Moderna shares plunge 25% on reported leak of melanoma vaccine results

The size of Moderna's move shows how much of its pipeline value rests on this personalized cancer vaccine, and the spillover to Merck and the biotech index reflects that the readout is treated as a sector-level signal, not just a company event. The critical detail is that the leak is unconfirmed, meaning the market is trading on rumor rather than data—a distinction that matters for how durable the repricing proves. What's worth watching is whether either company issues a formal statement, and whether the broader biotech weakness holds once the actual results are disclosed.