a melanoma doctor at Memorial Sloan Kettering Cancer Center and one of the investigators. The benefits may increase over time, he said. “It is likely that we haven’t seen the full therapeutic effect of all of this so far.”

Even after Thursday’s drop, Moderna shares are still far above where they were before the company announced in August that the shot worked, without presenting the full set of data. They rose 1.7% at 12:36 p.m. in New York on Friday.

Wall Street, however, reset some of its loftiest expectations for the biotech company after this week’s release. It had priced in the idea that the vaccine was a starting point for a broader oncology franchise Moderna has been seeking to develop, which could one day include similar treatments for lung, bladder and other cancers.

The limited data cast some doubt on how difficult that would be, showing the ability to prevent distant spread may not be as robust as expected. The information available didn’t instill confidence in some Moderna investors, who were betting on the company’s post-COVID pivot to becoming a major cancer company.

“While approval seems likely, commercial success is far less certain,” wrote Trung Huynh, an analyst at RBC Capital Markets, in a note to investors. The data raise “fundamental questions” about the vaccine’s value proposition compared to the current gold standard therapy, which has similar benefits at potentially much lower cost, he said.

The success of the vaccine in melanoma will also put Moderna and Merck under pressure to complete tests in other cancers, even as questions swirl about near-term sales and the potential broader market for the technology.

“This is an extraordinarily complex and potentially expensive way to manufacture medicines,” said Lennard Lee, an associate professor at the University of Oxford. “The logistics could make Amazon look simple.”

All of the patients in the study also received Merck’s best-selling cancer drug Keytruda, making the personalized vaccines designed for each patient an additive therapy. The additional benefits, and the lack of serious side effects, may persuade more patients to embrace treatment, said Sekwon Jang, director of melanoma research and therapeutics and trial investigator at the Inova Schar Cancer Institute in northern Virginia.

“With this combination I think the risk-benefit ratio actually is much more favorable,” Jang said. “I expect more of my patients will chose to receive this combination treatment,” if it gets approved, he said.

But investors were already expecting strong results after the companies issued a press release in August saying the study was successful — and little else. The news buoyed expectations for the use of Moderna’s mRNA technology, designed to let doctors prime the immune system to detect and destroy foreign invaders and other threats, including tumor cells.

After decades of disappointment, the findings could herald a real advance in the development of a cancer vaccine that targets solid tumors. Details of the study results, however, haven’t been fully presented to doctors or investors yet, and are needed to parse its potential and validate their expectations.

The results will be presented on Saturday in Madrid, where a broader swath of oncologists will weigh in on the findings.

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With assistance from Madison Muller.

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