Moderna shares drop 25% after leak of cancer vaccine data, despite positive trial results

Personalized melanoma vaccine reduces recurrence risk by 42% when paired with Keytruda

By LineZotpaper

Published22 October 2026

Updated22 October 2026

Read Time2 min

Sources4 outlets

Moderna shares plunged as much as 25% on Thursday after a leak of partial data from its closely watched cancer vaccine trial, even though the full results released later showed the treatment met Wall Street expectations. The personalized vaccine, developed with Merck, reduced the risk of recurrence or death in certain melanoma patients by 42% when combined with Merck's immunotherapy Keytruda, compared with Keytruda alone.

Moderna's stock fell sharply on Thursday following the leak of partial data on the company's experimental cancer vaccine, intismeran autogene. The full phase 3 trial results, released later in the day, showed the vaccine paired with Merck's Keytruda reduced the risk of cancer recurrence or death by 42% in certain melanoma patients, and lowered the risk of cancer spreading to distant parts of the body or death by 38%, compared with Keytruda alone. The data is scheduled to be presented at the European Society for Medical Oncology conference this weekend.

Despite the positive findings, Moderna shares dropped as much as 25% on Thursday. The stock had risen more than 530% year-to-date as of Thursday afternoon, reflecting high investor expectations for the vaccine, which is seen as a potential breakthrough treatment that could be adapted for other cancers. Merck shares fell about 1% on the day; Merck's stock has risen nearly 41% this year.

The personalized vaccine requires a complex manufacturing process that takes around six weeks for each patient. Merck CEO Robert Davis told CNBC last week that the company believes it can shorten that timeline in the future, potentially allowing broader patient access. For Moderna, the vaccine represents a critical commercial opportunity beyond its existing portfolio of vaccines for COVID-19, influenza and respiratory syncytial virus.

Analysis

Why This Matters

The trial results could pave the way for a new class of personalized cancer treatments, with potential applications beyond melanoma.
For Moderna, the vaccine is a key pipeline product that could diversify revenue beyond its COVID-19 and other respiratory vaccines.
Investors remain highly sensitive to data leaks and partial disclosures, as seen in the sharp stock reaction despite positive full results.

Background

Moderna, best known for its COVID-19 vaccine, has been expanding into other areas, including cancer. Its partnership with Merck combines Moderna's mRNA technology with Merck's established immunotherapy Keytruda. Personalized cancer vaccines like intismeran autogene are designed to train the immune system to target mutations specific to a patient's tumor, but manufacturing complexity and cost remain challenges.

Key Perspectives

Investors: The stock drop suggests that high expectations created a 'sell the news' reaction, or that the leaked partial data caused confusion. The 530% year-to-date gain indicates significant speculation on the vaccine's potential. Patients and oncologists: A 42% reduction in recurrence risk is a meaningful improvement in melanoma care, but the six-week manufacturing timeline and likely high cost could limit initial access. Moderna and Merck: Both companies emphasize the vaccine's potential and are exploring ways to speed up production. Merck's CEO has signaled confidence in shortening the manufacturing timeline.

What to Watch

The formal presentation of the phase 3 data at the ESMO conference this weekend, and any additional details on safety or subgroup outcomes.
Any updates from Moderna or Merck on manufacturing improvements or regulatory filing plans.
Share price movements in the coming days as the market digests the full data set.

Sources

Moderna shares plunge after leak of cancer vaccine data despite positive results · Business News
Moderna’s stock tumbles after reported leak of data for cancer vaccine · MarketWatch.com - Top Stories
Merck/Moderna cancer vaccine cuts melanoma death or recurrence risk by 42%, shares fall · Stock Market News
Moderna shares plunge after leak of cancer vaccine data despite positive results · US Top News and Analysis

Checked

12 claims in this story checked against 1 source: 7 supported.

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Partly supportedModerna shares fell as much as 25% on Thursday after a leak, despite positive trial results.US Top News and Analysis: “Moderna shares dropped as much as 25% on Thursday following the leak of partial data on its closely watched cancer vaccine developed with Merck. The stock fell even though the full data, released later Thursday following the leak, appeared positive relative to Wall Street expectations.”
Partly supportedThe personalized melanoma vaccine reduces recurrence risk by 42% when paired with Keytruda.US Top News and Analysis: “The personalized vaccine, known as intismeran autogene, paired with Merck's immunotherapy Keytruda reduced the risk of recurrence or death in certain melanoma patients by 42% compared with Keytruda alone.”
Partly supportedThe full phase 3 trial results released later showed the treatment met Wall Street expectations.US Top News and Analysis: “The stock fell even though the full data, released later Thursday following the leak, appeared positive relative to Wall Street expectations.”
SupportedThe vaccine and Keytruda reduced the risk of recurrence or death by 42% in certain melanoma patients compared with Keytruda alone.US Top News and Analysis: “The personalized vaccine, known as intismeran autogene, paired with Merck's immunotherapy Keytruda reduced the risk of recurrence or death in certain melanoma patients by 42% compared with Keytruda alone.”
SupportedThe combination reduced the risk of cancer spreading to distant parts of the body or death by 38% compared with Keytruda alone.US Top News and Analysis: “The data also showed the shot along with Keytruda reduced the risk of cancer spreading to distant parts of the body or death by 38%, compared with just the immunotherapy.”
SupportedThe data is scheduled to be presented at the European Society for Medical Oncology conference this weekend.US Top News and Analysis: “The detailed phase 3 trial results were set to be presented at the European Society for Medical Oncology conference this weekend.”
SupportedModerna shares had risen more than 530% year-to-date as of Thursday afternoon.US Top News and Analysis: “The stock has spiked more than 530% year-to-date as of Thursday afternoon.”
Partly supportedInvestors see the vaccine as a potential breakthrough that could be adapted for other cancers.US Top News and Analysis: “Investors see the targeted vaccine as a possible breakthrough treatment, with the potential to be used in other forms of cancer in the future.”
SupportedMerck shares fell about 1% on Thursday.US Top News and Analysis: “Shares of Merck, a much larger drugmaker, fell by about 1% on Thursday.”
SupportedThe personalized vaccine’s manufacturing process takes around six weeks for each patient.US Top News and Analysis: “Since the vaccine is personalized, the complicated manufacturing process can take six weeks.”
SupportedMerck CEO Robert Davis told CNBC last week that the company believes it can shorten the manufacturing timeline in the future, potentially allowing broader patient access.US Top News and Analysis: “But Merck CEO Robert Davis told CNBC last week that the company believes it can shorten that timeline in the future, potentially allowing the shot to reach more patients more quickly.”
Partly supportedFor Moderna, the vaccine represents a critical commercial opportunity beyond its existing vaccine portfolio.US Top News and Analysis: “It could also offer Moderna a critical additional commercial product in its pipeline, following vaccines for Covid-19, the flu and respiratory syncytial virus.”