WASHINGTON (AP) — A first-of-its-kind cancer vaccine is significantly reducing the chances that melanoma patients’ tumors come back, according to results of a major study released Thursday by drugmakers Moderna and Merck.

The vaccine was custom-brewed for each patient to match their tumor. Adding it to a standard post-surgery treatment reduced by 42% the risk of melanoma returning two years after surgery, according to the most advanced study to date of this new strategy for fighting cancer.

“The field has been looking for a win and I think this one is a big one,” said Dr. Hussein Tawbi of MD Anderson Cancer Center in Houston, one of the researchers in the international study.

The vaccine, called intismeran, is made with the same Nobel Prize-winning mRNA technology that was behind COVID-19 vaccines. Experimental personalized cancer vaccines made with mRNA and some other approaches also are being tested against other types of cancer.

Experts not involved with the new study say its success bolsters the field.

“This is a new frontier,” said Karen Knudsen, CEO of the Parker Institute, which helps fund cancer immunotherapy research.

“It’s a big step forward for science, for oncology, because this vaccine will apply to many different cancers,” said Dr. Michael Atkins, deputy director of Georgetown University’s Lombardi Comprehensive Cancer Center.

Moderna and Merck had announced in August that the late-stage trial of intismeran was successful but the detailed results were set to be released Saturday at a meeting of the European Society of Medical Oncology in Madrid. Some of the data was leaked publicly late Thursday, prompting a full release ahead of the weekend's scientific presentation.

Most people think of vaccines as preventing an infection, like measles or whooping cough. There are already two cancer-preventing vaccines, the HPV and hepatitis B shots that block infection from those tumor-causing viruses. And research is underway to prevent some types of inherited cancer such as Lynch syndrome-caused colorectal cancer.

But a personalized vaccine designed to block surgically-removed cancer from returning is very different. For the intismeran study, scientists hunted gene mutations unique to each patient’s tumor, mutations that create proteins which shouldn’t be in the body unless those cancer cells start to grow again. The custom vaccine — nine doses given three weeks apart — teaches the patient’s immune system to watch out for and attack those so-called neoantigens.

How did it work? The study included more than 1,100 people with high-risk melanoma that was surgically removed. Everyone was given the standard follow-up therapy: Merck’s immunotherapy named Keytruda. The participants were randomly assigned to also receive custom-brewed vaccine doses or a placebo.

Two years later, 82% of the people in the vaccine group were still living without a recurrence of their cancer compared to 70% who had gotten standard Keytruda alone. The trial is continuing to track participants’ longer-term outcomes.

The vaccine’s side effects were mainly the sore arm, fatigue and other reactions typical of mRNA coronavirus vaccination, Tawbi said.

Moderna shares dropped 18% Thursday after the study’s full results were made public, but they remained more than 200% higher than they were in late August before investors were first told of the initial results.

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