Details of Moderna’s Breakthrough Cancer Vaccine Got Leaked. Its Stock is Plunging.

Moderna's shares plunged on Thursday after data of its breakthrough cancer vaccine developed with Merck got leaked online.

The stock fell up to 25% in the afternoon even though the information was positive compared to what was expected by analysts.

The data showed that the vaccine, along with Merck's immunotherapy Keytruda, reduced the risk of recurrence or death in certain melanoma patients by more than 40% compared to just Keytruda.

The therapy also reduced the risk of the cancer spreading to distant parts of the body by 38% in comparison with just Keytruda.

Despite the stock's decline, Moderna has jumped more than 500% since the beginning of the year, largely due to expectations for the vaccine.

The clinical trial released in August focused on the deadliest form of skin cancer known as melanoma. The landmark study essentially allows customized cancer treatment meant to address the unique characteristics of each patient's illness. Bancel described the findings as a "pivotal moment" in the field of cancer research.

"For many years, the idea of creating an mRNA treatment designed specifically for an individual patient's cancer was aspirational. We are now helping turn that vision into a reality," he said. "Together with Merck, we have started to demonstrate the transformative potential of this technology to address critical unmet needs in the adjuvant melanoma setting."

Professor Georgina Long, the study's principal investigator and medical director of Melanoma Institute Australia, described the study as a "landmark moment" for adjuvant melanoma treatment.

She said it was the first Phase 3 study that showed intismeran, a treatment designed based on the unique mutational "fingerprint" of a patient's own tumor, can help reduce recurrence or death. The treatment was delivered in combination with pembrolizumab.

"Intismeran in combination with pembrolizumab has the potential to establish a new treatment paradigm in the adjuvant melanoma setting, helping patients remain cancer-free for longer," Long said.

Dr. Dean Y. Li, president of Merck Research Laboratories, said that one of the goals of adjuvant therapy is to increase the possibility of a cure in more patients.

"These first Phase 3 findings for intismeran in combination with KEYTRUDA as adjuvant therapy reinforce the promise of a more personalized approach to cancer treatment," he said.

He said the research has the potential to "redefine" how stage 4 melanoma patients are medically treated.

Merck CEO Robert Davis told CNBC last week that process to develop the vaccine can take six weeks but is confident that it can shorten the timeline soon.