Moderna loses 17.6% in a day after a leak of its melanoma vaccine data, which beat Keytruda alone

Moderna shares closed at $202.63 on Thursday, down $43.18 or 17.6% from Wednesday's $245.81, after partial data from its personalized melanoma vaccine appeared online before the company and its partner Merck released the full results, CNBC reported. The stock traded between $180.80 and $241.26 during the session, according to Nasdaq. Reuters put the one-day loss in market value at about $24 billion. Nasdaq lists Moderna's market value at roughly $80.9 billion.

What the trial showed

The vaccine, intismeran autogene, is made for each patient from the mutations in their own tumor and is given with Merck's immunotherapy Keytruda. Per CNBC's account of the full release, the combination cut the risk of recurrence or death in certain melanoma patients by 42% against Keytruda alone, and the risk of the cancer spreading to distant sites or death by 38%. About 82% of patients on the combination were alive without their melanoma returning at two years, against 70% on Keytruda alone. A separate analysis of patients who completed the six-month regimen showed a 58% reduction.

The companies said it is still too early to say whether the treatment extends overall survival. Detailed phase 3 results are due at the European Society for Medical Oncology conference this weekend.

Why a good result moved the stock down

The move was about expectations, not efficacy. CNBC reports Moderna shares were up more than 530% for the year going into the release. Nasdaq shows a 52-week range of $22.28 to $254.48, so the stock was trading near the top of a range that spans more than tenfold. BMO Capital Markets analyst Evan Seigerman wrote that the reaction "likely reflects inflated expectations into the full data release, rather than a true indictment" of the combination, and UBS analysts called it "overall a strong data set," as quoted by CNBC. Merck, a far larger company, fell about 1%.

The Financial Times and Reuters both framed the drop around expectations as well, while the WSJ and Bloomberg focused on the leak. We could not find a statement from either company on how the data became public early, so we do not report one.

Who feels it

For anyone holding Moderna options, this is the case where being right on the science still produced a large loss on the stock. Shares after hours were $198.30 at 6:50 PM ET, per Nasdaq, so the opening reaction on Friday is not settled. For biotech holders more broadly, it is a reminder that a trial read-out priced for a perfect result leaves little room: a 42% risk reduction on top of a strong standard of care is, by the trial investigators' account, a meaningful clinical result, but the share price had already counted on more.

What to watch

• The ESMO presentation this weekend, for subgroup data and side effects. The investigator CNBC quoted described milder toxicity than other cancer treatments.
• Overall survival data, which the companies say is not yet available.
• The vaccine studies in kidney, bladder and non-small cell lung cancer, which UBS said gain support from this data set.
• The manufacturing time. Each dose is custom made and the process can take about six weeks, per CNBC.

For the broader backdrop on Thursday, see our market close.

Sources: CNBC; Reuters, Financial Times, WSJ and Bloomberg coverage of the leak; Nasdaq quote and summary data for MRNA read Oct. 22, 2026 at 6:50 PM ET; filings on SEC EDGAR. Chart: Yahoo Finance price history. This is market information, not investment advice.