Moderna shares dropped as much as 25% on Thursday after screenshots of partial data on its closely watched cancer vaccine with Merck leaked on social media, prompting the European Society for Medical Oncology to release the full study details early, according to BioPharma Dive.

The stock fell even though the data, released later Thursday, appeared meaningful for patients and positive relative to Wall Street expectations, CNBC reports.

The selloff continued into Friday, with 24/7 Wall St. reporting Moderna down 4% to $193.62 in morning trading after an 18% plunge the prior session, while the iShares Biotechnology ETF held steady, isolating the damage to the company.

The reaction inverts the usual script: a landmark late-stage win triggered a violent reversal rather than a rally.

The personalized vaccine, intismeran autogene, paired with Merck's Keytruda cut the risk of recurrence or death in certain melanoma patients by 42% versus Keytruda alone, and reduced the risk of cancer spreading to distant parts of the body or death by 38%, according to CNBC.

Wall Street had expected a 40% to 50% relative reduction, and Moderna had said 35% would be clinically significant, per BioPharma Dive.

BMO analyst Evan Seigerman called the reduction clinically meaningful, and told investors the reaction 'likely reflects inflated expectations into the full data release, rather than a true indictment on the efficacy of the combo therapy.'

William Blair's Myles Minter framed it as a 'sell-the-news' reaction playing out, BioPharma Dive reports.

The full phase 3 results showed about 82% of patients who received the vaccine alongside Keytruda were alive without their melanoma returning at two years, compared with 70% on Keytruda alone, according to CNBC.

A separate analysis showed a 58% reduction in risk of recurrence or death after the six-month vaccine regimen.

BioPharma Dive notes the 42% figure falls a touch short of the 44% seen in a smaller Phase 2 trial, and RBC's Trung Huynh called the results 'low end' of expectations with a 'deterioration' from earlier testing.

The rally itself was enormous: Moderna shares had spiked more than 530% year-to-date as of Thursday afternoon, per CNBC, after the trial's August primary-endpoint announcement more than quadrupled the stock, BioPharma Dive reports.

The personalized cancer vaccine sits at the center of Moderna's growth story, so the gap between data and hype hits it hardest, 24/7 Wall St. explains. Every approved Moderna product today is a respiratory vaccine, leaving oncology to carry the long-term upside the market had been paying for.

For Merck, the program is an add-on to Keytruda, so its shares fell only about 1% on Thursday, per CNBC, and were down 0.8% at $146 on Friday, according to 24/7 Wall St. Merck Research Laboratories president Dean Li argued the combination showed added benefit on top of a therapy that works well, saying 'We have no problems of showing additional benefit here,' though he cautioned more work remains across tumor types.

The full presentation at the ESMO conference this weekend is Moderna's next test, and 24/7 Wall St. notes the stock goes into the event after a slide that has already stripped out much of the optimism built during the rally.

Moderna CEO Stéphane Bancel told CNBC the company is finishing a filing with regulators seeking full approval next year. The companies are studying the vaccine in non-small cell lung, bladder and kidney cancers, and UBS analysts said the dataset gives more confidence in those upcoming studies.

BioPharma Dive reports interim results from a mid-stage kidney cancer trial could come later this year or in the first half of 2027, while 24/7 Wall St. notes Merck's CEO believes the six-week personalized manufacturing timeline can be shortened to reach more patients faster.