Moderna shares plunge after cancer vaccine data leak unsettles investors

Moderna's personalized cancer vaccine had helped fuel a substantial stock rally in 2026. Then a reported leak of trial data sent shares down about 25%, according to Forbes.

The vaccine, developed with Merck, targets melanoma. MarketWatch reported that the companies were expected to share Phase 3 results at a medical conference this weekend. Partial data reached investors before that presentation.

On October 22, ABC reported that a user on X had circulated an unverified report about the vaccine. Its coverage put Moderna's share decline at as much as 24%.

Forbes described a 25% plunge, a sharp reversal from the rally built on optimism about the vaccine. Bloomberg reported that Merck shares also fell, describing investors as unconvinced that the findings established a new era of cancer treatments.

Full data were released later Thursday. According to CNBC, those results appeared positive against Wall Street expectations before the release, despite the selloff following the partial leak.

STAT News reported that detailed results confirmed the vaccine met key endpoints in the melanoma trial. Its summary described a 42% reduction in disease recurrence risk compared with standard treatment.

The coverage differs on the headline benefit. The Independent and Ars Technica Tech reported a 42% reduction in recurrence or death risk, while Financial Times reported 58% for recurrence or death. The supplied reports do not explain that discrepancy.

The clinical findings and the market response point in different directions. Associated Press reported that the major trial showed a significant reduction in the chance of melanoma returning. But Financial Times described the results as failing to exceed investors' lofty expectations.

The reporting describes a leak of partial data, not an established cybersecurity breach. ABC called the circulated material unverified, while CNBC distinguished the partial leak from the full results released later that day.

The next scheduled milestone is the weekend medical conference, where Moderna and Merck are expected to share Phase 3 data, according to MarketWatch. The presentation will provide another opportunity to examine the findings beyond the partial material that initially circulated.

A central unresolved issue in the supplied coverage is why reports give different risk reductions. Until that difference is explained, the 42% and 58% figures should not be treated as interchangeable.