Personalized mRNA vaccine cuts melanoma recurrence or death risk by 42%

By Daniel Okafor

A personalized mRNA melanoma vaccine cut the risk of recurrence or death by 42% when added to Keytruda in a late-stage, placebo-controlled trial of 1,137 patients, according to results Moderna and Merck released Thursday.

The cohort had a clear starting point. Every patient had melanoma surgically removed. Every patient also received Merck’s Keytruda, the immunotherapy that helps the immune system fight cancer.

Who received the experimental vaccine?

The study tested a tumor-specific add-on, not a replacement for surgery or established immunotherapy.

• 757 patients: received intismeran plus Keytruda.
• 380 patients: received placebo plus Keytruda.

Those groups were almost a two-to-one split, with twice as many patients in the vaccine group. The trial therefore asked whether individualized vaccination improved recurrence outcomes beyond Keytruda alone after resection.

Intismeran is an experimental vaccine made using genetic information from an individual patient’s tumor. It is designed to teach the immune system to recognize proteins on cancer cells and attack them if the cancer returns. The treatment is personalized because its starting material comes from the patient’s own tumor.

In short, intismeran was tested as an add-on to Keytruda, not as a replacement for surgery or immunotherapy.

What did the two-year results show?

Two years after treatment began, 82% of patients receiving intismeran with Keytruda had no cancer recurrence. The figure was 70% among patients receiving Keytruda with placebo.

That is a 12-percentage-point separation in the observed no-recurrence rates. For every 100 patients at that mark, about 12 more people in the vaccine group remained free of recurrence than in the placebo group.

The 42% headline figure was a relative reduction in the combined risk of melanoma returning or death. The 12-point figure was an absolute difference in two-year no-recurrence rates.

The BBC, citing the same results, also reported a 38% reduction in the risk of melanoma spreading. The released account did not define that endpoint in enough detail to equate it with recurrence or death.

In plain terms, adding intismeran produced a 12-point higher no-recurrence rate at two years than Keytruda with placebo.

What side effects were reported?

The results listed:

• injection-site soreness or pain
• fatigue
• mild fever

Because every participant received Keytruda, the summary does not separate reactions attributable to intismeran from those associated with Keytruda or the combination. The announcement also gave no event counts or severity grades, leaving the frequency and seriousness of these effects unavailable in the released summary.

The public safety picture therefore remains limited to three listed reactions, without enough numerical detail for a fuller comparison.

What remains uncertain before peer review?

The findings are scheduled for presentation Saturday at a European Society for Medical Oncology meeting in Madrid. The full results have not yet appeared in a peer-reviewed journal. Researchers will continue following the patients to assess longer-term outcomes, including whether the vaccine improves survival.

The trial offers a direct comparison: 757 vaccine recipients against 380 placebo recipients, all after surgery and on Keytruda. Still, peer review, fuller endpoint definitions and longer follow-up remain necessary before clinicians can judge how durable the benefit might be.

One design feature narrows the question. Every patient received Keytruda, so the trial cannot show whether intismeran works without standard immunotherapy. Nor do two-year recurrence findings establish a survival advantage; researchers specifically listed survival as an outcome still to assess.

The next evidence package must address those questions, not simply repeat the headline percentage.

How did the market respond?

Moderna shares fell 18% Thursday after the results became public, according to The Associated Press. The stock moved immediately after the Thursday disclosure, falling 18% even as the company reported an encouraging 42% reduction in the combined risk of melanoma recurrence or death.

The share price is not a clinical endpoint. The AP report linked the decline to the results becoming public but did not provide a detailed investor rationale.

Clinical signal and market reaction moved in opposite directions. The stock fall does not overturn the efficacy finding; it simply marks a sharp, immediate investor response to incomplete public data.

The next checkpoint is Saturday’s ESMO presentation in Madrid. Continued follow-up must then establish whether fewer recurrences translate into better survival. Until then, intismeran remains an investigational personalized vaccine with a striking two-year recurrence signal, not yet a peer-reviewed standard of care.